Even if quondam investigations have thoroughly tried to describe the pathophysiology of hypothyroidism-induced NAFLD, the fundamental matrix behind this distinct entity is far from entirely known [
24]. We know that low thyroid levels are commonly associated with hypometabolism. This state is defined by weight gain, reduced resting energy expenditure, reduced gluconeogenesis, and reduced lipolysis. THs dysfunction can instigate obesity, deficient lipid metabolism, and IR, which are elements of the metabolic syndrome also encountered in NAFLD [
75,
76,
77]. Perhaps via the three-known mechanisms of NAFLD (hepatic lipid accumulation, inflammatory status accompanied by oxidative stress, and subsequently defective liver repair and regenerative response), hypothyroidism may directly or indirectly contribute to NAFLD. Thus, researchers have studied the profound interaction and signaling between THs, thyroid hormone receptors (TRs), and liver function [
78,
79].
2.1. Thyroid Hormone–Liver Axis
The thyroid gland is a pivotal endocrine organ responsible for thermogenesis, adipogenesis, fat distribution, energy, lipid, protein, carbohydrate, and cell metabolism [
80]. The THs 3,5,3′-triiodothyronine (T3) and 3,5,3′,5′-tetraiodothyronine/thyroxine (T4) are important keys for tissue repair as they mediate cellular differentiation and interfere with cell-signaling mechanisms via protein–protein synergy through collaboration with nuclear receptors or binding to other proteins [
81,
82]. Production of THs from the thyroid gland is modulated centrally by thyrotrophs of the anterior pituitary that produce TSH, which in turn is regulated by the hypothalamus. The hypothalamus secretes the thyrotropin-releasing hormone (TRH). The secreted THs are then transported on cell membranes and their peripheral signaling is expressed in the adipose and hepatic tissue [
83,
84]. The TRs combine with another nuclear receptor and form the retinoid X receptor (RXR), which binds to thyroid hormone response elements (TREs) in regulatory portions of the target genes. The cytoplasmatic TRs regulate the noncanonical THs signaling, while the canonical signaling is considered the main mediator that controls, via TRβ, the hypothalamic–pituitary–thyroid axis. The TRs also control the actions of T3 [
85,
86,
87]. There are two key thyroid receptor isoforms represented by the thyroid hormone receptor α1 and β1 (THR-α and THR-β). THR-α isoforms are ubiquitously expressed, but mainly found in the cardiovascular system, white adipose tissue (WAT), and in bone structures, whereas THR-β is mostly expressed in the hepatic organ and cardiac ventricle [
85,
88]. In NASH patients who underwent bariatric surgery, researchers found that THR-β messenger RNA (mRNA) negatively correlated with steatosis activity, implying that during disease development, there is a progressive resistance to THs. Resistance to TH is defined as a syndrome in which tissues have diminished sensitivity to TH [
88]. THs exert different central and peripheral functions [
89] and are influential modulators in liver regeneration as they can crosstalk with growth factors and integrins to regulate physio-pathological responses. THs mediate the number and function of different peroxisomal enzymes, and can regulate mitochondrial biogenesis and function in the hepatic cells (HCs) [
90,
91]. These particular properties of THs in the liver may explain the involvement of THs in liver cancer onset [
92].
Tissue THs activity is controlled by deiodinases (D1, D2, and D3). Deiodinases are peroxidase seleno-dependent enzymes that are capable of activation or deactivation of THs in the peripheral tissues. Intrahepatic TH activity and homeostasis is regulated by serum TH levels and also by liver deiodinases [
93]. While substrate T4 is converted into active hormone T3 by D1, D3 transforms T4 and T3 into inactive products, rT3, and diiodothyronine (T2), respectively. More than that, via D1, rT3 is catalyzed to T2 [
94]. In healthy liver, hepatocytes express high levels of D1 while stromal cells express low D3 levels. Midst injury, a loss of D1 in hepatocytes is identified, while D3 expression in stromal cells increases, especially in fibrogenic myofibroblast. Intrahepatic hypothyroidism limits exposure to T3, which has an important role in cellular differentiation, is a greater regulator of metabolic systems than T4, and has oncosuppressor properties [
95]. T3 expression is restrained during early phases of embryogenesis through D3 raised levels. After birth, D1 and D2 levels increase while D3 levels decrease, therefore T3 is granted the possibility to incite cellular differentiation and tissue maturation [
96,
97]. Evidence shows that after partial hepatectomy, myocardial infarction or in injured skeletal muscle, the expression of D3 is upregulated [
98]. Additionally, in many chronic diseases, the hepatic D1 activity is reduced, whereas D2 along with TRs, uncoupling proteins, and beta-adrenergic receptors contribute to the development of obesity [
99]. As a secondary mechanism for fat deposition, there is a raised conversion of T4 to T3 via enhanced deiodinase activity [
100].
