Thyroid dysfunction is among the most prevalent endocrine disorders worldwide and exerts wide-ranging effects on metabolic and physiological processes. Prominent among these is the regulation of mineral metabolism, in which calcium homeostasis plays a pivotal role in maintaining skeletal integrity and normal cellular function [1]. Hypothyroidism, characterised by deficient thyroid hormone production, has been studied extensively for its influence on bone mineral metabolism [2], yet the interplay between thyroid hormones and calcium balance remains incompletely understood.

The epidemiological burden is considerable. The reported global prevalence of hypothyroidism ranges between 4% and 21%, varying with geographical location, dietary iodine intake and genetic predisposition [3,4]. In India the picture is equally striking: a multicentre epidemiological survey across eight cities estimated the prevalence of hypothyroidism in adults at approximately 10.95%, affecting urban and rural populations alike [3]. Inadequate dietary iodine, a rising incidence of autoimmune thyroiditis and environmental influences on thyroid function have all been implicated. This burden makes a comprehensive understanding of the metabolic consequences of hypothyroidism, including disturbances of calcium homeostasis, a matter of practical clinical importance.

Calcium is essential to neuromuscular transmission, coagulation, enzymatic activity and skeletal integrity [5]. Its serum concentration is tightly regulated by an endocrine network comprising parathyroid hormone, calcitonin and vitamin D, upon which thyroid hormones exert a modulatory influence [6]. Thyroid dysfunction has accordingly been associated with alterations in serum calcium that may predispose to metabolic bone disorders including osteoporosis, osteomalacia and secondary hyperparathyroidism [7]. Hypothyroid individuals frequently exhibit hypocalcaemia, attributed principally to impaired intestinal calcium absorption and reduced bone turnover [8], with altered renal calcium handling and disturbed vitamin D metabolism contributing further.

The mechanisms are multifactorial. Reduced circulating thyroid hormone influences parathyroid hormone secretion, vitamin D metabolism and renal tubular reabsorption of calcium [1,9]. Thyroid hormones act directly on osteoblasts and osteoclasts, governing the balance between bone formation and resorption; in the hypothyroid state, diminished bone turnover reduces calcium mobilisation from the skeleton and thereby aggravates hypocalcaemia [10]. The net consequence may be an increased risk of bone demineralisation, pathological fracture and impaired neuromuscular function [11].

Despite this coherent theoretical framework, empirical findings remain inconsistent. Several investigators report significant hypocalcaemia in hypothyroidism, whereas others describe only minimal variation, suggesting the involvement of multiple compensatory regulatory pathways [12,13]. Jat et al. documented significant mineral disturbance across both subclinical and overt hypothyroidism [14], while Charak et al., examining 419 subjects, established correlations between thyroid-stimulating hormone and a range of electrolyte and mineral parameters [15]. Sridevi et al. and Athokpham et al. likewise reported reduced calcium in hypothyroid cohorts [16,17]. The discrepancies between series — which differ in sample size, severity of thyroid failure and analytical method — highlight the need for further comparative analysis using well-matched controls.

Two aspects are particularly under-addressed. First, few studies quantify the magnitude of the calcium deficit in a way that permits clinical interpretation rather than mere statistical significance. Second, the relationship between the degree of thyroid failure, as indexed by thyroid-stimulating hormone, and the extent of calcium reduction is inconsistently reported. The present study was therefore undertaken to compare serum calcium concentrations between newly diagnosed overt hypothyroid patients and age- and sex-matched euthyroid individuals attending a tertiary care hospital, and to examine the correlation between thyroid-stimulating hormone and serum calcium.

