Thallium Toxicity: Mechanisms of Action, Available Therapies, and Experimental Models

Abstract

Thallium (Tl) is a non-essential and highly toxic heavy metal capable of replacing potassium (K⁺) in biological systems, leading to mitochondrial dysfunction, oxidative stress, and inhibition of protein synthesis. In humans, the estimated oral lethal dose ranges from 10 to 15 mg/kg, with acute mortality rates of 6–15% and chronic neurological sequelae in up to 55% of survivors. Environmental releases of thallium of up to 5000 metric tons annually from industrial and mining activities, combined with its high oral bioavailability and nonspecific multisystemic symptoms, underscore the urgent need for more effective therapeutic strategies. This review summarizes current evidence on Tl toxicity, including its mechanisms of action, clinical manifestations, and available treatments. It emphasizes the strategic selection of biological models: simple organisms such as Caenorhabditis elegans and Drosophila melanogaster enable high-throughput screening and early biomarker detection; zebrafish (Danio rerio) provide vertebrate-level evaluation of multi-organ effects; and rodent models offer systemic toxicokinetic and therapeutic validation. Human-derived organoids and induced pluripotent stem cell (iPSC) systems recreate tissue-specific microenvironments, allowing translational assessment of mitochondrial, neuronal, and cardiac toxicity. Integrating these models within a tiered and complementary framework, alongside environmental and clinical surveillance, can accelerate the development of targeted treatments and strengthen public health responses to Tl exposure.

1. Introduction

Thallium (Tl) is a heavy metal that is non-essential for living organisms and characterized by its high toxicity, even at low concentrations [1]. In recent decades, its presence as a trace contaminant has gained increasing relevance in both environmental and health contexts, driven by its expanding industrial use and the recognition of its adverse effects on human health and ecosystems. Although Tl can be released through natural processes, the main sources today are anthropogenic, including mining, non-ferrous metal smelting, coal combustion, and cement production [2,3].

A particularly concerning feature of many Tl compounds is their solubility in water and the absence of odor, taste, or color, which has facilitated both accidental and intentional poisonings [4]. In mammals, its toxicity exceeds that of other heavy metals, such as lead and mercury, and it manifests through nonspecific symptoms that complicate early clinical diagnosis. In humans, exposure can result in abdominal pain, alopecia, peripheral neuropathies, psychiatric disorders, cardiac abnormalities, and, in severe cases, death [2,5].

Because Tl can simultaneously affect multiple organs and systems, its study requires a multidisciplinary approach encompassing environmental toxicology as well as cellular and molecular biology. Although some therapeutic strategies have been approved, such as the use of Prussian blue, there are still no effective treatments to reverse mitochondrial and neurological damage [4,6]. In this scenario, biological models have become essential tools for elucidating the mechanisms of Tl toxicity and evaluating potential therapeutic interventions.

Traditional animal models, such as rats and mice, have been instrumental in describing tissue distribution, systemic effects, and toxicokinetics. However, the incorporation of alternative models, including simple organisms such as Caenorhabditis elegans (C. elegans), Drosophila melanogaster (D. melanogaster), and Danio rerio (D. rerio), has expanded research opportunities by enabling high-throughput assays and the identification of conserved molecular pathways [7,8,9,10]. More recently, human organoids and induced pluripotent stem cell (iPSC)-derived systems provide a translational bridge to human physiology, recreating relevant tissue microenvironments to directly assess mitochondrial, neuronal, and cardiac toxicity [11].

In this context, the present review synthesizes current knowledge on Tl toxicity, with an emphasis on the molecular mechanisms involved, clinical manifestations, and available therapeutic strategies. In particular, it highlights the biological models used to study Tl, analyzing their advantages, limitations, and complementary applications in toxicological research. By integrating mechanistic findings with experimental approaches, this work aims to provide a broader and more critical perspective on Tl as both an environmental and clinical risk, while guiding future research and therapeutic strategies.

2. Literature Search Strategy

This article is presented as a narrative review. The literature was retrieved mainly from the PubMed, Scopus, and Web of Science databases, and complemented by reports from regulatory agencies [The United States Environmental Protection Agency (EPA), World Health Organization (WHO), Secretaría de Medio Ambiente y Recursos Naturales (SEMARNAT)]. Articles were selected according to their relevance to Tl toxicity, mechanisms of action, clinical manifestations, therapeutic approaches, and biological models. Priority was given to studies published in the last 15 years, although seminal works and key historical reports were also included when relevant.

3. Characteristics of Tl

Tl is a soft, malleable heavy metal with a bluish-white color. Its atomic number is 81, and it has an atomic mass of 204.38. In nature, Tl exists in two oxidation states: I (Tl⁺) and III (Tl³⁺). Although the monovalent state is more stable than the trivalent form (Tl³⁺), both are highly toxic. The element’s name derives from the Greek word thallos, meaning “green shoot”, in reference to the bright green spectral line observed by the English chemist William Crookes during his discovery of Tl in selenium samples [12].

Despite being present in very low concentrations in the environment, Tl is classified as a non-essential and highly toxic element. The EPA has listed it as a priority pollutant [13].

The most common commercial forms of Tl are thallium acetate (TlCH₃COO) and thallium sulfate (Tl₂SO₄), in which Tl is present in the +1 oxidation state. These compounds have industrial applications with significant toxicological implications for human health. C₂H₆O₂Tl was historically used in pharmaceutical products for the treatment of fungal infections of the scalp. Tl₂SO₄, on the other hand, was used as a pesticide, particularly in syrups and gels for ant control [14,15].

4. Environmental Contamination by Tl

Tl represents a significant environmental concern due to its release during industrial activities such as mining, metal smelting, cement production, and coal combustion [2]. Although Tl occurs naturally, its abundance is generally low. Globally, the Earth’s crust contains approximately 0.7 mg of Tl per kilogram (0.7 ppm), primarily associated with potassium (K⁺)-rich minerals such as clays, granitic soils, and certain feldspars [3,16]. In uncontaminated freshwater systems, Tl concentrations generally remain below 1 µg/L, while in open-ocean waters, they rarely exceed 20 ng/L [17].

However, human activities have significantly increased Tl concentrations in air, soil, and water, with global emissions reaching up to 5000 tons per year. This poses a substantial risk of bioaccumulation in food chains [12,18]. For instance, in certain regions of China, soil concentrations of Tl have reached up to 124 mg/kg, and crops such as lettuce, carrots, and chili peppers have shown levels as high as 500 mg/kg (dry weight), far exceeding international safety limits and posing a threat to food security [19]. In the Silesia–Cracow region (Poland), surface soils have exhibited Tl concentrations of up to 150 mg/kg, with values ranging between 4.4 and 49.8 mg/kg in historically mined areas, averaging 20.8 mg/kg [20]. In old mining waste deposits near Olkusz, average levels reach 43 mg/kg, with localized peaks of 78 mg/kg [21]. Beyond these areas, the Allchar site (Republic of North Macedonia) is notable for its extreme concentrations, ranging from 0.11 to 20,000 mg/kg, with an average of 660 mg/kg in surface soils [20].

Moreover, documented incidents between 2010 and 2018 in major rivers highlight the ongoing contamination linked to mining and metallurgical activities, despite the fact that the use of Tl-containing minerals in industry dates back to the 1960s, when specific regulations were still lacking [22]. A notable case is Lake Sanliqi in Daye, which has experienced chronic contamination by Tl and other metals for more than 60 years due to copper smelting waste, affecting even nearby agricultural areas [23,24].

