Vitamin D Deficiency as a Risk Factor for Hypothyroidism: A Revised Evidence-Based Review

Abstract

Hypothyroidism is a common endocrine disease characterized by insufficient synthesis of thyroid hormones, which has become a serious health problem worldwide. At the same time, deficiency of vitamin D, an endemic nutritional deficiency is increasingly being associated with a variety of AIDs and endocrinopathies. The aim of this review is to summarize the current literature in relation to vitamin D deficiency and hypothyroidism, with specific emphasis on immunologic mechanisms and clinical significance. A review of the newer literature (2020-25) was performed, including observational studies, meta-analyses and interventional trials. The findings indicate a notable correlation between decreased serum vitamin D levels and an elevated risk of hypothyroidism., especially autoimmune hypothyroidism like Hashimoto's thyroiditis. Mechanistically, vitamin D exerts immunomodulatory impacts by regulating the balance between T-helper cells, enhancing regulatory T cell activity while inhibiting pro-inflammatory cytokine production, thereby influencing thyroid autoimmunity. Large-scale population studies, comprising the National Health and Nutrition Examination Survey (NHANES), have documented adjusted odds ratios of 1.6 to 1.7 for hypothyroidism in vitamin D deficiency individuals. Meta-analyses support these findings, indicating significantly reduce serum vitamin D levels in hypothyroidism and Hashimoto's thyroiditis patients relative to healthy controls. Vitamin D supplementation has demonstrated potential in lowering thyroid autoantibody levels and enhancing thyroid function in certain studies, the clinical benefits remain heterogeneous and appear to be most effective in euthyroid individuals with baseline vitamin D deficiency. This review concludes that vitamin D deficiency is a significant risk factor for hypothyroidism and highlights the potential of vitamin D supplementation as an adjunctive therapy, especially in population with a high prevalence of both conditions, for instance, in the Middle East and North Africa. To create conclusive clinical guidelines, more extensive randomized controlled trials are required.