Autoimmune thyroid disorders (AITD)—principally Hashimoto’s thyroiditis (HT) and Graves’ disease (GD)—arise from a breakdown in immune tolerance to thyroid antigens such as thyroid peroxidase (TPO) and thyroglobulin (Tg). In recent years, vitamin D has drawn intense interest for its immunomodulatory actions: the active metabolite 1,25(OH)2_22​D binds the vitamin D receptor (VDR) on dendritic cells, T and B lymphocytes, generally skewing responses away from pro-inflammatory Th1/Th17 pathways and enhancing regulatory T-cell activity, antigen tolerance, and dampened autoantibody production [1,2]. Observational syntheses consistently report lower serum 25(OH)D in AITD and particularly in HT compared with controls, although findings for GD are more heterogeneous by age and study design [3]. In a meta-analysis of observational studies (42 datasets), Taheriniya et al. showed significantly lower vitamin D in AITD and HT, with an age-dependent signal in GD (association significant only in ≥40 years) [3]. Mechanistic plausibility is further supported by genetic data implicating VDR polymorphisms (FokI, BsmI, ApaI, TaqI) and variants in vitamin D–metabolizing enzymes that may influence susceptibility or modify responses to supplementation [4]. Beyond association, randomized evidence suggests potential benefit: a 2023 meta-analysis of 12 RCTs in HT found vitamin D (especially calcitriol and courses >12 weeks) reduced TPOAb/TgAb and modestly improved thyroid function indices, though heterogeneity and trial quality warrant cautious interpretation [5]. Not all primary studies agree—some cohorts observe no vitamin D difference in HT versus controls—underscoring residual confounding (assay variability, season, BMI, ethnicity) and the need for well-designed prospective studies [6]. Overall, the biological rationale and accumulating epidemiologic and interventional data motivate examining vitamin D status when investigating AITD and exploring whether correction of deficiency could complement standard care [1,3–5].

A cross-sectional, observational study was conducted in the Department of General Medicine in collaboration with the Department of Biochemistry at Dr. Patnam Mahender Reddy Institute of Medical Sciences, Chevella, RR District, Telangana a tertiary care teaching hospital. The study aimed to evaluate the association between serum vitamin D levels and autoimmune thyroid disorders (AITD), including Hashimoto’s thyroiditis (HT) and Graves’ disease (GD). Ethical approval was obtained from the Institutional Ethics Committee before initiating the study, and informed written consent was obtained from all participants.

A total of 100 participants, aged between 18 and 60 years, were recruited and divided into three groups:

• Group I (Hashimoto’s Thyroiditis): 40 patients diagnosed with HT.
• Group II (Graves’ Disease): 30 patients diagnosed with GD.
• Group III (Controls): 30 age- and sex-matched euthyroid healthy individuals without any known autoimmune or endocrine disorders.

Inclusion Criteria

1. Patients clinically and biochemically diagnosed with autoimmune thyroid disorders (HT or GD).
2. Age 18–60 years.
3. Patients not receiving vitamin D supplementation for at least 3 months prior to sampling.

Exclusion Criteria

1. Patients with liver, kidney, or malabsorption disorders affecting vitamin D metabolism.
2. Individuals on corticosteroids, antiepileptics, or vitamin D/calcium supplements.
3. Pregnant or lactating women.
4. Patients with other autoimmune diseases such as rheumatoid arthritis, systemic lupus erythematosus, or type 1 diabetes mellitus.

Data Collection and Clinical Assessment

Detailed clinical history and physical examination were performed for all participants, including:

• Duration of thyroid illness, family history, and medication details.
• Anthropometric parameters: height, weight, and body mass index (BMI).
• Blood pressure measurement using a standard sphygmomanometer.

