Cross-Disciplinary Approach of Adrenal Tumors: Insights into Primary Aldosteronism-Related Mineral Metabolism Status and Osteoporotic Fracture Risk

Our objective was to overview the novel aspects in the field of adrenal gland neoplasms, namely, the management of bone status with respect to primary aldosteronism (PA). In the current narrative review, a PubMed study was conducted from inception until June 2023. The inclusion criteria were: human (clinically relevant) studies of any study design (at least 10 patients per study); English papers; and the following combination of key words within the title and/or abstract: “aldosterone” AND “bone”, “skeleton”, “osteoporosis”, “fracture”, “calcium”, “parathyroid”, “DXA”, “osteocalcin”, “P1NP”, “alkaline phosphatase”, “bone marker”, “trabecular bone score”, or “FRAX”. The exclusion criteria were in vitro or animal studies, reviews, and case reports/series. We screened 1027 articles and finally included 23 studies (13 of case-control type, 3 cross-sectional, 5 prospective, 1 observational cohort, and 1 retrospective study). The assessments provided in these studies were as follows: nine studies addressed Dual-Energy X-ray Absorptiometry (DXA), another study pointed out a bone microarchitecture evaluation underlying trabecular bone score (TBS), and seven studies investigated the bone turnover markers (BTMs) profile. Moreover, 14 studies followed the subjects after adrenalectomy versus medical treatment, and 21 studies addressed secondary hyperparathyroidism in PA patients. According to our study on published data during a period of almost 40 years (n = 23, N = 3965 subjects aged between 38 and 64, with a mean age 56.75, and a female-to-male ratio of 1.05), a higher PTH in PA versus controls (healthy persons or subjects with essential hypertension) is expected, secondary hyperparathyroidism being associated in almost half of the adults diagnosed with PA. Additionally, mineral metabolism anomalies in PA may include lower serum calcium and higher urinary calcium output, all these three parameters being reversible under specific therapy for PA, regardless medical or surgical. The PA subgroup with high PTH seems at higher cardiovascular risk, while unilateral rather than bilateral disease was prone to this PTH anomaly. Moreover, bone mineral density (BMD) according to central DXA might show a higher fracture risk only in certain adults, TBS being a promising alternative (with a still unknown perspective of diabetes’ influence on DXA-TBS results in PA). However, an overall increased fracture prevalence in PA is described in most studies, especially with respect to the vertebral site, the fracture risk that seems correctable upon aldosterone excess remission. These data recommend PA as a cause of secondary osteoporosis, a treatable one via PA intervention. There is still an area of debate the way to address BMTs profile in PA, the case’s selection toward specific bone evaluation in every day practice, and further on, the understanding of the potential genetic influence at the level of bone and mineral complications in PA patients.