In liver injury, the expression of the nuclear TRs in HSCs is repressed, hence the main hormone receptor becomes THR-α, which starts an elaborate fibrogenic response and higher contractility. Detection in vitro of the HSC activation revealed that T3 may assist in the resolution of liver fibrosis in rats by mediating transforming growth factor-beta (TGF-β)-induced collagen I gene expression [
101]. Injury-activated HSCs are crucial components for liver regeneration as they are major sources of myofibroblasts. These stromal cells promote mediators of angiogenesis, chemokines, and diverse growth factors [
102]. As they activate, HSCs induce mutual changes in D1 and D3 hepatic expression, especially in D3, which diminishes the active-THs accumulation. These mechanisms can cross-talk in their way to affect TH expression [
103]. Putative evidence is needed to elucidate the exact mechanism behind D1–D3 hepatic repair and the control behind TH homeostasis. The authors believe that D3 could represent a new therapeutic target in hepatic injury and fibrosis, and that rt3 could also be a novel biomarker [
104]. This shows how intricate the relationship between the liver, THs, and TRs is and how different their influence is on each other, depending on the primary disorder.
2.2. Lipid and Cholesterol Metabolism
Normally, there is an equilibrium between hepatic lipid synthesis and catabolism; however, an abundance of dietary intake leads to excess of plasma glucose, which enhances glycolysis and lipogenesis, with a subsequent rise in intracellular free fatty acids (FFAs) that can debilitate TR activity [
105]. When energy intake happens, ATP is formed from carbohydrate oxidation and glycogen stocks are restored in the skeletal muscle and liver. Glycogen could account for the major deposit source of hepatic energy. Even if the link between liver glycogen and hypothyroidism is still unclear [
106], the data show that low TH plasma concentrations may alter glycogen accumulation by decreasing the fatty acid synthase (FAS) and acetyl-CoA carboxylase (ACC) activity [
107].
In excess conditions, the carbohydrate is transformed into fatty acids (FAs) via “de novo lipogenesis” (DNL). Most of the hepatic lipid accumulation is represented by re-esterification of circulating FFAs, followed by DNL, and finally, by dietary fatty acids. FAs go through one of the three metabolic transformations: β-oxidation (hepatic desaturation and elongation), esterification, and accumulation as TG or transformation and secretion as lipoproteins. NAFLD subjects have high levels of diacylglycerols, triacylglycerols, and elevated saturated FAs composition. Triacylglycerol can accumulate as fat droplets, or can be added to very-low-density lipoprotein (VLDL) or help with cellular reparation [
108,
109]. It seems that the derivative 3,5-diiodothyronine can regulate the activity of hepatic lipases to raise lipid mobilization from fat droplets [
110]. DNL, along with triacylglycerol synthesis, are steadily coordinated by hormones and various crucial enzymatic processes [
111]. THs regulate lipogenesis by binding to the specific genes of TRs, which can then mediate the transport of FFAs into the liver cells with the help of protein transports like liver fatty acid binding proteins (L-FABPs), fatty acid transporter proteins (FATPs), and fatty acid translocase (FAT). In the hepatocytes via THR-β, THs can increase the mitochondrial oxidation of FAs, promote the intrahepatic lipolysis through lipophagy with subsequently decreased TG clearance and increased TG hepatic up-take [
112]. Studies show that TH enhances the activity and recruitment of Zinc-α2-Glycoprotein in hepatic cells that help facilitate lipolysis [
113]. Nonetheless, this lipidic surplus can lead to cellular stress, apoptosis, and consequent liver impairment. Hence, the biochemical panel of NAFLD/NASH patients often register elevated liver transaminase, low-density lipoproteins (LDL), cholesterol, and TG levels [
114].
Studies noted that NAFLD subjects have a lower concentration of THs [
92], that small elevated TSH levels are linked to higher risk and prevalence of metabolic syndrome [
115], and that TSH can hasten the onset and progression of NAFLD/NASH [
116]. A sufficient reduction of TH levels is enough to lower the response of adipose tissue to adrenergic signaling. In these conditions, insufficient adrenergic stimulation of lipolysis in adipose tissue contributes by reducing the FA transport to the liver [
117]. Additionally, THs stimulate the responses of catecholamine through enhancement of the expression of the uncoupling proteins in the mitochondria of skeletal muscle and obese cells by regulating the number of adrenergic receptors [
16]. While TSH directly affects the hepatocyte cell membranes, it can promote hepatic lipogenesis, gluconeogenesis, and diminish hepatic bile acid synthesis [
12,
118]. Raised TSH levels were correlated with higher hepatic lipoprotein lipase activity [
119,
120]. These processes are mediated by THs through genomic and non-genomic mechanisms [
121]. The first mechanism occurs with the help of the classical nuclear receptors THR-α and THR-β [
120]. TSH promotes the TG hepatocyte up-take and endorse liver steatosis through binding to TSH receptor, which sets the hepatic sterol regulatory element-binding transcription factor 1 SREBP-1c activity via the cyclic AMP (cAMP)/protein kinase A(PKA)/peroxisome proliferator-activated receptor-α (PPARα) (cAMP/PKA/PPARα) pathway, paralleling with decreased AMPK function and subsequently raised lipogenesis. TSH can restrain cholesterol biosynthesis by stimulating AMPK-mediated phosphorylation of 3-hydroxy-3-methyl-glutaryl coenzyme A reductase (HMGCR) [
122,
123].