At the regulatory level, there are significant differences between countries and regions regarding Tl regulation, which has been focused mainly on drinking water. In the United States, the EPA has set a maximum contaminant level (MCL) of 2 µg/L for Tl in drinking water, with a non-enforceable health goal (MCLG) of 0.5 µg/L. California has proposed a more stringent limit of 0.1 µg/L [25]. In Canada, the Canadian Council of Ministers of the Environment (CCME) recommends a guideline value of 0.8 µg/L [26]. In Europe, Directive 98/83/EC requires member states to establish limits for chemical contaminants in drinking water; for example, Italy has adopted a maximum limit of 2 µg/L in groundwater [27]. China enforces one of the strictest limits globally, at 0.1 µg/L for human consumption, reflecting a response to severe contamination events [28] In Mexico, although Tl and its salts have been banned as rodenticides for decades, there is no specific Official Mexican Standard (NOM) for Tl in drinking water. The only applicable standard is NOM-147-SEMARNAT/SSA1-2004 [29], which establishes a limit of 5.2 mg/kg in soils for agricultural, residential, or commercial use. However, increasing industrial activity, illegal mining, and inadequate wastewater disposal in remote areas may contribute to a rise in Tl contamination in the country. In contrast, the WHO has not established a specific guideline value for Tl in drinking water.

In terms of future trends, although emissions from traditional industries may decline with the shift to clean energy and stricter environmental controls, the rapid expansion of lithium and battery production poses a new challenge [30]. Minerals used in these industries often contain Tl, potentially increasing contamination risks if proper regulations are not enforced [30]. Furthermore, the persistence of Tl in marine sediments and agricultural soils represents a long-term hazard, especially under changing redox conditions that may remobilize the metal into water systems, enhancing its bioavailability and bioaccumulation [31]. Given these concerns, effective Tl management requires harmonized regulations, continuous monitoring, and the implementation of efficient remediation technologies to minimize both environmental and public health impacts.

5. Statistics and Reported Cases of Tl Poisoning in Humans

Unlike other heavy metals, there are currently no consolidated global statistics on Tl poisoning, likely due to its low incidence. However, its effects can be severe: the acute mortality rate is estimated to be between 6% and 15%, with a lethal dose of approximately 10–15 mg/kg in humans, although deaths at lower doses have been documented. Additionally, between 33% and 55% of survivors develop chronic neurological or visual sequelae [32,33].

One factor that complicates surveillance is the lack of adequate toxicological monitoring systems in many countries, which contributes to the underreporting of cases, delayed diagnoses, or confusion with other syndromes of neurological or hepatic origin. In the United States, the National Poison Data System reported 60 unique cases of Tl exposure in 2023, mostly in adults over the age of 20. No deaths were reported, and only one case presented severe complications [34]. This figure represents an increase compared to 2019, when 49 exposures were recorded, also without fatalities [35].

Various clinical studies have documented the manifestations and progression of Tl poisoning. A case reported in 2023 by the Journal of Yeungnam Medical Science described a 51-year-old Argentine woman who developed myalgia, vertigo, jaundice, and alopecia. Urinary Tl levels of 540 µg/g creatinine were detected (normal range: 0.4–10 µg/g). She was successfully treated with D-penicillamine, achieving full clinical recovery after one year [36].

In China, a retrospective study (2000–2010) analyzed 13 patients initially diagnosed with other pathologies, such as Guillain–Barré syndrome. Most exposures were accidental or due to deliberate poisoning. In some cases, the source was contaminated water or food, including edible mushrooms harvested from Tl-contaminated soils. The average hospital stay was 44 days. One person died, and among the survivors, long-term sequelae were documented: peripheral neuropathy, cognitive impairment (33%), anxiety (50%), and depression (42%) [37].

That same year, also in China, the poisoning of a family of 14 people was reported after they consumed Tl-contaminated bread. Although initial symptoms were mild to moderate (vomiting, painful neuropathy, alopecia, fatigue, among others), one patient with pulmonary fibrosis died due to respiratory failure. During follow-up, some survivors developed complications such as venous thrombosis and residual paresthesia, although most recovered without permanent sequelae [38].

One of the most notorious cases of deliberate Tl exposure is that of Zhu Ling, a student at Tsinghua University (Beijing), who was chronically poisoned between 1994 and 1995. She presented with abdominal pain, alopecia, coma, and severe neurological damage. Although she initially survived, she was left with permanent disabilities and died in 2013. Her case was pioneering in the use of telemedicine for diagnosis, as it was internationally discussed in online medical forums. She received treatment with plasmapheresis, hemodialysis, and Prussian blue, although she developed severe complications during hospitalization [39].

In Mexico, the case of a 19-year-old pregnant woman who ingested Tl-containing rat poison with suicidal intent was reported. Within a week, she developed characteristic symptoms such as alopecia, Mees’ lines, and palmar erythema. Her newborn presented with stupor, distal cyanosis, and alopecia. Although the child showed favorable progression, psychomotor delays were detected at three years of age [40].

Taken together, clinical cases reflect variability in routes of exposure and clinical manifestations, but also highlight frequent signs, such as abdominal pain, vomiting, tachycardia, fatigue, alopecia, and neurological disturbances. Mees’ lines on the nails and generalized alopecia are distinctive findings, although they appear late (2 to 3 weeks after exposure) [36,37,38,39,41]. Neurological complications include peripheral neuropathies, ataxia, motor dysfunction, and psychiatric disorders such as anxiety, depression, or hallucinations [42,43].

Table 1. Representative cases of thallium (Tl) poisoning in humans.