Introduction
Primary aldosteronism (PA), also known as Conn syndrome, characterized by reninindependent aldosterone secretion, represents the most common cause of secondary hypertension among middle-aged adults.PA is either sporadic or familial, being classified into two major subtypes: unilateral (traditionally named an aldosterone-producing adenoma, responsible for 50-60% of cases) and bilateral (bilateral adrenal hyperplasia/disease involving 30-35% of all patients diagnosed with PA), as well as rare forms (unilateral hyperplasia, adrenocortical carcinoma, or familial hyperaldosteronism) [1,2].The 2022 WHO (World Health Organization) classification recommended an immunohistochemistry analysis (HISTALDO) in addition to the traditional hormonal assessment in order to describe CYP11B2 (aldosterone synthase) activity amid adrenal lesions with potential aldosterone excess involvement [3].
Various pathogenic variants have been detected in PA, including somatic mutations, such as KCNJ5, CACNA1D, ATP1A1, ATP2B3, and CTNNB1 genes [9,10].The algorithm of approaching PA is based on successive tests (screening followed by confirmatory tests), as well as imaging tests for uni/bilateral disease (including adrenal vein sampling in selected cases) [11].Screening protocols (aldosterone-to-renin ratio) are recommended for a selective subgroup, e.g., in cases with standard therapy-resistant hypertension, high blood and hypokalemia, hypertension onset at <40 years, people with a family history of hypertension or stroke at a young age, atrial fibrillation, and incidental adenoma.After a positive screening test, confirmatory tests such as the fludrocortisone suppression test, oral sodium loading test, saline infusion test, and captopril challenge test are required depending on the patient [12][13][14].
Persistent aldosterone excess plays a key role in cardio-renal injury and the development of cardio-metabolic complications, through the pro-fibrotic, pro-inflammatory effects, oxidative stress, and insulin resistance [15][16][17].Untreated PA is associated with an increased morbidity due to coronary artery disease, left ventricular hypertrophy, atrial fibrillation, heart failure, stroke, diabetes mellitus, and obstructive sleep apnea that contribute to an overall decreased quality of life and a higher mortality rate than the general population [18][19][20][21].
PA management includes adrenalectomy (nowadays, laparoscopic access is preferred to open surgery) in unilateral forms and medical treatment with mineralocorticoid receptor antagonists (MRAs) such as spironolactone and eplerenone in bilateral disease in addition to the multidisciplinary management of comorbidities.New therapeutic perspectives are under development such as macrophage antibiotics or aldosterone synthase inhibitors [22][23][24][25].
Recently, it has been recognized that mineralocorticoids may also play a role in bone turnover.Some authors suggest that an excess of aldosterone may be a cause of secondary osteoporosis [26,27].The underlying mechanisms involve oxidative stress, inflammation, and a potential genetic influence concerning bone strength through various genes like NR3C2, PIK3R1, PRKCH, and SCNN1B [28,29].Another pathway relates to aldosteronemediated increased 24 h (hours) urinary calcium and consecutive secondary hyperparathyroidism in addition to a low vitamin D status [30,31].Animal models showed that MRA might induce a bone strength recover in PA [32,33].In humans, MRs have been identified at the level of bone cells, confirming that aldosterone has a direct effect on bone aldosterone-mediated increased 24 h (hours) urinary calcium and consecutive secondary hyperparathyroidism in addition to a low vitamin D status [30,31].Animal models showed that MRA might induce a bone strength recover in PA [32,33].In humans, MRs have been identified at the level of bone cells, confirming that aldosterone has a direct effect on bone metabolism [34,35].Moreover, MRs expression has been confirmed in parathyroid glands, suggesting that aldosterone may directly regulate parathyroid hormone (PTH) synthesis and secretion [36,37] (Figure 1). Figure 1.Potential common pathogenic traits in bone anomalies with respect to PA patients [26][27][28][29][30][31][32][33][34][35][36][37].
However, which is the exact extend of bone involvement in PA still represents an open issue.Moreover, the practical importance of assessing the bone status in PA in order to address a specific intervention against osteoporosis and fracture risk reduction is less clear representing a topic of individual decision rather than large screening protocols and it should be taken into consideration other contributors, as well, such as age, menopausal status, concurrent diseases that are considered prone to fragility fractures, etc.
Aim.Our objective was to overview published data concerning the bone status, including mineral metabolism and fracture risk evaluation, in patients confirmed with adrenal tumors underling PA.The synopsis of data followed several aspects such as DXA results in PA patients, TBS in these subjects, the bone turnover markers profile, prevalent fragility fractures, as well as the assessment of mineral metabolism, particularly PTH assays.
Methodology.This is a narrative review.A PubMed study was conducted with regard to the bone profile in PA from inception until June 2023.Inclusion criteria were: human (clinically relevant) studies of any study design (at least 10 patients per study); English papers; the following combination of key words within the title and/or abstract: "aldosterone" AND "bone", "skeleton", "osteoporosis", "fracture", "calcium", "parathyroid", "DXA", "osteocalcin", "P1NP", "alkaline phosphatase", "bone marker", "trabecular bone score" or "FRAX".Exclusion criteria were: in vitro or animal studies, reviews, case reports or series.We screened 1027 articles according to the mentioned However, which is the exact extend of bone involvement in PA still represents an open issue.Moreover, the practical importance of assessing the bone status in PA in order to address a specific intervention against osteoporosis and fracture risk reduction is less clear representing a topic of individual decision rather than large screening protocols and it should be taken into consideration other contributors, as well, such as age, menopausal status, concurrent diseases that are considered prone to fragility fractures, etc.
Aim.Our objective was to overview published data concerning the bone status, including mineral metabolism and fracture risk evaluation, in patients confirmed with adrenal tumors underling PA.The synopsis of data followed several aspects such as DXA results in PA patients, TBS in these subjects, the bone turnover markers profile, prevalent fragility fractures, as well as the assessment of mineral metabolism, particularly PTH assays.
Methodology.This is a narrative review.A PubMed study was conducted with regard to the bone profile in PA from inception until June 2023.Inclusion criteria were: human (clinically relevant) studies of any study design (at least 10 patients per study); English papers; the following combination of key words within the title and/or abstract: "aldosterone" AND "bone", "skeleton", "osteoporosis", "fracture", "calcium", "parathyroid", "DXA", "osteocalcin", "P1NP", "alkaline phosphatase", "bone marker", "trabecular bone score" or "FRAX".Exclusion criteria were: in vitro or animal studies, reviews, case reports or series.We screened 1027 articles according to the mentioned strategy, analyzed the papers through the key words, excluded the duplicates, and finally included 23 studies (Figure 2).strategy, analyzed the papers through the key words, excluded the duplicates, and finally included 23 studies (Figure 2).