This hospital-based comparative observational study was conducted in the Department of General Medicine, McGann Teaching District Hospital, Shivamogga Institute of Medical Sciences, Shivamogga, Karnataka. Both outpatient attendees and admitted patients were eligible for inclusion. Study duration: The study was carried out over a period of 12 months. Ethical considerations: Institutional Ethics Committee clearance was obtained before commencement of the study. Written informed consent was obtained from every participant prior to enrolment, and confidentiality of personal and clinical information was maintained throughout. Study population and grouping: A total of 160 subjects were enrolled and allocated to two groups of 80 each: newly diagnosed overt hypothyroidism, and euthyroid controls. Controls were matched to cases for age and gender. Inclusion criteria: Subjects aged above 18 years with newly diagnosed overt hypothyroidism formed the case group. Age- and gender-matched euthyroid individuals formed the control group. Exclusion criteria: Subjects were excluded if they had chronic kidney disease or hepatic disease — both of which independently disturb calcium metabolism — if they were receiving mineral supplementation, or if they had undergone thyroid surgery. Sample size estimation: The sample size was derived from a previous study, taking an anticipated between-group difference in the mineral parameter of interest of 0.4 with a pooled standard deviation of 0.73. With an alpha error of 5% and 80% power, the minimum required sample was calculated as 52 subjects per group. Eighty subjects were recruited into each group, comfortably exceeding this requirement and providing additional power. Sample collection and handling: Following informed consent, 2 mL of venous blood was collected from each participant using a plain vacutainer. Samples were allowed to clot and were then centrifuged to separate the serum, which was stored at 4–8 °C until analysis. Standardised pre-analytical handling was applied uniformly to both groups to eliminate systematic bias in mineral estimation. Biochemical estimation: Serum free T3, free T4 and thyroid-stimulating hormone were estimated using the fully automated Cobas e411 analyser. Serum calcium was estimated using the fully automated Cobas c311 analyser. Overt hypothyroidism was defined biochemically by an elevated thyroid-stimulating hormone concentration accompanied by reduced free T4, and the euthyroid state by thyroid function tests within the reference range; the thyroid profile therefore served both to define group allocation and to quantify the severity of thyroid failure for correlation analysis. Study variables: The primary outcome variable was serum total calcium concentration, compared between the two groups. Secondary variables comprised serum thyroid-stimulating hormone, free T3 and free T4, together with age and sex, which were recorded to confirm adequacy of matching. Statistical analysis: Data were expressed as mean ± standard deviation. Categorical variables were summarised as frequencies and percentages. An unpaired Student's t-test was used to compare parameters between cases and controls, with the t statistic and corresponding p value reported for each. Pearson's correlation coefficient was used to assess the relationship between the thyroid profile and serum calcium. A p value below 0.05 was regarded as statistically significant, and a value below 0.0001 as highly significant. Standardised mean differences (Cohen's d) were derived from the reported group means and standard deviations to express the magnitude of between-group differences independently of sample size, and the coefficient of determination was calculated from the correlation coefficient to quantify shared variance. (Word count: 513)

One hundred and sixty subjects were studied — 80 with newly diagnosed overt hypothyroidism and 80 euthyroid controls.

Table 1. Age distribution of study participants (n = 160)

Participants spanned the range 18 to 70 years, with a mean age of 46.49 ± 12.87 years. The largest single band was 40–51 years, containing 45 subjects (28.13%), followed by 51–62 years with 38 (23.75%). Taken together, 83 subjects (51.88%) fell between 40 and 62 years, confirming that the study population was predominantly middle-aged. The youngest band, 18–29 years, contributed the fewest at 24 subjects (15.00%). This distribution reflects the recognised tendency of hypothyroidism to declare itself in the fourth to sixth decades, and identifies middle age as the period in which screening for associated mineral disturbance is most likely to be productive.

Table 2. Sex distribution by study group

Women constituted the great majority of the study population at 126 of 160 subjects (78.75%), giving a female-to-male ratio of approximately 3.7:1 and reflecting the well-established female preponderance of thyroid disease. Critically for the validity of the comparison that follows, the two groups were identically constituted, each containing 63 women and 17 men. Sex is therefore fully controlled as a confounder, and any between-group difference in serum calcium cannot be attributed to differing sex composition.