Case (Location/Year)	Source of Exposure	Detected Doses	Diagnosis (Methodology)	Main Symptoms	Treatment	Outcome	Ref.
Zhu Ling (Beijing, China/1994–1995)	Unknown (criminal suspicion)	Urine: 275 µg/L; blood: 31 µg/L; hair: 531 µg/kg; nails: 22,824 µg/kg	ICP-MS in blood, urine, hair, nails, and CSF; lumbar puncture; epidemiological tracing via internet; time reconstruction in hair using laser ablation ICP-MS	Abdominal pain, alopecia, intense leg pain, paresthesia, paralysis, coma	Plasmapheresis (seven sessions, ≈10,000 mL total), ventilatory support, mechanical ventilation, Prussian blue, chelators, and metabolic support	Chronic lower limb paralysis, optic atrophy, generalized brain atrophy, permanent intellectual and physical impairment	[39]
14 family members (mean age 36) with late hospitalization (China/2008–2018)	Ingestion of contaminated food (accidental/criminal)	At 2 weeks: Blood: 219–1414 µg/L (median 535); urine: 956–11,285 µg/L (median 7460)	Atomic absorption or ICP in blood and urine; EMG, brain imaging/EEG in neurological cases	Vomiting, pain, painful neuropathy, alopecia, fatigue, hyperpigmentation, mild hepatitis, confusion or coma	First: chelator sodium dimercaptosulfonate (5 mg/kg IM 1–4x/day); second: oral Prussian blue (250 mg/kg/day in 50 mL 20% mannitol, divided in four doses); daily hemodialysis (4–6 h/day)	One died due to previous pulmonary fibrosis/ARDS, one developed deep vein thrombosis in left leg, and one had residual paresthesia and generalized anxiety. Most recovered without sequelae	[38]
40-year-old male (China, 2014)	Acute unknown ingestion (possible criminal exposure)	Blood: “Supralethal” levels > estimated lethal range of 10–15 mg/kg in humans	Tl monitoring in blood and urine; neurological and metabolic clinical assessment	Severe abdominal pain, intense vomiting, ascending dystrophic neuropathy, confusion	Hemoperfusion (three rounds; reduced Tl in blood by 20–35%), followed by continuous CVVH (5 rounds; 40–64% reduction each round), chelation with 2,3-DMS (250 mg/day IM), nutritional and metabolic support (including Prussian blue 250 mg/kg/day orally)	Fully recovered without significant neurological sequelae	[44]
51-year-old female (Buenos Aires, 2018)	Unidentified accidental exposure	Urine: 540 µg/g creatinine (healthy range: 0.410 µg/g)	Normal heavy-metal screening (Pb, As, Hg, Cu); thallium measured in urine	Myalgia, vertigo, asthenia, abdominal pain, jaundice, confusion, asterixis, thrombocytopenia, alopecia (eyebrows, scalp, armpits), plantar ulcers	Oral D-penicillamine 1000 mg/day (preferred over Prussian blue due to contraindication); supportive care and annual follow-up	Full resolution after one year, normalized urinary excretion, no apparent sequelae	[36]
18-year-old male (USA, 2024)	Ingested elemental thallium bar (~90% pure–100 g) purchased online, suicide attempt	Peak serum: 423.5 µg/L (normal <5.1); urine: 1850.5 µg/g creatinine (normal <0.4); metal fragment ~100 g	Serial ICP-MS in serum and urine; abdominal X-ray; colonoscopy for fragment extraction	No classical symptoms (no vomiting, diarrhea, alopecia, neuropathic pain, limb weakness, or systemic signs)	Activated charcoal, oral Prussian blue (3 g TID), endoscopic fragment removal, intestinal lavage with polyethylene glycol	Completely asymptomatic; negative serum and urine at 44 days; no alopecia or neurological deficits	[45]
Glass factory worker (Japan, 1998)	Daily exposure to raw materials containing Tl at glass plant (chronic occupational exposure, ~4 years)	Hair: ~20 ng/g (above healthy range ~5–10 ng/g)	ICP-MS in hair sample; neurological exam; nerve conduction velocity; work history	Mild alopecia, chronic diarrhea, colicky abdominal pain, paresthesia in hands/feet, glove-and-stocking neuropathy	Immediate removal from exposure and symptomatic treatment; no chelators reported	Persistent mild neuropathy; no progression or severe sequelae	[46]
Adulterated herbal infusion, family (Granada, Spain/1985–1987)	Infusion consumed by all; adulterated with thallium thiosulfate (presumed criminal act)	Not quantified (clinical information based on symptoms and family evolution)	Clinical exam + family history; diagnosis confirmed by thallium lab analysis (unspecified method)	Vomiting, abdominal pain, GI bleeding, severe headache, paresthesia, sensory–motor polyneuritis	Symptomatic treatment (decontamination, fluids, neuromuscular support); Prussian blue use not always documented	Gradual recovery in 3–9 weeks without lasting sequelae in most survivors	[47]
Five patients (Mexico, 1989)	Three suicide attempts; two unknown accidental ingestions	Detectable Tl in blood and urine (not publicly quantified)	Blood and urine analysis; scalp skin biopsy (telogen phase, pigment in keratin, hypotrophic follicles)	Diffuse alopecia (all cases); GI and neurological symptoms in accidental cases. Radiopacities in tibial metaphyses in children	Oral D-penicillamine (dosage not specified); dermatological follow-up	Partial restoration of neuropathy in some cases	[48]
13 cases (Sichuan, China/2000–2010)	Oral ingestion of thallium (criminal or accidental, including contaminated water, food, and mushrooms from Tl-rich soil)	Elevated Tl levels in urine and blood (specific values not reported)	Clinical–toxicological diagnosis; majority initially misdiagnosed (e.g., Guillain–Barré syndrome); later confirmed via toxicological analysis	Peripheral neuropathy, cognitive impairment, alopecia, anxiety, depression, digestive symptoms	Supportive care, Prussian blue, hemodialysis in some cases	One death during hospitalization; follow-up up to 12 years: persistent neurological sequelae in several patients (33% cognitive decline, 50% anxiety, 42% depression)	[37]

ARDS: Acute Respiratory Distress Syndrome, CVVH: Continuous Veno Venous Hemofiltration, DMS: Dimercaptosuccinic Acid, EEG: Electroencephalogram, EMG: Electromyography, ICP MS: Inductively Coupled Plasma Mass Spectrometry, TID: ter in die.

summarizes the most representative clinical cases described in the recent literature.

6. Toxicokinetics of Tl

Tl enters the body through various routes, including the skin, inhalation, and primarily the gastrointestinal tract, where its absorption is particularly efficient, reaching an oral bioavailability close to 100% [49,50]. Water-soluble Tl⁺ salts can enter the body using the same cellular transport mechanisms as K⁺, due to their ionic similarity [50]. It is important to note that both Tl⁺ and Tl³⁺ are present under physiological conditions. The estimated oral lethal dose of Tl for humans ranges from 10 to 15 mg/kg [32].

The stability of Tl⁺ in aqueous solutions is notably influenced by pH and redox potential (Eh). Tl⁺ is highly soluble and mobile across a wide pH range (approximately 3 to 12), whereas Tl³⁺ rapidly reduces to Tl⁺, and its solubility is limited by hydrolysis reactions [51]. Monovalent Tl⁺ is the predominant and thermodynamically most stable form in aquatic systems, whereas Tl³⁺, although it can exist under specific conditions, is typically unstable [3].

In the gastrointestinal tract, pH varies significantly: it is acidic in the stomach (pH 1.5–3.5), increases in the duodenum to approximately pH 5–6, and reaches pH 7–8 in the distal small intestine. Although specific studies on gastric pH’s influence on Tl⁺ absorption are limited, the high solubility of Tl⁺ ions in water suggests that they may remain in solution under acidic conditions, facilitating absorption in the stomach [52]. Once absorbed, Tl is widely distributed throughout the body, with the highest concentrations found in the kidneys, although it also accumulates in the liver, heart, and brain [5]. Its transport is facilitated by binding to proteins such as transferrin, and it exhibits a high volume of distribution (3–10 L/kg) [53]. Experimental studies have shown heterogeneous distribution within the brain, with greater accumulation in the hypothalamus [54]. Furthermore, Tl has been shown to cross both the blood–brain barrier and the placental barrier, increasing its toxicological risk, particularly during pregnancy [55,56]. Regarding metabolism, Tl exposure induces the expression of metallothionein (MT), a cysteine-rich protein that plays a protective role by neutralizing the metal and scavenging free radicals (FRs) [57,58,59]. It is noteworthy that Tl does not undergo enzymatic or chemical transformations (as many xenobiotics do).