Study-Sample-Based Analysis
These studies (n = 23) were of various designs: 13 of case-control type, 3 cross-sectional, 5 prospective, 1 observational cohort, and 1 retrospective study.The smallest study included 10 patients, while the largest was of 2533 patients.A total of 3965 subjects, aged between 38 and 64, with a mean age 56.75 years (female-to-male ratio of 1.05) were registered.
The assessments provided within these studies were as follows: 9 studies addressed dual-energy X-ray absorptiometry (DXA), another study pointed out a bone microarchitecture evaluation underlying trabecular bone score (TBS), and 7 studies investigated the bone turnover markers (BTM) profile.Moreover, 14 studies followed the subjects after adrenalectomy versus medical treatment, and 21 studies addressed secondary hyperparathyroidism in PA patients (Table 1).

Study-Sample-Based Analysis
These studies (n = 23) were of various designs: 13 of case-control type, 3 cross-sectional, 5 prospective, 1 observational cohort, and 1 retrospective study.The smallest study included 10 patients, while the largest was of 2533 patients.A total of 3965 subjects, aged between 38 and 64, with a mean age 56.75 years (female-to-male ratio of 1.05) were registered.
The assessments provided within these studies were as follows: 9 studies addressed dual-energy X-ray absorptiometry (DXA), another study pointed out a bone microarchitecture evaluation underlying trabecular bone score (TBS), and 7 studies investigated the bone turnover markers (BTM) profile.Moreover, 14 studies followed the subjects after adrenalectomy versus medical treatment, and 21 studies addressed secondary hyperparathyroidism in PA patients (Table 1).Abbreviations: APA: aldosterone-producing adenoma, BAH: bilateral adrenal hyperplasia, EH: essential hypertension, F: female, HS: healthy subjects, M: male, N: number of patients, PA: primary aldosteronism, y: year (of note, the terms were used according to each original study).As mentioned, nine studies examined the association between bone mineral density (BMD) and PA (eight case-control studies and one cross-sectional analysis); these studies provided two types of approaches: BMD results in patients with PA versus controls or osteoporosis/osteopenia prevalence (n = 8) [44][45][46]48,50,52,57,60] or PA prevalence among subjects with osteoporosis (n = 1) [51].
In three studies, the PA group had lower BMD compared to non-PA patients [44,46,60]; similar BMD between PA and non-PA individuals was found in another three studies [48,50,52].Additionally, a cohort showed no statistically significant different BMD at the lumbar spine and femoral neck between unilateral and bilateral PA [57].Ceccoli et al. [45] reported in 40 patients with PA an improvement of lumbar spine BMD after treatment of aldosterone excess (p < 0.005) [45].
The earliest study we were able to identify was published in 2012, describing the association between PA and low BMD.Z-score-based BMD (at all central sites) was lower in the PA group when compared to the control group (lumbar spine: p = 0.003; femoral neck: p = 0.011, and total neck p = 0.012).Additionally, the prevalence of osteoporosis was higher in PA versus controls (72.7% versus 20.0%, p = 0.015).PA patients had a risk of osteoporosis according to an odds ratio (OR) of 15.4 (95% CI: 1.83-130; p = 0.012) [44].This aspect was confirmed by another study that also evaluated the impact of aldosterone excess on DXA results (73 PA patients versus 73 subjects with essential hypertension versus 40 healthy controls); PA individuals had a statistically significant higher prevalence of osteopenia and osteoporosis (38.5% and 10.5%) than the second group (28% and 4%), respectively, the third group (25% and 5%) [46].
On the contrary, other studies did not confirm a lower BMD in PA versus non-PA subjects, neither a higher prevalence of osteoporosis in these patients.For instance, Notsu et al. [48] showed that lumbar spine and femoral neck BMD were similar in 56 PA patients compared with 56 (non-PA) controls.These results did not exclude a potential aldosterone excess influence on bone microarchitecture as reflected by TBS, and not by BMD [48].Similarly, Kim et al. [50] suggested that aldosterone may trigger bone loss by deteriorating bone microstructure rather than by reducing the BMD; therefore, BMD was similar between 72 PA patients and 335 subjects diagnosed with non-functional adrenal incidentalomas [50].
Alternatively, quantitative computed tomography (QCT) was used to reflect trabecular bone micro-architectural effects (QCT being an effective tool to measure the volumetric BMD that allows the assessment of volume density of the cancellous and cortical bone).BMD (as measured by QCT) was lower in PA patients versus non-PA controls (141.9 ± 34.0 versus 158.9 ± 55.9 g/cm 3 , p = 0.047).Additionally, Lv et al. [60] found a similar prevalence of osteopenia between the two mentioned groups: 31.6%(PA group) versus 30% (the patients with essential hypertension) [60] (Table 2).On the other hand, Salcuni et al. [51] evaluated the prevalence of PA among patients with osteoporosis or osteoporotic fractures and found ratios of 5.2% and 6.9%, respectively [51] (Table 3).TBS, a useful and practical indicator of bone microarchitecture, serves as a fracture risk predictor independently of BMD.TBS seems to be a better pointer than DXA-BMD in PA patients according to Kim et al. [50].TBS was statistically significantly lower in females with PA (N = 72) than controls (N = 335) after adjustment for confounder factors (p = 0.007).The authors show that plasma aldosterone concentration was correlated with a lower TBS in women, but not in men regardless of BMD [50].Another study reported a higher prevalence of fractures in the PA group versus non-PA, but similar BMD at any site [48].Overall, the level of statistical evidence concerning TBS assessment in PA remains low (n = 1), and further studies are necessary to pinpoint if PA has a greater impact on bone microarchitecture rather than BMD in adults.

