Association of Vitamin D Deficiency and Thyroid Function in Postmenopausal Women

Although there are reports of vitamin D (VitD) insufficiency in immune-mediated hypothyroidism, an association between VitD and thyroid-stimulating hormone (TSH) levels has yet to be shown. We aim to examine VitD and TSH levels among postmenopausal women, as both conditions are more prevalent in elderly women. The clinic records of postmenopausal women during their routine maintenance visits were reviewed. All patients were examined for the symptoms related to thyroid function and osteoporosis. Participants were divided into three subgroups according to their TSH levels (below <0.5 mIU/L, 0.51-4.0 mIU/L and >4.0 mIU/L). Patient characteristics and VitD levels were compared between these subgroups. Multivariate linear regression model was constructed using serum VitD and serum TSH as the dependent variables to identify factors independently associated with these laboratory values. Two-hundred and nighty nine postmenopausal women were included. Average age was 62.2±7.5 years old. VitD was insufficient (10-30 ng/mL) in 12.0% and deficient (<10 ng/mL) in 60.9% of the participants. In 11.3%, TSH was low and in 7.6% of women, TSH was high, while the remaining 80.1%, had normal TSH levels. Subjects with low TSH had significantly higher VitD concentrations (34.2±29.1 ng/mL) compared to the other two groups (P-value: 0.039). In multivariate regression analysis, TSH was not a contributing factor, as age was the only significant predictor of VitD levels. Meanwhile, no predictor (including age and VitD) was identified for TSH levels in linear regression analysis. Age was the only independent predictor of serum VitD in this study population. Though suppressed TSH was associated with higher VitD levels, the association was not linear between TSH and VitD in postmenopausal women.