Tl elimination primarily occurs through urine (26%) within the first 24 h; however, over time, feces become the predominant excretion route (51%) [5]. Under normal conditions, urinary Tl concentrations in humans do not exceed 1 μg/g of creatinine [12]. Additionally, Tl can be excreted via saliva, tears, sweat, and breast milk. Due to its strong affinity for cysteine residues in keratin, Tl is deposited in tissues such as hair and nails, leading to very slow elimination and enabling its detection even months after exposure [49,60]. This slow excretion results in a risk of accumulation even at low levels, and its biological half-life can extend up to 30 days, potentially causing long-lasting or irreversible neurological consequences [5,60] (Figure 1).

7. Mechanisms of Toxicity

Exposure to Tl poses a significant risk to human health due to its ability to induce systemic and cellular alterations in a progressive and silent manner. Although its initial clinical manifestations may be nonspecific, the toxicity of this heavy metal extends to various organs and tissues, affecting key physiological functions and compromising cellular integrity. Its chemical similarity to K⁺, particularly the comparable ionic radius and monovalent charge of Tl, allows it to enter cells through K⁺ transport systems. Once inside, Tl⁺ can partially substitute for K⁺ in metabolic processes but fails to sustain its essential biochemical functions. This misreplacement disrupts critical processes such as energy production, protein synthesis, and redox balance, ultimately leading to cellular dysfunction and toxicity. These properties make it a highly harmful agent with both clinical and molecular implications [61,62].

Below, the main effects of Tl in the body are described in detail at the organ level, along with metabolic alterations that characterize its toxic mechanism.

7.1. Cellular Effects

Tl enters cells through the same channels and transporters as K⁺, disrupting ionic gradients and altering membrane potential, processes essential for the excitability of neurons and muscles. Once inside the cell, Tl promotes the production of reactive oxygen species (ROS); however, the biochemical mechanisms are not yet fully understood. Some studies suggest that Tl interferes with the cellular antioxidant system [63,64]. For example, Tl(OH)₃ has been reported to inhibit the activity of key enzymes such as glutathione peroxidase (GPx) and glutathione reductase (GR) in rat brain tissue [63,64]. The increased generation of ROS leads to the oxidation of fatty acids in the membrane, which in turn enhances membrane fluidity [65]. In fact, the interaction of Tl with membrane phospholipids, especially those found in brain tissue, such as phosphatidylcholine and phosphatidylserine, alters the physical properties of the bilayer. This is important because these changes may affect various membrane-dependent processes, such as the activity of associated enzymes and intracellular transport, which could contribute to the neurotoxicity associated with Tl poisoning [66].

Additionally, the increase in ROS production due to Tl exposure appears to be a key factor in disrupting the cell cycle. It has been shown that incubation with C₂H₆O₂Tl halts cell cycle progression at the G2/M phase, which is associated with decreased expression of cyclin-dependent kinase 2 (CDK2) and upregulation of p21 [67]. More recently, it was reported that Tl, even at concentrations that do not affect cell viability, disrupts the cell cycle in response to DNA base oxidation mediated by oxidative stress [68].

7.2. Effects on Organelles

Mitochondria are among the primary subcellular targets of Tl toxicity, largely be-cause these organelles rely on ATP-sensitive K⁺ channels, which can also permit Tl entry [69]. Once inside the mitochondria, Tl has been shown to induce loss of mitochondrial membrane potential [70,71]. This is relevant because it prevents the maintenance of the electrochemical gradient required for oxidative phosphorylation.

It has also been demonstrated that in certain cells, Tl reduces the activity of mitochondrial complexes in the electron transport chain (ETC) [72], which likely explains, at least in part, the significant increase in hydrogen peroxide and other ROS reported [73].

Furthermore, Tl has been documented to promote the opening of the mitochondrial permeability transition pore (mPTP) [74]. This pore is known to form under conditions of extreme mitochondrial stress, allowing the free entry of water and small molecules and causing changes in mitochondrial morphology, such as organelle swelling. Another consequence of mPTP opening is the release of cytochrome c and other pro-apoptotic proteins into the cytosol, leading to activation of the intrinsic apoptotic pathway and subsequent cell death [73].

Taken together, these alterations converge in the disruption of adenosine triphosphate (ATP) synthesis. Given the high energy demand of nervous tissue, Tl-induced mitochondrial dysfunction has deleterious effects on neuronal function.

Moreover, Tl negatively affects protein synthesis in animals by interfering with ribosome stability. For example, administration of thallium nitrate (TlNO₃) to mice results in reduced incorporation of amino acids into proteins in vivo, as well as disaggregation of hepatic polyribosomes [75]. It was reported that Tl significantly reduced levels of the 60S ribosomal subunit. This reduction was found not to result from increased degradation, but rather from a blockade in subunit biogenesis. The 60S subunit plays a crucial role in peptide elongation and overall protein synthesis, and its disruption can severely compromise cellular function. Consequently, ribosome assembly is impaired, leading to a global halt in protein synthesis and underscoring the importance of the 60S subunit in maintaining cellular homeostasis and viability [74].

Additionally, Tl also affects the endoplasmic reticulum (ER). Following metal exposure, structural changes, such as organelle dilation, increased membrane abundance, and even fragmentation, have been observed. Recently, it was found that Tl may induce ER stress, as significant increases in two key ER stress response proteins were detected 24 h after exposure in Madin–Darby canine kidney (MDCK) cells [76,77,78].

7.3. Effects on Metabolic Pathways

Due to Tl’s interference with metabolic processes that depend on K⁺, this metal can alter the enzymatic activation of not only Na⁺/K⁺ ATPase but also other enzymes that require the presence of K⁺ ions to function properly. Among these is pyruvate kinase [79], a key enzyme for cellular energy production and essential for the proper maintenance and metabolism of red blood cells.

Another major mechanism of Tl toxicity is related to its high affinity for the sulfhydryl groups present in many proteins. The binding of Tl to these sulfhydryl groups interferes with the active sites of several enzymes critical to cellular metabolism, including hydrolases, transferases, and oxidoreductases [80]. In particular, inhibition of pyruvate dehydrogenase (PDH) and succinate dehydrogenase (SDH) has been reported [81]. Both enzymes are located in mitochondria and are essential for energy metabolism. PDH inhibition limits the conversion of pyruvate into acetyl coenzyme A (CoA), thereby reducing the oxidative metabolism of glucose. SDH inhibition, since this enzyme is central to both the Krebs cycle and the electron transport chain, leads to decreased ATP production.

Tl also interferes with sulfhydryl groups in cysteine residues, preventing the formation of disulfide bonds necessary for keratin synthesis. This disruption compromises the structural integrity of the protein and clinically manifests as abnormalities in hair (alopecia) and nails (the appearance of Mees’ lines) [82].

Figure 2 summarizes the main toxic mechanisms exerted by Tl at the cellular level.

7.4. Other Mechanisms of Toxic Action

7.4.1. Effect of Tl on Calcium Homeostasis

Calcium is an essential regulator of cellular function, and its imbalance can lead to severe alterations. It has been reported that Tl significantly interferes with this homeostasis, causing mitochondrial dysfunction, cytoplasmic calcium overload, and alterations in the expression of key regulatory genes. The following studies illustrate these effects.

