The Effects of Vitamin D Supplementation on Thyroid Function in Hypothyroid Patients: A Randomized, Double-blind, Placebo-controlled Trial

BACKGROUND: Data on the effects of vitamin D supplementation on thyroid function in hypothyroid patients are scarce. OBJECTIVE: This study was done to evaluate the effects of vitamin D supplementation on thyroid function in hypothyroid patients. MATERIAL AND METHODS: This randomized double-blind, placebo-controlled trial was conducted on 201 hypothyroid patients aged 20–60 years old. Subjects were randomly assigned into two groups to intake either 50,000 IU vitamin D supplements (n = 102) or placebo (n = 99) weekly for 12 weeks. Markers of related with thyroid function were assessed at first and 12 weeks after the intervention. RESULTS: After 12 weeks of intervention, compared to the placebo, vitamin D supplementation resulted in significant increases in serum 25-hydroxyvitamin D (+26.5 ± 11.6 vs. 0.0 ± 0.0 ng/mL, P < 0.001) and calcium (+0.4 ± 0.7 vs. 0.1 ± 0.6 mg/dL, P = 0.002), and a significant decrease in serum thyroid-stimulating hormone (TSH) levels (−0.4 ± 0.6 vs. +0.1 ± 2.0 μIU/mL, P = 0.02). A trend towards a greater decrease in serum parathyroid hormone (PTH) levels was observed in vitamin D group compared to placebo group (−3.8 vs. +1.9, P = 0.07). We did not observe any significant changes in serum T3, T4, alkaline phosphatase (ALP) and albumin levels following supplementation of vitamin D compared with the placebo. CONCLUSION: Overall, the current study demonstrated that vitamin D supplementation among hypothyroid patients for 12 weeks improved serum TSH and calcium concentrations compared with the placebo, but it did not alter serum T3, T4, ALP, PTH, and albumin levels.