PA and BTMs Profile
BTMs were assessed in seven studies (four case-control, two prospective studies, and one observational longitudinal cohort study), a total of 388 patients (female-to-male ratio of 1.06, and mean age of 52.3 years) [41,43,45,48,52,56,60].Mostly, the following BTM were analyzed: osteocalcin, procollagen I N-terminal propeptide (PINP) and bone alkaline phosphatase (bALP); respectively, bone resorption markers: urinary deoxypyridinoline (Ur-DPD), urine type I collagen cross-linked N-telopeptide (Ur-NTX), tartrate-resistant acid phosphatase 5b (TrAP), and C-terminal telopeptide of collagen type I (CTX)/CrossLaps.Loh et al. [52] found that both CTX and P1NP levels were statistically significant higher in the PA group compared to controls (p = 0.005, and p = 0.045) on a small cohort (N = 18, female-to-male ratio of 7:11, with an average age of 50 years) [52].Also, Adolf et al. [56] identified a slight increase in osteocalcin in 36 menopausal females with PA compared to controls (p = 0.023), while the TrAP value was unaltered (p = 0.189) [55].Ceccoli et al. [45] reported that serum CTX was higher and bALP was lower in subjects with PA (N = 116) compared with those diagnosed with essential hypertension (N = 110), but these data did not reach a statistical significance [45].Similarly, Notsu et al. [48] revealed similar Ur-NTX levels in 56 patients with PA versus 56 controls [48], while two other studies found the same results with regard to Ur-DPD [41,43].
These heterogeneous aspects regarding BTM based on the cross-sectional data were associated with changes in the BMT profile during follow-up from different perspectives.For instance, one study showed a statistically significant reduction in CTX at 3 months following PA therapy (p = 0.012); however, bALP remained unchanged [52].Another study revealed that patients with bilateral adrenal disease undergoing MRA treatment were associated with a statistically significant decrease in osteocalcin (p = 0.018), PINP (p = 0.007), bone ALP (p = 0.004), and TrAP (p = 0.028) after 1 year of follow-up, unlike patients who underwent an adrenalectomy [56].This aspect might suggest that MRA might negatively interfere with bone formation.Yet, Ceccoli et al. [45] studied a cohort of PA patients (40% of them having a unilateral disease) and detected similar BTMs (CTX-I, and bALP) after 24 months of follow-up [45] (Table 4).