Laboratory Investigations

After overnight fasting, 5 mL of venous blood was collected under aseptic conditions. Serum was separated and stored at –20°C until analysis. The following parameters were measured:

• Thyroid Profile: Serum thyroid-stimulating hormone (TSH), free triiodothyronine (FT₃), and free thyroxine (FT₄) were estimated using chemiluminescent immunoassay (CLIA).
• Autoimmune Markers: Anti-thyroid peroxidase antibody (Anti-TPO Ab) and anti-thyroglobulin antibody (Anti-Tg Ab) levels were quantified using enzyme-linked immunosorbent assay (ELISA).
• Vitamin D Status: Serum 25-hydroxyvitamin D [25(OH)D] levels were measured using ELISA.

Interpretation of Vitamin D Levels

According to Endocrine Society guidelines:

• Deficient: <20 ng/mL
• Insufficient: 20–29 ng/mL
• Sufficient: ≥30 ng/mL

Statistical Analysis

Data were entered into Microsoft Excel and analyzed using SPSS version 25.0 (IBM Corp., Armonk, NY). Quantitative variables were presented as mean ± standard deviation (SD), and comparisons between two or more groups were performed using the Independent t-test or one-way ANOVA, as appropriate. Qualitative variables were expressed as percentages and proportions, and associations between categorical variables were assessed using the Chi-square test. The Pearson’s correlation coefficient (r) was applied to evaluate the relationship between serum vitamin D levels and various thyroid parameters, including TSH, FT₃, FT₄, anti-thyroid peroxidase (anti-TPO) antibodies, and anti-thyroglobulin (anti-Tg) antibodies. A p-value less than 0.05 was considered statistically significant for all analyses.

Table 1 : Demographic Profile of Study Participants

This table describes the demographic distribution of participants among the three groups—Hashimoto’s thyroiditis (HT), Graves’ disease (GD), and healthy controls. The mean age of participants was comparable across groups (39.8 ± 8.2 years in HT, 37.6 ± 7.9 in GD, and 38.4 ± 6.8 in controls; p = 0.62), showing that the study groups were age-matched. Females constituted the majority in all groups, accounting for 82.5% in HT, 76.7% in GD, and 73.3% in controls, reflecting the known female predominance in autoimmune thyroid disorders. The mean BMI was slightly higher in the HT group (26.3 ± 3.1 kg/m²) compared to GD (25.8 ± 3.4) and controls (24.9 ± 2.9), although the difference was not statistically significant (p = 0.08). Importantly, a family history of thyroid disorders was reported in 35% of HT and 30% of GD patients compared to only 10% among controls (p = 0.04), suggesting a significant hereditary or genetic predisposition in autoimmune thyroid conditions

Table 2 : Thyroid Function Tests among Study Groups

Table 2 compares thyroid hormone profiles across the three groups. Patients with Hashimoto’s thyroiditis showed a markedly elevated mean TSH level (8.74 ± 2.16 µIU/mL) and decreased FT₃ (2.13 ± 0.52 pg/mL) and FT₄ (0.61 ± 0.14 ng/dL), consistent with hypothyroid status. Conversely, Graves’ disease patients exhibited suppressed TSH (0.12 ± 0.08 µIU/mL) and elevated FT₃ (6.42 ± 1.16 pg/mL) and FT₄ (2.74 ± 0.38 ng/dL), representing hyperthyroid activity. The control group had values within normal reference ranges (TSH 2.31 ± 0.79 µIU/mL, FT₃ 3.12 ± 0.67 pg/mL, FT₄ 1.24 ± 0.29 ng/dL). The intergroup differences for all three parameters were highly significant (p < 0.001), with high F-values confirming the robustness of association

Table 3 : Autoantibody Profile in Study Groups

This table highlights the autoimmune antibody titers in the study population. The mean anti-thyroid peroxidase (Anti-TPO) antibody level was significantly elevated in Hashimoto’s thyroiditis (482.4 ± 95.2 IU/mL), moderate in Graves’ disease (271.3 ± 82.1 IU/mL), and minimal in controls (36.8 ± 15.6 IU/mL), with p < 0.001. Similarly, anti-thyroglobulin (Anti-Tg) antibody levels were highest in HT (367.5 ± 78.3 IU/mL), followed by GD (188.6 ± 64.5 IU/mL), and lowest in controls (28.4 ± 10.2 IU/mL). These results reaffirm the autoimmune nature of both HT and GD, with greater antibody activity in Hashimoto’s thyroiditis, which aligns with the chronic lymphocytic infiltration seen histologically in HT.