In addition, there is proof that highlights the indirect regulation of the liver lipid metabolism by the thyroid gland through the central nervous system [
124]. TH indirectly drives the transcriptional regulation of liver lipogenesis as a result of its effect on carbohydrate-responsive element-binding protein (ChREBP) and liver X receptors. They can lower the VLDL, LDL, high-density lipoproteins (HDL), and apolipoprotein B100 levels [
109]. TH can lower LDL by reducing the proprotein convertase subtilisin/kexin type 9 (PCSK9) levels. PCSK9 is controlled by SREBP-2, and data show that hypothroidism is associated with increased levels of PCSK9 [
125]. Among other functions, TH instigates the expression of hepatic β-hydroxy β-methylglutaryl-CoA (HMG-CoA) reductase, which through different pathways, maintain constant sterol levels, and of cholesterol 7 alpha-hydroxylase (Cyp7A1) [
62,
118]. Lower CYP7A1 activity is correlated with higher LDL levels [
72]. Experimental studies showed that a THR-β and T3 agonist can reduce Cyp7A1 levels and that Cyp7A1-TRs may represent a new therapeutic target [
126]. Interestingly, the administration of synthetic THR-ß agonist (KB2115) reduced LDL serum levels [
127]. Furthermore, the THR-β agonist (e.g., eprotirome, assobetirome) can lower circulating LDL-cholesterol levels without cardiac secondary effects [
128]. A study on rodents showed that mice with a THR-α (ThrαPV/PV) mutation had lower hepatic lipid accumulation and decreased lipogenesis, while a negative mutation in THR-β (ThrβPV/PV) led to decreased fatty acid b-oxidation and raised PPARδ signaling, with subsequently increased lipid up-take and liver steatosis [
129]. Additionally, THR-α knockout mice seem to be sheltered against diet-induced hepatic steatosis and peripheral IR [
130].
THs perturb the regulation of HDL via enhancement of plasma protein factors like cholesteryl ester transfer protein (CETP), lecithin-cholesterol acyltransferase (LCAT), and hepatic lipase and decrease the synthesis of liver phospholipid (phosphatidylserine and cardiolipin) and sphingolipid species [
131]. Dysregulations in this protein metabolism act in synergy with the severity of hypothyroidism [
132]. THs increase cholesterol efflux from peripheral tissues through the enhancement of genes such as Apolipoprotein A1 (Apo A1) and scavenger receptor class B member 1 (SRB1) [
133]. Another complex mechanism by which THs can modulate lipid metabolism are represented by transcriptional, non-transcriptional, and microRNA mechanisms [
134]. For example, via miR181d, THs decrease and import a transcription factor that can modulate the activity of sterol O-acyltransferase 2 (SOAT2), which has a crucial role in the conversion of cholesterol to specific esters [
135]. T3 may have protective effects against lipotoxic derivatives and its action is regulated via mRna and the activity of liver lipases. Additionally, T3 via hepatic lipophagy transports lipids to lysosomes and enhances FA oxidation, which promotes long term hepatic cell autophagy and damage [
136,
137]. Previous treatment with a high dose of the T3 hormone led to a decrease in cholesterol levels and promoted weight loss, however, its use was limited because of its serious cardiac side effects [
138].
These processes emphasize the hepatic lipidic balance and wealth that is induced and sustained by THs. It is clear that lipidomic signatures have a decisive part in NASH evolution/resolution [
139,
140], that lipid and cholesterol metabolism alteration is beyond complex, and represent the hallmarks of NAFLD; however, supplementary and sustainable evidence to describe these changes in hypothyroidism and how exactly they connect with TH levels are desired.