Prevalent (Osteoporotic) Fractures among the Patients with PA
Interestingly, despite rather equivocal results with respect to BMD changes in PA, the fracture prevalence was found to be increased in PA patients according to most of the five studies we identified to specifically address this issue (four case control studies and one cross-sectional cohort), with a total of 2725 patients (female-to-male ratio of 1.16; mean age 56.13 years) being assessed [44,48,49,51,57].
The highest prevalence of vertebral fractures was of 45.5% (N = 11 patients with PA) versus 13.3% in non-PA subjects (N = 15; p = 0.095) [44].Also, an analysis revealed a vertebral fracture prevalence of 45% in the PA group when compared to the non-PA group of 23% (p = 0.05).Severe vertebral fracture prevalence was statistically significantly higher in PA subjects versus controls (23% versus 4%, p = 0.01).PA was found to be a risk factor for vertebral fractures (OR of 3.13; 95%CI: 1.30 to 7.51, p = 0.05) independently of blood pressure, glycated hemoglobin, and lipid profile.This aspect seemed to be correlated with microarchitecture damage rather than BMD changes [48].
Similarly, Umakoshi et al. [57] showed that PA subjects (N = 113) had a higher prevalence of vertebral fractures than those without PA (N = 58) of 29% versus 12% (p = 0.011).Moreover, subjects with unilateral adrenal disease (N = 37) had a higher prevalence of the same fractures than bilateral PA (N = 76), of 46% versus 20% (p = 0.021).In this study, all vertebral fractures in PA patients were asymptomatic.Unilateral PA was an independent risk factor for vertebral fractures (OR = 3.16; 95% CI: 1.12-8.92;p = 0.017) regardless of age and sex.Patients with vertebral fractures tended to be older than those without them, but only a tendency of statistical significance was registered (p = 0.087) [57].
Evidence from German Conn's Registry showed that 54% of the PA patients had secondary hyperparathyroidism [53]; another study reported that 37% of the PA subjects presented an abnormally high PTH level [59]; early studies also suggested a PTH elevation amid PA [38].Rossi et al. [39] showed that serum intact PTH was increased in PA patients versus subjects with normal blood pressure (p < 0.01) while ionized serum calcium was statistically significantly lower (p < 0.01) with similar levels of 24 h urinary calcium.An increase in ionized serum calcium and a decrease in PTH was associated with spironolactone administration (p < 0.001) and surgical treatment (p < 0.05) [39].
Lenzini et al. [54] studied the role of the renin-angiotensin-aldosterone system in PTH regulation.In primary cultures of human parathyroid cells, aldosterone and angiotensin II increased PTH secretion by acting through MRs and angiotensin II type 1 receptors.This further supported the idea that mild hyperparathyroidism is a characteristic of PA and that it may potentially be corrected by adrenalectomy [54].Recent data suggest that PTH plays a role in PA by triggering and maintaining aldosterone excessive secretion and further stimulating adrenocortical cell proliferation, but more evidence is necessary [41].
Based on our analysis, three studies showed an abnormal mineral metabolism in PA as reflected by higher PTH levels, lower serum calcium, and increased 24 h urinary calcium output when compared to controls.For instance, the control group was represented by subjects with essential high blood pressure (N = 110, mean age of 55 years, with female predominance) in one study; these patients had an increased serum calcium and decreased 24 h urinary calcium versus PA subjects (N = 116, with an average age of 51.6 years, and a slightly higher male predominance) [45].Another analysis showed statistically significantly lower plasmatic total calcium levels (p < 0.001) and higher calcium excretion values (p < 0.001) in association with increased plasma PTH levels (p < 0.001) in PA subjects versus individuals diagnosed with essential hypertension and healthy controls [46].Similar data were reported by Jiang et al. [47] (p < 0.001, p < 0.001, and p < 0.001) [47], and by Loh et al. [52] concerning lower serum Ca levels (p < 0.013) and increased PTH (p < 0.027) compared to individuals with essential hypertension (24 h urinary calcium was not assessed in this particular cohort) [52].Another two studies showed the same serum calcium and PTH profile, but no statistically significant difference was confirmed regarding urinary calcium output [41,42].A cross-sectional study (PA: N = 10, and essential hypertension: N = 182) found that PA contributed to secondary hyperparathyroidism, independently of vitamin D level (as reflected by serum 25-hydroxyvitamin D) [42].Of note, two other studies found higher PTH and 24 h urinary calcium, but no different serum calcium in PA group when compared with different controls [44,60].
A single study (published in 2019) showed among 503 patients (retrospective PA group) a prevalence of 1.2% with regard to primary hyperparathyroidism, while this prevalence was 2.1% in the (prospective PA group).Overall, the prevalence of secondary hyperparathyroidism was 54.6% (in a population aged between 42 and 56 years, an average of 47) [53].Further evidence is necessary to highlight the association with primary PTH increase in PA.
PTH was found useful to distinguish between patients with unilateral versus bilateral adrenal disease (PTH of 113.4 ± 45.7 ng/L versus 81.7 ± 29.9 pg/mL, p = 0.026) in one study; a value of PTH at 80 ng/mL was associated with a sensitivity and a specificity of 74% and 82%, respectively, to identify secondary hyperparathyroidism in PA [43].Another three studies did not identify a difference regarding PTH levels among these two mentioned subgroups of uni/bilateral adrenal involvement [47,57,59].