Table 4 : Serum 25(OH) Vitamin D Levels among Study Groups

Table 4 assesses the vitamin D status among the study participants. A significant proportion of patients with autoimmune thyroid disorders were vitamin D deficient. Specifically, 70% of Hashimoto’s thyroiditis and 60% of Graves’ disease patients had vitamin D deficiency (<20 ng/mL), compared with only 23.3% among controls. Sufficient vitamin D levels (≥30 ng/mL) were present in just 10% of HT and 13.3% of GD cases, while 43.4% of controls maintained sufficient levels. The mean serum 25(OH)D levels were significantly lower in the AITD groups—17.4 ± 4.3 ng/mL in HT and 18.6 ± 5.2 ng/mL in GD—compared to 28.2 ± 6.5 ng/mL in controls (p < 0.001). The chi-square test (χ² = 21.46) indicated a strong and statistically significant association between vitamin D deficiency and autoimmune thyroid disorders.

Table 5 : Correlation between Serum Vitamin D and Thyroid Parameters

This table presents the correlation analysis between serum vitamin D levels and thyroid function parameters. Vitamin D showed a significant negative correlation with TSH (r = –0.42, p = 0.001), anti-TPO (r = –0.46, p < 0.001), and anti-Tg antibodies (r = –0.40, p = 0.001), suggesting that lower vitamin D levels are associated with higher autoimmune activity and TSH elevation. In contrast, positive correlations were observed with FT₃ (r = +0.35, p = 0.004) and FT₄ (r = +0.39, p = 0.002), indicating that adequate vitamin D levels correspond with improved thyroid function and lower disease severity. These correlations reinforce the possible immunomodulatory role of vitamin D in thyroid autoimmunity.

Autoimmune thyroid disorders (AITDs) such as Hashimoto’s thyroiditis (HT) and Graves’ disease (GD) represent the most prevalent organ-specific autoimmune diseases. They are characterized by the presence of circulating autoantibodies directed against thyroid antigens—thyroperoxidase (TPO) and thyroglobulin (Tg)—leading to either hypothyroidism or hyperthyroidism. The current study assessed the association between serum vitamin D status and autoimmune thyroid disorders, revealing significant findings across demographic, biochemical, and immunological parameters.

In the present study, the mean age of patients with HT and GD was comparable to that of healthy controls, consistent with the middle-aged predominance typical of autoimmune thyroiditis. Females constituted the majority in all groups (82.5 % in HT, 76.7 % in GD), confirming the strong female preponderance seen in AITDs. Similar gender trends were observed by Kivity et al., who reported that women are affected up to ten times more frequently than men due to hormonal and genetic susceptibility factors [7]. A positive family history was noted in 35 % of HT and 30 % of GD patients compared with only 10 % in controls (p = 0.04), supporting the concept of familial clustering. Antonelli et al. also emphasized the role of genetic predisposition and HLA-linked susceptibility in familial autoimmune thyroiditis [9].

The present study demonstrated markedly elevated TSH and reduced FT₃ and FT₄ in HT, consistent with primary hypothyroidism, whereas GD showed suppressed TSH and elevated thyroid hormone levels. Similar biochemical trends were described by Song et al. in a Korean cohort, where patients with Hashimoto’s thyroiditis exhibited mean TSH levels above 8 µIU/mL and FT₄ levels below 0.8 ng/dL [10]. Likewise, Unnikrishnan et al. in an Indian population reported that 11.7 % of thyroid disorders were autoimmune in nature, with the majority presenting as hypothyroidism [11]. The significant differences in thyroid profiles among groups (p < 0.001) reaffirm the biochemical distinctiveness between hypothyroid and hyperthyroid AITD subtypes.

