Effects of vitamin D supplementation on TSH and thyroid hormones: A systematic review of randomized controlled trials

Approximately 200 million people have been diagnosed with thyroid disease worldwide. Associations between thyroid disorders and nutritional factors have been established in former studies.

The Web of Science, PubMed, and Scopus databases and Google Scholar were searched without date restriction until June 2023 by using relevant keywords. All original articles written in English that studied the effects of vitamin D supplementation on TSH and thyroid hormones were eligible for the present review.

The present review included a total of 16 randomized controlled trials (RCTs) with 1127 participants (intervention group, 630; control group, 497). Based on the findings made, vitamin D supplementation had no significant effects on serum TSH levels in 9 cases (56.2%) reported in the trials. However, TSH levels were elevated in 4 studies (26.6%) and low in 3 (18.7%) cases after vitamin D administration. Four cases reported in 6 RCTs and 4 cases reported in 5 trials indicated no significant effects on thyroxine (T₄) and triiodothyronine (T₃) levels after vitamin D supplementation, respectively. Moreover, 8 cases of 11 included RCTs reported that vitamin D intake had no significant effects on fT4, and 5 cases of 7 RCTs demonstrated no significant effects on fT3 levels after vitamin D supplementation.

Most findings suggest that vitamin D supplementation had no significant effects on thyroid hormones and more than half indicated no significant effect on TSH levels. Nevertheless, high-quality research is further required to prove these achievements and evaluate vitamin D effects on other markers of thyroid function.