Lower serum vitamin D levels are found in people with autoimmune thyroid diseases (AITD), Hashimoto’s thyroiditis (HT), hypothyroidism (HD) and Graves disease (GD),  but researchers considered the link to GD significant only in people aged 40 years and older. The researchers’ findings were recently published in BMC Endocrine Disorders.

Vitamin D has anti-inflammatory effects through inhibition of the production of cytokines as well as diminishing T cell-dependent immune responses in autoimmune diseases.

Researchers conducted a systematic review and meta-analysis regarding the correlation of vitamin D levels to adult thyroid disorders. They reviewed four databases, including PubMed, ISI Web of Science, Google Scholar, and Scopus. Studies that involved non-human or irrelevant datasets, pregnant or lactating women, case reports, reviews, editorials, poster abstracts, and non-original studies were excluded.  


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Altogether, the researchers used 35 studies (42 arms) between the years 1980 and 2018 in their meta-analysis. They extracted pertinent data from selected studies and determined study quality as low, moderate, or high based upon selection, comparability, and outcome criteria. The investigators also performed a sensitivity analysis on each study as well as sub-group analyses based on age, gender, geographical region, study design, and population health status.

Serum vitamin D was markedly lower in people with AITD (weighted mean difference [WMD] -3.1 ng/dl; 95% confidence interval [CI], −5.57 to −0.66; P = .013; I2 = 99.9%), HT (WMD  -6.05 ng/dl; 95% CI, −8.35 to −3.75; P < .001; I2 = 91.0%) and hypothyroidism (WMD -13.43 ng/dl; 95% CI, -26.04 to -0.81; P = .03; I2 = 99.5%), but not in subjects with GD (WMD -4.14 ng/dl; 95% CI, -8.46 to 0.17; P = .06; I2 = 97.5%).  This is in comparison to those with Graves disease aged ≥40 years (WMD -8.79 ng/dl; 95% CI, -15.87 to -1.72; 12 = 98.1), according to the findings of one subgroup.

Limitations of the study included a significant amount of heterogeneity among subjects, which may have affected the results and diminished the ability to generalize the outcomes.  There was also no information on the dietary intake of vitamin D among the people enrolled in the analyzed studies, which could have affected serum levels.

“Overall, vitamin D could contribute to the prevention or correction of hypothyroidism and improvement of thyroid function,” the authors stated, suggesting that future prospective cohort studies and clinical trials would  improve the understanding of the relationship between vitamin D levels and thyroid disorders.

Reference

Taheriniya S, Arab A, Hadi A, Fadel A, Askari G. Vitamin D and thyroid disorders: a systematic review and meta-analysis of observational studies. BMC Endocr Disord. 2021;21:71. doi:10.1186/s12902-021-00831-5