Vitamin D in Endocrine Disorders: A Broad Overview of Evidence in Musculoskeletal, Thyroid, Parathyroid, and Reproductive Disorders

Vitamin D is well established for its skeletal effects, being a cornerstone of several endocrine disorders. In recent years, it has come under investigation as a potential disease-modifying drug in several endocrine disorders through its immune modulatory and anti-tumorigenic action, particularly in thyroid disease, gynecologic disorders, and general fertility. Vitamin D supplementation is well established in the treatment of osteoporosis, osteomalacia, hypoparathyroidism, and primary hyperparathyroidism. In autoimmune thyroid disease, there is a negative correlation between 25(OH)D3 levels and prevalence. Currently available data are inconclusive on supplementation as a disease-modifying treatment. In Hashimoto's thyroiditis, while some found improved thyroid function, a decline in progression, and antibody titers, these findings were not consistent, and some found no improvements. Painless postpartum thyroiditis severely lacks evidence. Interventional studies failed to demonstrate benefits in Graves' disease. The literature consistently reports lower vitamin D levels in infertility, polycystic ovarian syndrome (PCOS), and endometriosis. In PCOS, data suggest that vitamin D supplementation is beneficial; however, results in exact benefits vary and there is no consensus on dosing. Current guidelines support supplementation as part of preconception nutritional care. In general, for female infertility and endometriosis, the results are conflicting, with a lack of high-quality evidence. The literature suggests there is a possible benefit regarding sperm motility, but not in testosterone levels for males. In conclusion, while in vitro studies and animal models are promising, the available evidence is often contradictory, with high heterogeneity in study designs and populations. Our paper highlights the need for further high-quality research to resolve current controversies.