Exposure to low concentrations of Tl (1 µg/L) for 48 h has been shown to induce a significant increase in cytoplasmic calcium levels in the murine hippocampal neuronal cell line HN9.10e. These results were accompanied by early mitochondrial dysfunction, evidenced by alterations in both mitochondrial ROS production and mitochondrial membrane potential. Based on these findings, the authors emphasize the central role of mitochondria in calcium storage and release, warning that prolonged mitochondrial dysfunction may lead to cytosolic calcium overload, thereby contributing to severe cellular dysfunction [33]. Furthermore, it has been reported that mitochondrial calcium overload, together with the presence of Tl, may promote the mPTP. In particular, in the case of Tl, this opening has been reported to depend on a prior calcium load within the mitochondria [83]. The above highlights the close relationship between the metal’s toxicity and calcium imbalance, both in the cytosol and in the mitochondrial matrix.

Furthermore, transcriptomic analyses of zebrafish embryos exposed to 100 µM Tl revealed significant alterations in calcium signaling pathways. In total, 46 differentially expressed genes (DEGs) were identified, among which slc8a3, slc8a4a, and atp2b3b stand out as key regulators of calcium homeostasis. The authors discuss that the downregulation of these genes compromises calcium efflux, promoting intracellular calcium overload. To further investigate, the researchers employed the Tg(elavl3:GCamp6f) transgenic zebrafish line, which expresses GCamp6f, a fluorescent calcium indicator protein in neuronal cells under the control of the elavl3 gene. Fluorescence imaging revealed notable changes in calcium levels in the brain and spinal cord of embryos exposed to Tl. These results were corroborated by quantification using Fluo-4 AM, a fluorescent dye used to measure intracellular calcium, which showed a 1.3-fold increase in Tl-treated embryos (100 μg/L) compared to the control group. Additionally, the researchers noted that increased intracellular calcium could disrupt both mitochondrial and endoplasmic reticulum function, triggering oxidative stress and subsequently apoptosis. To confirm this, acridine orange staining was used, revealing a higher proportion of apoptotic cells in the head region of Tl-exposed embryos. Finally, ultrastructural analysis by transmission electron microscopy confirmed typical apoptotic features, such as nuclear condensation and morphological alterations in mitochondria, observed in the Tl-treated group [84].

At the systemic level, Tl toxicity affects key organs involved in calcium handling, such as the kidney. Intraperitoneal administration of Tl (30 mg/kg) in rats has been shown to induce severely compromised renal function. Low-vacuum scanning electron microscopy revealed calcium deposits in the renal medulla, specifically in the thick ascending limb of the loop of Henle. Furthermore, a reduction in the expression of essential transporters for calcium reabsorption, such as the Na⁺–K⁺–2Cl⁻ cotransporter 2 (NKCC2), was observed, affecting overall body ion regulation and contributing to acute kidney injury. In addition, ROS generation and bioenergetic dysfunction further exacerbate calcium dysregulation, intensifying renal damage [85]. Based on the above, it appears that exposure to Tl triggers a disruption of calcium homeostasis, which, as discussed, manifests at multiple levels, from mitochondrial to cellular and systemic. Although further studies are still needed, this calcium imbalance may emerge as a key mechanism underlying metal-induced toxicity.

7.4.2. Effects of Tl on Myelin

Tl can cause severe neurological damage, particularly in the peripheral nervous system. Some authors have sought to characterize the metal’s ability to induce structural alterations in nerve fibers, with particular attention paid to its impact on myelin, given that it is essential for the efficient conduction of nerve impulses. The following summarizes the available evidence from the literature regarding these effects.

A case was reported of a man who, after ingesting 5 to 10 g of thallium nitrate, died nine days later. On the seventh day, a sural nerve biopsy was performed, revealing that the large myelinated fibers were the most affected. Many axons appeared swollen, were devoid of organelles, and contained large vacuoles; however, in most cases, the surrounding myelin sheaths remained intact or exhibited only mild alterations [86]. In another report, a 40-year-old man was admitted to the neurology unit with symptoms of neuropathy after attending a party and noticing a peculiar taste in a candy he was offered. On day 18 of the illness, blood Tl levels were confirmed at 40,980 µg/mL. A sural nerve biopsy performed on day 40 revealed axonal loss and active axonal degeneration, and Kulchitsky Pal staining showed associated demyelination [87].

Experimental studies available in the literature addressing the effects of Tl on myelin are scarce and mostly date from the 1970s to 1990s. For example, in 1973, it was demonstrated that in functionally coupled organotypic cultures of dorsal root ganglia, peripheral spinal nerves, and muscles, exposure to Tl salts (C₂H₆O₂Tl and Tl₂SO₄, 10–20 µM) in a liquid medium caused significant alterations. After two hours of exposure, enlarged mitochondria were observed in the axons of peripheral nerve fibers, which progressively swelled. This led to the transformation of the mitochondria into axonal vacuoles. The resulting swelling of the fibers caused retraction of myelin from the nodes of Ranvier, but not degeneration. Impulses were still able to propagate along the nerve fibers during the experiment [88].

In a second study, myelinated dorsal root ganglia were used, and it was observed that treatment with Tl (around 100 µM) inhibited neurite outgrowth; however, its ability to block primary myelination was very low compared to that observed for other heavy metals, such as lead [89]. In animal models, a single intraperitoneal administration of TlCH₃COO (16 mg/kg) to newborn Wistar rats caused, by day 8, a moderate reduction in large and medium sural nerve fibers, with early signs of myelin degeneration along the axons. By day 50, the myelinated fibers exhibited tortuous courses and fragmentation. Additionally, axonal and myelin alterations were observed along the entire length of the axon, suggesting a progressive distal axonopathy [90].

Taken together, these findings suggest that Tl (at least in the early stages) causes axonal damage with secondary myelin loss, without primarily blocking the myelination process.

7.4.3. Mutagenic and Genotoxic Effects of Tl

The genotoxic and mutagenic effects of Tl have not been conclusively characterized. Researchers have evaluated the metal’s capacity to induce DNA damage and have sought markers of genetic instability following its administration. A summary of the available evidence from the literature is presented below.

Results from bacterial studies are contradictory. In Rec assays with Bacillus subtilis, Tl (1 mM) showed a moderate effect, indicating that the metal may cause DNA damage in recombination-deficient bacteria. However, in reversion (mutagenicity) tests with Escherichia coli and Salmonella, Tl did not produce positive results [91].

In eukaryotic cell models, the findings are more consistent, although not entirely uniform. In rodent embryonic fibroblasts, Tl induced single-strand DNA breaks and reduced cell survival in a dose-dependent manner after 24 h of exposure [92]. In human cell hemocultures, Tl (0.5–100 µg/mL) was reported to reduce the mitotic and replicative indices in a concentration-dependent manner. Furthermore, a significant increase in chromosomal aberrations was observed, and its clastogenic effect and induction of oxidative DNA damage were confirmed using the single-cell gel electrophoresis assay [93].

In peripheral lymphocytes from 13 patients poisoned by Tl, chromosomal alterations were observed, mainly chromatid aberrations. However, in the subgroup of eight individuals evaluated with the micronucleus assay, only one showed a markedly increased frequency of micronuclei. No correlation was found between the frequency of chromosomal aberrations and the results of the micronucleus test [94]. On the other hand, in this same model, genotoxicity was evaluated for six metal salts using the micronucleus assay and fluorescence in situ hybridization (FISH) analysis. It was found that all compounds, except Tl, produced a significant increase in micronuclei at least at one dose. FISH analysis, in turn, revealed that only antimony (Sb) and mercury (Hg) were highly genotoxic [95]. In conclusion, Tl exhibits genotoxic properties that some authors have considered weak and inconclusive. These properties have been more extensively studied in eukaryotic cells, yet they lack consistency across studies. This complicates the classification of the metal as a mutagen and underscores the need for standardized and comparative investigations.

