Vitamin D and thyroid disorders: a systematic review and Meta-analysis of observational studies

Background: The contribution of vitamin D to thyroid disorders has received paramount attention; however, results are mixed. Hence, we designed a systematic review and meta-analysis to obtain a definitive conclusion. Methods: The search included PubMed, ISI Web of Science, Scopus, and Google Scholar databases up to March 2021 to collect available papers reporting the relationship between serum levels of vitamin D and thyroid disorders. The pooled effect was reported as weighted mean difference (WMD) and 95% confidence interval (CI). Results: Out of 6123 datasets, 42 were eligible to get into this systematic review and meta-analysis. Serum vitamin D was markedly lower in autoimmune thyroid diseases (AITD) (WMD − 3.1 ng/dl; 95% CI, − 5.57 to − 0.66; P = 0.013; I² = 99.9%), Hashimoto’s thyroiditis (HT) (WMD − 6.05 ng/dl; 95% CI, − 8.35 to − 3.75; P < 0.001; I² = 91.0%) and hypothyroidism patients (WMD − 13.43 ng/dl; 95% CI, − 26.04 to − 0.81; P = 0.03; I² = 99.5%), but not in subjects with Graves’ disease (GD) (WMD − 4.14 ng/dl; 95% CI, − 8.46 to 0.17; P = 0.06; I² = 97.5%). Conclusions: Our findings suggested lower vitamin D levels in patients with hypothyroidism, AITD, and HT compared to healthy subjects. However, the link between serum vitamin D and GD was only significant among subjects ≥40 years old.