Unveiling the Link Between Vitamin D, Hashimoto's Thyroiditis, and Thyroid Functions: A Retrospective Study

Hashimoto's thyroiditis (HT) is an autoimmune disease influenced by genetic factors and environmental triggers that affect immune system function. Data suggest that vitamin D may also play a role in the etiopathogenesis of HT.

This retrospective study included patients admitted to the Endocrinology and Metabolic Diseases Outpatient Clinic. Data from individuals aged 18 years and older were analyzed, including serum levels of thyroid-stimulating hormone (TSH), free triiodothyronine (fT3), free thyroxine (fT4), anti-thyroid peroxidase (anti-TPO), anti-thyroglobulin (anti-TG), and vitamin D. HT was diagnosed based on the presence of anti-TPO and/or anti-TG antibodies, while individuals with negative results for both were classified as non-HT. Thyroid function was categorized as euthyroid if TSH levels were between 0.55 mU/L and 4.78 mU/L and fT4 levels were between 0.89 ng/dL and 1.76 ng/dL; hypothyroid status was defined as TSH > 4.78 mU/L. Vitamin D levels were classified as deficient (<50 nmol/L), insufficient (50-74.9 nmol/L), or sufficient (≥75 nmol/L).

Of the total participants, 25,018 did not have HT, while 27,800 were diagnosed with HT. Vitamin D level was significantly higher in the HT group than the non-HT group (41.43 nmol/L and 39.44 nmol/L, p < 0.001). Vitamin D deficiency was present in 65.5% of the non-HT group and 62.1% of the HT group (p < 0.001). Subgroup analyses based on thyroid function showed that vitamin D levels were highest in the euthyroid HT group and similar in the euthyroid non-HT, hypothyroid non-HT, and hypothyroid HT groups (p < 0.001).

In conclusion, while vitamin D levels were higher in the HT group compared to the non-HT group, no clinically significant association between vitamin D levels and HT or autoantibody positivity was observed. Vitamin D deficiency was more prevalent in the hypothyroid group compared to the euthyroid group. This study suggests that although vitamin D deficiency may not be directly involved in the pathogenesis of HT, it may still play a role in modulating immune activity or influencing the disease phenotype..