Dynamics of the Mineral Metabolism and DXA Results in Treated PA Patients
The follow-up data among the mentioned studies represent an inhomogeneous spectrum with respect to three main points to our awareness: firstly, some studied analyzed the effect of therapeutic intervention for PA in terms of (adrenal) surgery or medical treatment such as MRA; secondly, the panel of assessments that were under longitudinal check-up varied from serum and urinary levels of calcium, PTH, BTMs, and BMD-DXA [39,[41][42][43][44][45]47,[52][53][54]59].A third point is represented by the specific intervention for osteoporosis and fracture risk reduction in PA subjects, but this aspect remains theoretical since we did not identify any study to particularly address this issue so far.
Post-adrenalectomy changes in mineral metabolism were initially reported in 1985 (based on our PubMed strategy of research): postoperatively, total serum calcium levels increased in six patients diagnosed with unilateral tumors (p < 0.01) [38].Subsequently, Rossi et al. [39] showed an increased serum ionized calcium level and a decreased PTH level after both MRA (spironolactone) administration (p < 0.001) and adrenalectomy (p < 0.05) [39].Maniero et al. [41] found that adrenal removal normalized PTH levels (p = 0.002) and raised the serum ionized calcium level (p < 0.001) in 44 patients with single adrenal disease [41].After treatment of PA with either adrenal surgery or MRA, PTH concentrations decreased (p = 0.023) according to another cohort [42]; PTH was corrected by adrenalectomy (p = 0.02) in a group of 46 PA subjects with unilateral disease, in association with a statistically significant decrease in 24 h urinary calcium excretion (p = 0.038) [43].
Similarly, 9 out of 11 PA patients included in a small-sized study experienced a statistically significant reduction in urinary calcium output at 6 months after the initiation of specific PA treatment (surgery or spironolactone) (p < 0.01), respectively, of PTH (p < 0.01); additionally, 5 out of 11 PA patients were associated with a significant lumbar BMD increase at 1 year after starting PA therapy [44].A BMD elevation was confirmed at the lumbar spine, total hip, and femoral neck in 40 PA subjects after 24 months of post-intervention follow-up in association with a tendency of CTX decrease and bALP increase (N = 16 individuals underwent an adrenalectomy, and N = 24 subjects were treated with MRA) [45].Also, serum CTX was reduced by 20% (p = 0.012) and P1NP decreased by 18% (p = 0.036) after unilateral adrenalectomy (N = 3) or MRA treatment (N = 12) based on other data [52].Therefore, bone turnover may be improved by MRA or adrenal surgery, but the results are limited to relatively small studies [44,45,52].

Unilateral versus Bilateral Adrenal Disease in PA: Mineral and Bone Reflections
As presented, some studies provided data with respect to unilateral versus bilateral PA.This may have a direct or indirect bone reflection depending on different genetic backgrounds and even clinical panels, but the differences lie mostly in management, with unilateral tumors being mostly referred to adrenalectomy, while bilateral disease was followed under MRA exposure.Whether adrenalectomy or medical therapy have distinct skeleton effects is still an open issue.
One study suggested a quite opposite approach, namely PTH serving as a discriminator factor between unilateral and bilateral adrenal involvement in PA; a level above 80 pg/mL may indicate the need to perform adrenal venous sampling for further differential diagnosis between these subtypes [43].Whether vertebral fracture prevalence is higher in unilateral versus bilateral lesions is relatively difficult to confirm at this point [48], although one study investigated found this result (46% versus 20%, p = 0.021) [57].
A few studies showed an increase in PTH levels in unilateral versus bilateral lesions with a reversible pattern following the management of aldosterone over-production [42,45,53,55].On the other hand, some authors found a similar mineral metabolism profile (for instance, calcium and PTH) among these two PA subtypes [46,47,59].