8. Treatment of Tl Poisoning in Humans

Various therapeutic regimens have been used to treat Tl poisoning; however, there is no consensus among physicians regarding the effectiveness of different strategies, and no single treatment has been proven effective in severe cases of toxicity. As a result, combined therapies have been employed, including gastric lavage, potassium ferric hexacyanoferrate (commonly known as Prussian blue), activated charcoal, and extracorporeal drug removal methods such as hemodialysis and forced diuresis [12]. In some cases, D-penicillamine has also been used, although its efficacy is limited and it is not recommended as a first-line treatment; however, it may be considered an alternative when Prussian blue or other more effective options are unavailable [44,48]. On the other hand, exogenous administration of MT has been shown to be effective in reducing Tl-induced liver damage in animal models, acting as a metal-chelating and antioxidant agent [59].

Prussian blue is the antidote of choice and is approved by the U.S. Food and Drug Administration (FDA) for the treatment of Tl poisoning. Prussian blue is a crystalline network with high affinity for Tl and cesium. The drug is available in both soluble (potassium ferric hexacyanoferrate) and insoluble (ferric hexacyanoferrate) forms. Both forms have been used therapeutically, although the literature highlights the greater efficacy of the soluble form in Tl poisoning [96,97].

The mechanism of action of Prussian blue involves binding to Tl as it passes through the gastrointestinal tract, exchanging it for K⁺ on the surface of the crystal in the intestinal lumen. This process reduces the gastrointestinal reabsorption of the metal and facilitates the excretion of the insoluble complex. The K⁺ that is exchanged originates from within the drug’s own crystalline lattice [98]. In Mexico, it has been reported that administration of 3 g/day of Prussian blue accelerates Tl elimination from the body, resulting in a 70% reduction in plasma concentrations in approximately four days [40]. Although Prussian blue has few adverse effects, one of its limitations is that it can only bind Tl present in the gastrointestinal tract, and its attributable effect remains unclear since it is typically administered in combination with other therapies [98].

On the other hand, activated charcoal is a processed form of common charcoal that results in a porous network capable of trapping other chemicals and preventing their absorption by the body, which is why it has been used as a decontaminant. Activated charcoal binds to certain toxic substances, such as Tl, and retains them within the gastrointestinal tract, thereby reducing systemic absorption and promoting toxin elimination from the body [99]. The administration of multiple doses (two or more sequential doses) of activated charcoal, in combination with Prussian blue, has been shown to be effective in treating Tl poisoning [99].

Among the experimental strategies recommended for managing acute Tl poisoning are forced diuresis and hemodialysis [100]. Although this therapy has been one of the most commonly used in acute poisoning scenarios, complications such as fluid overload, pulmonary edema, and acid–base disturbances have led clinicians to reduce its use in recent years [101]. However, there are reports of successful recovery in patients treated with a combination of Prussian blue and forced diuresis [96,102].

By contrast, hemodialysis has proven to be more effective than forced diuresis in treating Tl poisoning [103]. On average, a standard session lasts about four hours. However, long-term hemodialysis (200 h over 10 days) in combination with forced diuresis and oral Prussian blue administration has resulted in the complete recovery of a patient who ingested Tl₂SO₄. Moreover, hemodialysis has been reported to be effective even in the third week after Tl exposure [87].

In addition to Prussian blue, several antioxidant compounds have been investigated for their potential to reverse Tl-induced oxidative stress. These compounds offer promising advantages, such as free radical scavenging, preservation of glutathione (GSH) levels, and prevention of lipid peroxidation in tissues, thus complementing the chelating action of Prussian blue. Among the antioxidants evaluated are N-acetylcysteine (NAC), α-tocopherol, ascorbic acid, and S-allyl-L-cysteine, all of which have demonstrated improvements in cell viability, reductions in oxidative stress markers such as malondialdehyde (MDA), and the preservation of antioxidant enzyme activities such as superoxide dismutase (SOD) in cell and animal models [53,104,105].

However, these compounds have limitations, including their inability to significantly reduce the total body burden of Tl, lack of standardized dosing, and absence of robust clinical evidence in humans. In a comparative study in mice, Meegs et al. [105] evaluated NAC (200 mg/kg i.p.) versus Prussian blue (50 mg/kg oral) after a lethal dose of Tl. Although NAC improved survival (35% vs. 10%), it did not match the efficacy of Prussian blue and did not provide additional benefit when combined with it.

Table 2. Antioxidant compounds vs. thallium (Tl) toxicity.

Model	Tl Compound	Antioxidant (Dose)	Key Findings	Ref.
Female Swiss albino mice	Subcutaneous Tl acetate 70–85 mg/kg (LD90–LD100)	Prussian blue (50 mg/kg oral); NAC (200 mg/kg i.p.)	Prussian blue increased survival from 10% to 50% (p = 0.014); NAC to 35% (p = 0.13); NAC + Prussian blue showed no additional benefit over Prussian blue alone	[105]
GT1-7 neuronal cells	Tl(I), Tl(III)	Mannitol, ascorbic acid, α-tocopherol (dose not specified)	Protection: ↑ viability, ↓ apoptosis, ↓ MDA, SOD stabilization	[104]
Adult rats (i.p.)	Tl+ acetate 32 mg/kg	MK-801 (1 mg/kg)	Reduced lipid peroxidation and GSH depletion in brain	[106]
Rat synaptosomes/mitochondria (ex vivo)	Tl+ 5–250 µM	S-allyl-L-cysteine (100 µM)	Mitochondrial and lipid peroxidation protection; less effective than neuroexcitatory antagonists	[53]
Rats	Tl (dose not specified)	Diallyl sulfide, curcumin	Hepatoprotective effect: ↓ inflammatory cytokines and liver enzymes	[107]
Isolated hepatocytes/mitochondria	Tl(I)	α-tocopherol, deferoxamine, carnitine, L-glutamine, fructose, xylitol	Inhibition of caspase-3 and apoptosis	[108]

µM: micromolar, GSH: glutathione, GT1-7: immortalized neuronal cell line derived from the mouse hypothalamus, i.p.: intraperitoneal administration, LD₉₀–LD₁₀₀: lethal dose for 90–100% of the animals, MDA: malondialdehyde, MK-801: NMDA receptor antagonist (dizocilpine), NAC: N-acetylcysteine, SOD: superoxide dismutase, Tl(I), Tl(III): oxidation states of thallium, +1 and +3, respectively; ↑ indicates an increase, and ↓ indicates a decrease.

lists the antioxidants evaluated against Tl, and

Table 3. Comparison between Prussian blue and antioxidant compounds studied against thallium (Tl) toxicity.

Aspect	Prussian Blue	Antioxidants
Main mechanism	Reduces Tl absorption in the intestine by increasing excretion	Cellular protection by reducing oxidative stress
Preclinical evidence	Strong: animal models and human case reports	Limited: in vitro and cell-based studies only
Clinical evidence	Yes: supported by multiple clinical applications	No clinical evidence in humans to date
Potential benefit	Removes Tl from the body	May reduce residual Tl-induced organ damage
Substitute or complement?	Clinically validated substitute	Complementary therapy

compares their effects with the standard treatment using Prussian blue.