Table 3. Comparison of thyroid function between groups

The biochemical separation between groups was unequivocal. Thyroid-stimulating hormone was more than eight times higher in the hypothyroid group (24.19 versus 2.89 µIU/mL), reflecting the compensatory pituitary response to failing hormone synthesis, while free T3 fell by 32.6% and free T4 by 45.8%. All three differences were highly significant (p<0.0001) and of large magnitude, with standardised effect sizes of 2.19 for thyroid-stimulating hormone, 1.66 for free T3 and 2.77 for free T4. The considerably wider standard deviation of thyroid-stimulating hormone in the hypothyroid group (13.70 against 1.30) indicates a broad spread of disease severity within that group — a feature that makes the correlation analysis in Table 5 informative rather than merely confirmatory.

Table 4. Comparison of serum calcium between groups

This table addresses the primary objective. Mean serum calcium was 8.51 ± 0.55 mg/dL in hypothyroid patients against 9.59 ± 0.52 mg/dL in euthyroid controls — an absolute reduction of 1.08 mg/dL, or 11.3% below the control mean, which was highly significant (t = 12.76, p<0.0001). Two features deserve emphasis. First, the magnitude is clinically as well as statistically meaningful: the hypothyroid group mean sits close to the lower limit of the conventional reference range, so that a substantial proportion of individuals within that group must have been frankly hypocalcaemic. Second, the standardised effect size of 2.02 indicates that the two distributions overlap very little — a difference of more than two pooled standard deviations. The narrow and comparable standard deviations in both groups (0.52 and 0.55) show that this was a consistent shift affecting the whole hypothyroid group rather than an artefact produced by a few outlying values.

Table 5. Correlation between thyroid-stimulating hormone and serum calcium (n = 160)

Across the whole study population, thyroid-stimulating hormone correlated inversely and significantly with serum calcium (r = −0.539, p<0.0001). The relationship is of moderate strength, the coefficient of determination indicating that approximately 29% of the variance in serum calcium is shared with variation in thyroid-stimulating hormone. In practical terms, the higher the thyroid-stimulating hormone, the lower the serum calcium — a graded relationship rather than a simple categorical difference between hypothyroid and euthyroid states, which supports a genuine dose-dependent effect of thyroid failure on calcium homeostasis. That roughly 71% of the variance remains unexplained is equally instructive, and is consistent with the multiple parallel regulators of calcium — parathyroid hormone, vitamin D status, albumin concentration and dietary intake — that were not measured here.

Table 6. Comparison of serum calcium findings with published studies

*Median with interquartile range.

The direction of effect was identical across every comparator series, and the magnitude of the present finding sits squarely within the published range. The reduction of 1.08 mg/dL observed here is almost exactly that reported by Srinivas et al. (1.07 mg/dL), somewhat smaller than the 1.46 mg/dL of Sridevi et al. and larger than the 0.75 mg/dL of Athokpham et al. The absolute values in the hypothyroid groups cluster tightly between 8.2 and 8.96 mg/dL across all five studies. This consistency across cohorts differing in geography, sample size and analytical platform lends considerable weight to hypocalcaemia as a reproducible biochemical feature of overt hypothyroidism.