Why Bone Considerations Next to the Aldosterone Excess?
Most of the mentioned data highlighted secondary hyperparathyroidism in individuals with PA (single-tumor-related PA or association with bilateral adrenal disease).While BMD and even BTMs analysis left us with no clear picture, a higher prevalence of the vertebral fractures in PA groups versus different controls was identified in adult patients.Moreover, TBS seems a promising tool, but limited data have been published so far, while improvement of mineral metabolism (especially serum calcium and PTH) and fracture risk parameters might be registered under surgical or medical therapy for PA.
The underlying mechanisms might include the fact that negative calcium balance through higher urinary/fecal calcium excretion causing hypocalcemia induces secondary hyperparathyroidism, with consecutive bone resorption in PA [61,62].Also, MRs from human bone cells may be activated by aldosterone excess [35].Recent studies supported a bidirectional functional link between the adrenocortical zona glomerulosa and the parathyroid gland, MR being present in the parathyroid gland whereas the PTH receptor type 1 was identified in the adrenal gland [63,64].Primary cultures of human parathyroid cells showed that both aldosterone and angiotensin II may stimulate PTH secretion via angiotensin II type 1 receptors and MR [54,65].Moreover, oxidative stress might play a role in bone remodeling [66].Lv et al. [60] showed a higher superoxide dismutase status (p = 0.011) in the PA group that could act as an important indicator of oxidative stress in these patients with potential negative impact on BMD [60].Common pathways of aldosterone signaling and bone strength (involving various genes such as NR3C2, PIK3R1, PRKCH, and SCNN1B) might explain bone and mineral metabolism changes in PA [29].Moreover, cortisol co-secretion in PA has been reported in 4% to 77% of patients with PA; this might cause supplementary bone damage in addition to a higher cardio-metabolic risk through glucocorticoid receptors [67][68][69][70][71].
Another aspect relates to PA management and potential bone health benefits [56,72,73].Spironolactone may reduce BTMs, as previously shown [56].Its anti-mineralocorticoid action causes the tubular reabsorption of calcium, thus higher serum calcium levels and a decrease in PTH, in accordance with the effects of thiazide diuretics [74].Another point is represented by the reduced urinary excretion of magnesium and potassium with potential bone-protective effects in patients receiving MRA [75].Also, experimental studies found that eplerenone reduced glucocorticoid-induced osteopenia [76].In addition, vitamin D receptor activity might be correlated with aldosterone excess in PA and interfere with secondary hyperparathyroidism; thus, vitamin D deficiency should be adequately replaced [77,78].On the other hand, with respect to specific anti-osteoporotic intervention in PA, so far, the traditional management is applicable in these cases since no specific interventional studies addressed the matter of PA-related secondary osteoporosis.

Glucose Profile Anomalies in PA: How about the Fracture Risk?
Perhaps the co-presence of glucose anomalies in PA might explain the heterogeneous panel of BMD results [41,43,45,48,52,56,60] and the potential use of TBS [51] to assess fracture risk, particularly in menopausal women.Glucose metabolism anomalies, regardless the co-diagnosis of cortisol excess, affects up to 25% of PA patients, and it should be taken into consideration not only to address the cardiovascular and metabolic risk, but also the fracture risk, as similarly seen in type 2 diabetes mellitus [79][80][81].So far, according to our methods of research, we did not identify specific studies to focus on the BMD-TBS profile in the diabetic versus non-diabetic subgroups of PA, and this topic needs a further extension.
Since its release in daily practice, TBS has found its way among different pathogenic forms of diabetes mellitus, not only of type 2, but also in relationship with endocrine tumors such as acromegaly, endogenous Cushing's syndrome, or primary hyperparathyroidism [82][83][84][85][86].Of note, primary hyperparathyroidism has been reported in 1.2% to 2.1% of the subjects confirmed with PA according to the German Conn's Registry [53].On the other hand, primary hyperparathyroidism may embrace associations with other types of adrenal tumors as seen in multiple endocrine neoplasia type 1 and 2 underlying well known pathogenic variants of MEN1 and RET [87][88][89][90][91]. Whether a subgroup of patients diagnosed with PA also displaying a synchronous parathyroid tumor harbor a common genetic background is yet to be discovered.