9. Biological Models for the Study of Tl Poisoning

Understanding the mechanisms underlying Tl toxicity and the development of effective therapies largely depends on the biological models employed. The use of animal models has been invaluable in characterizing Tl distribution within the body, with rodents being the most used.

Table 4. Thallium (Tl) accumulation in the organism according to the route of administration and dose.

Study Models	Tl Compound	Route of Administration	Administered Dose	Time	Tl Accumulation in the Organism	Ref.
Rats	Tl 204	Oral	0.76 mg/kg	Approximately 7 days	Highest amount in the kidneys, followed by the salivary glands, testes, muscles, bones, lymph nodes, gastrointestinal tract, spleen, and liver.	[109]
Rats (Wistar)	Tl2SO4	Intraperitoneal	16 mg/kg	24 h after	Greater accumulation of Tl in the kidneys, followed by the testes, spleen, lungs, heart, and liver.	[110]
Hamster	C3H2O4Tl2 and Tl2SO4	Oral (single dose)	C3H2O4Tl2: 12.5 mg/kg Tl2SO4: 12.35 mg	12, 24, and 72 h after	The Tl content was higher in the kidneys regardless of the post-treatment time, followed by the testes (starting at 24 h post-treatment), the heart, and the liver.	[111]
Rats (Wistar) (newborn)	C2H6O2Tl	Intraperitoneal	16 mg/kg	24 h after	Highest content in the testes, heart, and kidneys.	[54]
Albino rats	Tl2SO4	Intraperitoneal	30 mg/kg	4 days	The Tl content was highest in the kidney, followed by the ileum, stomach, and liver.	[112]
Rats (Wistar)	Tl 3+	Oral	20 mg/kg/día	60 days	Greater amount in the kidney, followed by the liver, spleen, intestines, and heart.	[113]

TINO₃: thallium nitrate. Tl₂SO₄, thallium sulfate. C₃H₂O₄Tl₂, thallium malonate. C₂H₆O₂Tl, thallium acetate.

summarizes various studies conducted in these models, encompassing different Tl compounds, doses, and exposure times.

While rodents have formed the cornerstone of research, the integration of simpler models, organoids, and systems derived from iPSCs offers new opportunities for mechanistic analysis and preclinical evaluation. This section outlines the advantages, limitations, and potential applications of both traditional and emerging models, emphasizing their complementary role in the study of toxicity and the development of therapeutic strategies.

9.1. Traditional Models: Rodents

Both rats and mice remain the standard for studying tissue distribution, toxicokinetic, and the systemic effects of Tl. The advantages of these models include their physiological similarity to humans, the availability of surgical techniques, and established genetic models [74,76]. However, they also present several limitations, such as high costs, ethical considerations, and the extended duration required for experimentation.

9.1.1. Rats

The rat has been the preferred biological model due to its larger size compared to the mouse, which facilitates handling and allows for a wide range of surgical procedures [72,114]. This model has been pivotal in characterizing Tl accumulation, particularly in the kidneys, the main organ of deposition, and in elucidating its mechanisms of toxicity at the cellular and subcellular levels.

In hepatocytes and isolated mitochondria from rats, Tl³⁺ has been shown to be more toxic than Tl⁺, increasing ROS production, lipid peroxidation, and membrane potential loss, effects that can be mitigated by lipid antioxidants such as α-tocopherol and hydroxyl radical scavengers such as mannitol [71,115]. Oxidative damage has also been observed in isolated brain mitochondrial fractions, along with marked inhibition of Na⁺/K⁺-ATPase, underscoring the metal’s impact on energy metabolism [116].

Studies in rats have further confirmed that Tl can substitute for K⁺ in the activation of key enzymes such as Na⁺/K⁺-ATPase and pyruvate kinase, displaying an affinity even greater than that of K, which explains its strong capacity to disrupt essential biochemical processes [79]. The activation or inhibition of these enzymes is concentration-dependent; at higher Tl concentrations (>10 mM), enzymatic inhibition has been observed [56]. Additionally, Tl has been shown to inhibit sulfhydryl-dependent enzymes, such as porphobilinogen synthase (PBGS), through direct oxidation of cysteine residues. However, this effect can be significantly attenuated by the addition of glutathione and DL-dithiothreitol, both reducing agents that protect sulfhydryl groups [80].

Taken together, the evidence from rat models has been fundamental in understanding Tl toxicity, from mitochondrial dysfunction and oxidative stress to enzymatic inhibition, cementing this model as a cornerstone in toxicological research and the evaluation of potential therapies.

9.1.2. Mice

The high genomic similarity between mice and humans, the availability of a wide range of genetic and molecular tools, as well as their small size and ease of handling, have established the mouse as a highly valuable biological model for studying the toxic effects of Tl [72].

At the hepatic level, administration of thallium nitrate at lethal doses (100 and 150 mg/kg body weight) in mice caused polyribosome disaggregation and marked inhibition of protein synthesis [75]. Similarly, exposure to different cationic forms of Tl (10 ppm) for two weeks led to its accumulation in the liver and severe histological damage [105].

The utility of the mouse model has also extended to the study of Tl reproductive toxicity. Oral exposure reduced sperm viability and increased mortality, in addition to inducing DNA damage in spermatozoa [117]. Finally, in the immune system, Tl was observed to inhibit the expression of key genes involved in B cell development in the bone marrow [118].

These findings confirm that the mouse is a versatile and sensitive model for elucidating the toxic mechanisms of Tl in critical organs and functions.

9.2. Simple Biological Models

The use of simple biological models (lower animals) offers advantages over mammalian models for toxicological research. Lower animals have short generation times, are cost-effective to maintain, and pose fewer ethical constraints, allowing for multiple concentration–response tests for a given substance [119]. Additionally, the main signaling pathways in these organisms are highly conserved, and their genetic homology with humans exceeds 60%. Among these model organisms are the zebrafish (D. rerio), the fruit fly (D. melanogaster), and the soil nematode (C. elegans) (Figure 3).

9.2.1. Zebrafish (D. rerio)

The zebrafish (D. rerio) has established itself as a valuable biological model for assessing toxicity during embryonic development and detecting early cardiovascular or neurological effects. Its ability to absorb compounds directly from water enables high-throughput toxicity assays, which, combined with its transparent embryos developed ex vivo, facilitates controlled substance exposure and direct observation of morphological and functional abnormalities [120,121]. Considering that Tl can enter aquatic ecosystems through erosion and mining leachates, atmospheric deposition, or industrial wastewater discharges [122], this model provides a realistic approximation of environmental exposure scenarios.

Various studies have shown that exposure of embryos and larvae to Tl can lead to reduced hatching rates and malformations, as well as cardiac alterations, neuronal damage, and oxidative stress [7,8]. In juveniles and adults, Tl increased mortality and caused reproductive dysfunction, gill and liver lesions, and changes in the intestinal microbiota [123].