The present study demonstrates a significant reduction in serum total calcium among newly diagnosed overt hypothyroid patients, from a control mean of 9.59 ± 0.52 mg/dL to 8.51 ± 0.55 mg/dL (p<0.0001). The age range of 18 to 70 years and mean age of 46.49 ± 12.87 years correspond closely to comparable Indian series: Jat et al. studied 210 subjects across the same age range [14], and Srinivas et al. reported mean ages of 46.7 ± 7.3 and 43.2 ± 6.4 years in their hypothyroid and euthyroid groups [18]. The marked female preponderance of 78.75% accords with the established epidemiology of thyroid disease and is supported by Unnikrishnan et al., Meng et al. and Jessy et al. [3,20,21]. Athokpham et al. reported a comparable 71.1% and 74.8% female representation in their control and hypothyroid groups respectively [17]. Because the two groups here were identically matched at 63 women and 17 men, sex is fully excluded as an explanation for the calcium difference — a methodological strength relative to several published series in which group composition differed. Thyroid function testing confirmed unequivocal group separation, with thyroid-stimulating hormone elevated more than eightfold and free T4 reduced by nearly half. These patterns are consistent with the interpretive framework described by Koulouri and Gurnell [22], and with the observation of Fatourechi and of Yoo and Chung that thyroid-stimulating hormone is the most sensitive marker of thyroid dysfunction [23,24]. The magnitude of the calcium reduction merits emphasis. An absolute fall of 1.08 mg/dL corresponds to a standardised effect size of 2.02, meaning the two distributions barely overlap. This is closely comparable to the 1.07 mg/dL reduction reported by Srinivas et al. [18] and lies between the 1.46 mg/dL of Sridevi et al. [16] and the 0.75 mg/dL of Athokpham et al. [17]. Saxena et al. similarly documented a fall in median calcium from 9.1 to 8.2 mg/dL [19]. The mechanistic explanation is well established: reduced thyroid hormone diminishes osteoblastic and osteoclastic activity, lowering bone turnover and thus the mobilisation of skeletal calcium, while intestinal absorption and renal tubular handling are additionally impaired [1,7,8]. The inverse correlation between thyroid-stimulating hormone and calcium (r = −0.539, p<0.0001) indicates that the deficit is graded rather than categorical. Srinivas et al. reported a stronger inverse relationship (r = −0.8368) [18] and Athokpham et al. stronger still (r = −0.891) [17], while Sridevi et al. found r = −0.79 [16]. The weaker coefficient here may reflect the wider spread of thyroid-stimulating hormone values in the present hypothyroid group, and indicates that thyroid failure accounts for roughly 29% of calcium variance, the remainder attributable to parathyroid hormone, vitamin D and albumin status. Limitations: Serum albumin was not measured, so calcium values were uncorrected and a component of the observed difference could reflect differences in protein binding. Parathyroid hormone and 25-hydroxyvitamin D were not assayed, precluding mechanistic attribution. Ionised calcium was not measured. The correlation was computed across the pooled cohort, so that the group difference itself contributes to the coefficient; within-group analysis would be more informative. Finally, the single-centre design and absence of follow-up after levothyroxine replacement preclude any conclusion about reversibility.

In this comparative study of 160 subjects — 80 with newly diagnosed overt hypothyroidism and 80 age- and sex-matched euthyroid controls — serum total calcium was significantly and substantially lower in the hypothyroid group, at 8.51 ± 0.55 mg/dL against 9.59 ± 0.52 mg/dL (p<0.0001). The absolute deficit of 1.08 mg/dL represents an 11.3% reduction and corresponds to a large standardised effect size of 2.02, indicating minimal overlap between the two distributions. The finding was consistent across the group rather than driven by outliers, as reflected in the narrow and comparable standard deviations.

The deficit was graded rather than categorical: thyroid-stimulating hormone correlated inversely with serum calcium across the whole cohort (r = −0.539, p<0.0001), accounting for approximately 29% of the variance in calcium concentration. This dose-dependent relationship supports a genuine biological effect of thyroid failure on calcium homeostasis rather than a chance association, while the substantial unexplained variance is consistent with the several parallel regulators of calcium that were not measured.

The study population was predominantly middle-aged, with 51.9% aged between 40 and 62 years, and overwhelmingly female at 78.75%. These findings argue for measurement of serum calcium at the time of diagnosis in every patient with newly detected overt hypothyroidism, and particularly in middle-aged women, in whom both conditions converge. Given the established role of thyroid hormone in bone turnover, sustained hypocalcaemia in this group carries potential implications for skeletal health that extend beyond the immediate biochemical abnormality.

Future work should incorporate albumin-corrected or ionised calcium, simultaneous measurement of parathyroid hormone and 25-hydroxyvitamin D to establish mechanism, and prospective reassessment after restoration of euthyroidism with levothyroxine, in order to determine whether the calcium deficit is reversible with adequate replacement.

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