Effect of vitamin D supplementation on TSH levels in euthyroid subjects with autoimmune thyroiditis

Purpose: The impact of vitamin D supplementation on thyroid function is not clear and the relationship between hypovitaminosis D and autoimmune thyroiditis (ATD) incidence and evolution is still a matter of debate. The aim of this study was to retrospectively evaluate the impact of vitamin D supplementation on thyroid function in subjects with and without ATD. Methods: One hundred and ninety-eight euthyroid subjects, with diagnosis of “hypovitaminosis D” (<30 ng/mL) who had been taking supplementation therapy with cholecalciferol for different time periods, were included. They were divided in two groups according to the previous diagnosis of ATD: “ATD-neg” group including subjects without ATD [n = 103 (52%)]; “ATD-pos” group including subjects with a confirmed diagnosis of ATD [n = 95 (48%)]. For both groups, we considered TSH and 25 hydroxyvitamin D (25OHD) levels before (T0) and after (T1) cholecalciferol supplementation. We also considered the treatment duration and the monthly dose of cholecalciferol expressed as IU/month. Results: In hypovitaminosis D subjects with ATD, TSH levels significantly decreased after therapy with cholecalciferol 100.000 IU/month [mean ± SD, TSH at T0: 2.67 ± 1.21 vs. TSH at T1: 2.28 ± 0.86, p = 0.028]. No significant TSH variation was observed in ATD-neg group, irrespective of treatment dose and duration. As expected, 25OHD levels significantly improved with all monthly doses and especially in the group receiving 100.000 IU/month. Conclusions: Cholecalciferol supplementation improved thyroid function in euthyroid ATD-pos subjects affected with severe hypovitaminosis D. In particular, a significant reduction in TSH levels was observed in subjects with very low baseline 25OHD levels, after taking high monthly doses of cholecalciferol.