However, the use of zebrafish presents important limitations: its physiology differs in several aspects from humans, it lacks certain organs present in mammals (e.g., lungs), and its xenobiotic metabolism is not always comparable to that of mammals, which can complicate the direct extrapolation of doses and effects [121,124]. Nevertheless, the combination of its ecological relevance and experimental versatility positions zebrafish as a highly sensitive model for investigating toxicity in aquatic systems.

9.2.2. Fruit Fly (D. melanogaster)

The fruit fly (D. melanogaster) stands out as an ideal biological model for toxicological studies due to its extremely short life cycle, approximately 60 to 80 days, allowing the assessment of Tl effects across multiple generations with high efficiency [125]. Moreover, these organisms share metal tolerance mechanisms similar to those in humans, including homologous genes involved in metal transport, storage, and excretion [126].

Despite the limited research on Tl toxicity in this model, its potential for detecting mutagenic effects has been demonstrated using the somatic mutation and recombination test (SMART), in which 600 µM of thallium sulfate induced genotoxicity with 87.6% somatic recombination and cytotoxicity by interfering with cell division in the wing imaginal discs [127].

These findings highlight a significant gap in the study of Tl toxicity in this model, representing an opportunity to further investigate its mechanisms of action.

9.2.3. Nematode (C. elegans)

The nematode (C. elegans) has established itself as a powerful biological model for studying environmental toxicology due to its fully characterized genome, high genetic homology with mammals, and sensitivity to contaminants [128]. Its small size, short life cycle, and body transparency enable high-throughput assays and direct observations of cellular and molecular processes, ranging from morphological changes to the activation of fluorescent reporter genes [9,129,130]. These characteristics make it a highly valuable tool for elucidating the toxic mechanisms of Tl, a highly toxic metal that has been little studied in this organism.

Available studies indicate that Tl critically affects the survival, development, and reproduction of C. elegans. The main mechanisms observed include ROS generation and the activation of highly conserved antioxidant pathways such as SKN-1/Nrf2 and DAF-16/FoxO [131,132]. This molecular parallelism with humans gives the nematode remarkable predictive potential for understanding toxic mechanisms and exploring therapeutic interventions. For instance, exposure to S-allylcysteine (SAC) was found to attenuate Tl-induced effects in C. elegans [10].

Despite its advantages, C. elegans has certain limitations that must be considered when interpreting results. As an invertebrate, it lacks organs and systems found in mammals, such as a circulatory system, lungs, and an adaptive immune system, and exhibits differences in xenobiotic metabolism, which limits the direct extrapolation of doses and effects [10,131]. Additionally, exposure to toxicants typically occurs in aqueous media, which does not always replicate the routes of entry and distribution in higher organisms [133,134]. Nevertheless, its unique combination of experimental simplicity and molecular relevance positions it as a strategic model for investigating Tl toxicity in a mechanistic context with translational applications.

9.3. Emerging Models: Organoids and iPSC-Derived Systems

Three-dimensional organoids (liver, heart, and brain) and human iPSC-derived systems provide tissue microenvironments with architecture and functions closely resembling those of humans, thereby enhancing translational relevance and enabling direct assessment of mitochondrial and neuronal toxicity, oxidative stress, inflammation, and cell death [135]. In screening platforms, these models facilitate the identification of early biomarkers through metabolomics/proteomics and the evaluation of combination therapies (e.g., chelators with antioxidants) under controlled conditions.

The utility of these models has already been demonstrated in the study of metals: in human liver and cardiac organoids, Tl was identified as the most toxic compound among several environmental toxins, with a mean inhibitory concentration (IC₅₀) of 13.5 µM in liver and 1.35 µM in heart [136]. This study validates the discriminatory power of these biological models for risk prioritization and mechanistic target identification. Recent reviews on organoid use also highlight the finding of Tl as the most toxic compound within tested panels in these models [137]. Furthermore, brain organoids have been systematically employed to model neurotoxicity induced by arsenic and lead, including proteomic alterations [138].

In the case of iPSCs, brain microphysiologies derived from them have been developed to study diseases and toxicity, reinforcing their utility for the exploration of mechanisms and biomarkers with potential for clinical translation [11].

10. Strategic Selection of Biological Models for Tl Toxicity: A Tiered and Complementary Approach

The appropriate selection of biological models should align with the specific objectives of the research; however, a tiered and complementary approach is recommended to maximize experimental outcomes and optimize resource use when studying Tl or other metal toxicities. This integrated framework combines models of increasing complexity, resulting in an effective combination of speed, functional validation, and translational relevance.

First, for initial screening, the use of simple models such as C. elegans and D. melanogaster is advised. Both organisms are characterized by short life cycles, high reproductive rates, and the conservation of key cellular pathways involved in oxidative stress responses [125]. These features make them ideal tools for high-throughput screening and early detection of molecular biomarkers.

The second stage corresponds to functional validation using zebrafish (D. rerio), a vertebrate animal that allows the assessment of multi-organ effects and behavioral alterations within a single organism, thanks to its transparency, rapid development, and high genetic homology with humans [120,121]. This platform has been widely used to study environmental toxicity and to develop phenotypic and genetic screens [139].

Finally, for mechanistic confirmation, the use of rodents and/or human iPSC-derived organoids is recommended. Rodents enable systemic, pharmacokinetic, and therapeutic efficacy studies, while organoids and iPSCs provide a human tissue context with relevant architecture and functionality [135]. Additionally, organoids represent sensitive platforms for testing toxicity through viability assays and functional changes (such as beating activity in cardiac organoids), and allow the exploration of strategies with a high degree of translatability [136] (Figure 4).

11. Conclusions and Future Perspectives

Tl toxicity remains a persistent challenge in clinical and environmental toxicology due to its unique ability to substitute for K⁺ and disrupt essential cellular processes. This property profoundly alters mitochondrial function, generating oxidative stress and causing loss of mitochondrial membrane potential, which severely compromises cell viability in high-energy-demand tissues, such as the nervous system and the heart. Moreover, the lack of specific symptoms and the limited availability of effective treatments hinder timely diagnosis and clinical management, underscoring the need for innovative therapeutic strategies.

Recent advances demonstrate that the understanding and management of Tl toxicity largely depends on the proper selection and combination of biological models. Traditional models remain essential for studying toxicokinetics and therapeutic efficacy; however, the incorporation of simple models, such as C. elegans, D. melanogaster, and D. rerio, offers a cost-effective and agile platform for compound screening, biomarker identification, and the analysis of conserved molecular pathways.

In parallel, human organoids and iPSC-derived systems represent a key bridge toward translational relevance by recreating human tissue microenvironments and enabling the direct evaluation of mitochondrial, neuronal, and cardiac toxicity. These emerging models not only facilitate the validation of results obtained in animal models but could also enable the application of omics tools to uncover metabolic and proteomic alterations prior to the onset of clinical symptoms.

Advancing toward more effective treatments requires a tiered and integrated approach that uses simple models for initial screening, vertebrate models for functional validation, and organoids/iPSC-derived systems for mechanistic confirmation and the development of combination therapies. Furthermore, the use of these models should be accompanied by environmental and clinical surveillance strategies, with standardized protocols for Tl detection and regulatory limits based on the most recent scientific evidence.

Overall, the strategic integration of multiple biological models—from nematodes to human organoids—will not only accelerate the discovery of novel therapeutic interventions but also strengthen the prevention of and response to Tl exposure, closing the gap between mechanistic knowledge and its clinical and environmental application.