From Facts to Further Expansion
There are several clusters of data regarding mineral metabolism and fracture risk in PA.One of them involves central DXA evaluation.Controversies remain as to whether or not BMD alone is sufficient to adequately assess BMD and predict osteoporotic fracture risk in PA.An interesting observation was the fact that plasma renin activity was inversely correlated with TBS only in females, but not in males, but BMD was not statistically significant different between PA cases and controls in both sexes according to one mentioned study [50].As a lumbar DXA derivate, we mention that the use of TBS should be particularly encouraged in menopausal women with PA-related glucose profile anomalies [92,93].
Another cluster involves vertebral fractures in PA.Grossly, one out of 2 to 5 adults diagnosed with PA display prevalent vertebral fractures (as shown by one study, with mostly an asymptomatic presentation) [44,48,49,51,57]; that is why screening X-rays of the thoracic-lumbar spine profile should be taken into consideration, particularly in cases with other well-known fracture risks such as older age, larger duration of menopausal period, diabetes mellitus, prior fragility fracture, and increased risk of fall due to blood pressure and glycaemia variations, and hypovitaminosis D.Moreover, a collateral hypothesis suggested a higher risk of sarcopenia in females with PA via aldosterone excess or metabolic components, a condition that is prone to fall as part of the general fracture risk panel [94][95][96].
A third group of interest in PA-bone interplay is represented by BTM assays.A heterogeneous BTM picture was described [41,43,45,48,52,56,60], but this might not come as a surprise since the diabetes contribution blunts BTM values; there are already wellknown inter-individual and intra-individual BTM variations, while the mentioned BTM studies covered a 2-decade period and the types of BMT tests suffered a great variation over this time [97,98].
A fourth category is represented by not only lower calcium levels, but also mostly by secondary hyperparathyroidism in PA.This subgroup of patients (that might be up to half of the PA subjects) [53] seems at higher cardiovascular risk; the condition affects more cases of unilateral adenomas than bilateral hyperplasia, and it remits upon specific therapy for PA; vitamin D deficiency is an additional contributor, but aldosterone excess exhibits its own effect on parathyroid cells independently of serum 25-hydroxyvitamin D.
Overall, larger longitudinal trials are necessary to point out the role of DXA, TBS, and X-ray scan as screening tools for vertebral fractures, as well as sarcopenia assessment amid PA.Of note, a subgroup of patients diagnosed with PA might be actually at higher risk of osteoporotic fractures, particularly in relationship with traditional panels with respect to menopausal status and older age, and with particular aspects in PA such as the co-presence of cortisol excess or secondary diabetes.Whether PTH functions as a surrogate marker to distinguish between unilateral and bilateral disease is yet to be clarified.However, placing the PTH assay among the PA panel of investigations seems a logical step based on the mentioned studies; identifying the subgroup with secondary hyperparathyroidism helps the cardio-metabolic risk assessment as well.
We are aware of the limitations that come with a narrative review; however, for more than 4 decades, the approach of osteoporotic fracture risk and mineral metabolism status in PA involved different parameters and various perspectives; thus, we intended not to restrict the specific criteria according to a systematic review.Also, choosing a lower cut-off (at least 10 PA patients per study) might bring a potential bias, but the topic of PA-bone is far from generous with regard to the level of statistical evidence.Of note, three of the included studies were published before 2000 [38][39][40], but we intended to adequately highlight the potential PA-bone connections according to the manner in which they were studied over time and to pinpoint the fact that the mentioned cross-domain has been a subject of research for more than 3 decades.
Moreover, despite the fact, as mentioned, that there is still enough space for further studies to highlight the current gaps, recently, this trans-disciplinary topic of the bone-adrenal bridge represents a dynamic point of interest, as also reflected by two meta-analyses from 2020 (n = 15) and 2022 (n = 18) [99,100].Thus, we can only present our considerations through a sample-based study (n = 23, N = 3965), one of the largest analysis on published data so far.Based on these, we expect not only growing evidence in this particular matter in addition to new molecular insights explaining the complex common pathogenic loops, but also an expansion of the practical recommendations on daily basis to address the skeleton profile in PA.Future lines of research might include controlled studies to address the bone impact of adrenal surgery versus medical treatment; longitudinal data with respect to the post-operatory bone outcome; genetic influence underlying both PA and bone damage; distinct fracture risk subgroups among PA individuals; studies to highlight the specific bone influence due to the co-presence of diabetes mellitus and autonomous cortisol secretion in PA; interventional trials to reveal the best anti-osteoporotic intervention strategy in PA; and potential benefits in terms of fracture risk reduction of the medication targeting aldosterone excess as seen in cardiovascular outcomes.

Conclusions
Published data during a period of almost 40 years revealed 23 studies with 3965 subjects (aged between 38 and 64, with a mean age 56.75 years, and a female-to-male ratio of 1.05) in which a higher PTH in PA versus controls (healthy persons or subjects with essential hypertension) was expected, secondary hyperparathyroidism being associated in almost half of the adults diagnosed with PA.Additionally, mineral metabolism anomalies in PA may include lower serum calcium and higher urinary calcium output, all these three parameters being reversible under specific therapy for PA, regardless of this therapy being medical or surgical.The PA subgroup with high PTH seems at higher cardiovascular risk, while unilateral rather than bilateral disease is prone to this PTH anomaly.Moreover, BMD might show a higher fracture risk only in certain patients, TBS being a promising alternative (with still unknown perspective of diabetes' influence on DXA-TBS results in PA).However, overall higher fracture prevalence in PA is described in most studies, especially with respect to vertebral fractures, a risk that seems correctable upon aldosterone excess remission.These data recommend PA as a cause of secondary osteoporosis, a treatable one via PA intervention.There is still an area of debate regarding the way to address the BMT profile in PA, with case selection toward specific bone evaluation in everyday practice, and further on, the understanding of the potential genetic influence at the level of bone and mineral complications in PA patients.

Figure 1 .
Figure 1.Potential common pathogenic traits in bone anomalies with respect to PA patients [26-37].

Table 1 .
Studies included in the final analysis (n = 23) according to the mentioned strategy (N ≥ 10 patients per study); the display is between 1985 and 2022; the table provides the study design and studied population.

Table 1 .
Studies included in the final analysis (n = 23) according to the mentioned strategy (N ≥ 10 patients per study); the display is between 1985 and 2022; the table provides the study design and studied population.