Both anti-TPO and anti-Tg antibodies were significantly higher in AITD patients compared with controls, with HT showing the highest titers. The mean anti-TPO level in HT (482.4 IU/mL) was almost double that seen in GD (271.3 IU/mL). These results are in line with observations by Bozkurt et al., who found elevated anti-TPO and anti-Tg antibodies in 92 % and 74 % of HT cases, respectively [12]. Similarly, Kim et al. demonstrated that autoantibody titers correlate inversely with serum vitamin D, suggesting that vitamin D deficiency may exacerbate autoimmune activation [13]. The higher antibody levels in HT also reflect the chronic inflammatory lymphocytic destruction typical of this disorder.

A striking finding in this study was the significantly lower mean 25(OH)D levels in AITD patients—17.4 ng/mL in HT and 18.6 ng/mL in GD—compared with 28.2 ng/mL in controls (p < 0.001). Vitamin D deficiency (< 20 ng/mL) was observed in 70 % of HT and 60 % of GD patients, compared with only 23 % among healthy controls. These findings are consistent with the meta-analysis by Taheriniya et al., which demonstrated that patients with AITD had significantly lower vitamin D levels than controls (weighted mean difference = –6.3 ng/mL, 95 % CI –8.6 to –4.0) [3]. In India, Goswami et al. also reported widespread vitamin D deficiency among patients with autoimmune hypothyroidism, independent of seasonal variation [14]. Furthermore, Bozkurt et al. identified vitamin D deficiency in 92 % of HT cases and established an inverse relationship between 25(OH)D and anti-TPO levels [12].
However, a few studies have yielded divergent findings. Ke et al. found no significant difference in vitamin D levels between AITD and controls after adjusting for confounders such as BMI and sun exposure [15]. Such variability underscores the multifactorial influences—dietary intake, sunlight exposure, and ethnicity—on vitamin D metabolism.

Correlation analysis in this study revealed a negative relationship between vitamin D and TSH (r = –0.42, p = 0.001) as well as thyroid antibodies (anti-TPO * r* = –0.46, p < 0.001; anti-Tg * r* = –0.40, p = 0.001), while positive correlations were observed with FT₃ and FT₄ (r = 0.35 and 0.39, respectively). These findings suggest that lower vitamin D levels may promote thyroid autoimmunity and dysfunction. Similar correlations were demonstrated by Muscogiuri et al., who proposed that vitamin D exerts immunomodulatory effects by reducing Th1 cytokine production and enhancing T-regulatory cell activity [16]. In another Indian study, Singla et al. reported a significant inverse correlation between 25(OH)D and anti-TPO titers (r = –0.44, p < 0.01) [17]. Tang et al. (2023) further supported this association through a meta-analysis of 12 randomized trials, showing that vitamin D supplementation (> 12 weeks) significantly lowered anti-TPO and anti-Tg antibody levels and modestly improved thyroid function [18].

The present study demonstrates a significant association between vitamin D deficiency and autoimmune thyroid disorders (AITDs), particularly Hashimoto’s thyroiditis and Graves’ disease. Patients with AITD exhibited markedly lower serum 25(OH)D levels and higher anti-thyroid antibody titers compared to healthy controls. Furthermore, an inverse correlation between vitamin D levels and anti-TPO, anti-Tg, and TSH, along with a positive correlation with FT₃ and FT₄, suggests a potential immunomodulatory role of vitamin D in thyroid autoimmunity. These findings highlight the importance of assessing and correcting vitamin D deficiency as an adjunctive measure in the management of autoimmune thyroid diseases. However, larger prospective and interventional studies are warranted to establish a causal relationship and therapeutic benefit.

Acknowledgement : None

Conflict of Interest : None

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