The Effect of Vitamin D Supplementation on Thyroid Hormone Levels in Patients With Autoimmune Thyroid Disease: A Systematic Review

Autoimmune thyroid diseases (AITDs) pose significant challenges in clinical practice, representing one of the most common endocrine abnormalities. Vitamin D deficiency has been linked as one of the contributing factors to the etiology of AITDs. This systematic review evaluates the effects of vitamin D supplementation on thyroid-stimulating hormone (TSH), triiodothyronine (T3), and thyroxine (T4) levels in adults with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines, seven relevant studies were identified from an initial pool of 1,469 articles. The population comprised individuals with thyroid autoimmunity, as evidenced by at least one elevated positive thyroid autoimmune marker and intervention involved the supplementation of vitamin D, regardless of the dose or method of administration. All randomized clinical trials within the last 10 years, which fit the study criteria, were included. These studies showed varying results based on follow-up duration. Short-term studies (three months or less) demonstrated no significant changes in mean TSH, T3, or T4 levels compared to the control group with vitamin D supplementation. However, all of the long-term studies (greater than three months) indicated significant improvements compared to the control in mean TSH, T3, and T4 levels. Additionally, all long-term studies that compared TSH, T3, and T4 to baseline levels revealed significant changes by the trial's end. Despite these promising findings, the review highlights limitations, including small sample sizes, short study durations, and the need for further research to establish optimal dosing and treatment duration for vitamin D in AITD management. The overall findings suggest that vitamin D supplementation may play a part in thyroid hormone regulation in AITD, particularly with prolonged administration.