2.4. Oxidative Stress and Inflammation
Oxidative stress and inflammation are one of the items that unlock and sustain many cardiovascular, endocrinological, and liver diseases, hence, early evaluation of markers that indicate their presence should be evaluated. The inflammatory pattern in NAFLD/NASH is usually represented by high TNF-α, interleukin-6 (IL6), and chemokine levels [
146]. Along with an accumulation of hepatic FFAs and inflammation, malfunction of the mitochondria appears. The altered mitochondria, also called the “powerhouse” of the cell [
147], leads to excessive oxidation and overflow of reactive oxygen species (ROS). These ROS products impair the activity of deiodinases [
148], enhance lipid peroxidation, activation of the KCs, and of specific pro-inflammatory cytokines. Among them, the TNF-α and transforming growth factor β (TGF-β) activate hepatic satellite cells and endorse fibrosis [
149,
150]. THs modulate the hepatocyte mitochondrial function by the uncoupling of oxidative phosphorylation [
151], and generate ROS through Ca
2+–calcium/calmodulin-dependent protein kinase kinase 2 (CAMKK2)–5′-AMP-activated protein kinase (AMPK) circuit. The hepatic autophagic removal of mitochondria and their biogenesis are both modulated by THs. More than that, by activating Ca
2+–AMPK and the cAMP–protein kinase A (PKA) pathways, THs can mediate lipid metabolism [
152].
In hypothyroid rodents, researchers have found an inflammation and decreased level of ceramides, which account for decreased cell differentiation and apoptosis [
153]. Hashimoto’s thyroiditis patients with hypothyroidism have increased serum markers of oxidative stress (e.g., malondialdehyde) [
154]. Metabolic products of inadequate fatty acid oxidation set off and worsen liver inflammation, fibrosis, and necrosis [
155,
156]. Recently the 3,5-diiodo-L-thyronine (3,5-T2) molecule has attracted more attention because of its metabolic effects, which are similar to T3, but mainly because of its beneficial actions on mitochondrial function and oxidative stress, which suggest its future introduction as a potential clinical drug [
157]. More data are needed regarding inflammation and oxidative stress in HIN, which may bring interesting possibilities of new therapeutic targets or biomarkers.
2.5. Adipokines and Hormones
One of the mechanisms possibly involved in the hypothyroidism-NAFLD pathogenesis could be represented by adipokine metabolism. Low circulating levels of thyroid hormones influence certain adipocytokines levels such as adiponectin or leptin. Adiponectin can enhance fatty acid oxidation and inhibit DNL through activation of AMP-activated protein kinase [
158,
159]. Hypothyroid subjects have altered adiponectin levels that can contribute to IR development. The modified adipocytokines registered have hepatotoxic properties, can promote oxygen radical release, and furnish liver inflammation and fibrosis [
160,
161]. Leptin is one hormone that is in charge of appetite modulation, can promote hepatic collagen synthesis hepatic IR, and is involved in hepatic fibrogenesis [
162]. High levels of this hormone were found in patients with thyroid disfunction and seems to be correlated with TSH levels and with BMI [
163]. Another adipokine studied in NAFLD and hypothyroidism is visfatin. In NAFLD, and especially in NASH subjects, low levels of visfatin have been described, however, an exact explanation behind this association is still unclear [
164]. Recently, it was proposed that visfatin could be a promising serum biomarker for monitoring liver disease in younger obese population [
165].
In addition to the hormonal branches that are explored in HIN pathogenesis, we mention fibroblast growth factor-21 (FGF-21). FGF-21 is a pleiotropic hormone, and specifically a member of the FGF endocrine subfamily, which expresses hormone-like activities and is considered a major regulator of energy homeostasis [
166]. Previously, it was described that NAFLD patients have higher FGF-21 levels compared to heathy subjects and that this can fasten NAFLD progression [
167]. More than that study shows that hypothyroid subjects have increased levels of FGF-21 independently of lipid profile [
168]. Experimental studies have shown that administration of FGF-21 led to improved glucose clearance, insulin sensitivity, and scaled down TG levels [
169]. Few studies investigated how hormonal changes influence HIN. Menopause can induce changes in hormonal status and lipid metabolism. Reduced estrogen production leads to increased plasma cholesterol levels, these changes being similar to those encountered in overt hypothyroidism. Recently, the SardiNIA study found that a postmenopausal status influences the relationship between lipid profile and TSH levels [
170]. The particular role of adipokines and other hormones such as estrogens in HIN need further exploration.
There is a fine balance between certain key elements found in metabolic syndrome that are also common seen in NAFLD and/or hypothyroidism. This sophisticated relationship, which incorporates alteration in the lipid, TG, cholesterol metabolism, IR, inflammation and oxidative stress, and the dysfunction of the hepatic hormone, is by some degree shaped by THs (
Figure 1). We hope that with a better understanding of HIN pathophysiology, advanced technology can help us find new therapeutic approaches.