Can a vitamin D deficiency affect your hormones?

Vitamin D is itself a steroid hormone in its active form and influences hundreds of genes, including those involved in immune defence. The central hormonal effect of a deficiency is the loss of suppression on parathyroid hormone (PTH). Normally, vitamin D keeps PTH production in check; without that brake, PTH rises, which disrupts calcium and phosphate balance and contributes to high blood pressure and kidney damage. This relationship is well established and is considered causal.

That elevated PTH also has knock-on effects on your bones. Less vitamin D means less calcium absorption from the intestines, more PTH, and ultimately accelerated bone breakdown. This mechanism is a recognised cause of osteoporosis. In addition, vitamin D deficiency contributes to loss of muscle strength, which plays a role in age-related muscle loss. Supplementation is therefore recommended alongside strength training in this context, although the effect on muscle strength is smaller than that of testosterone.

The effect on sex hormones such as testosterone or oestrogen is less clear. In a small study of forty women with polycystic ovary syndrome (PCOS), daily supplementation of 3200 IU for three months did not lead to measurable changes in testosterone or the protein that transports sex hormones (SHBG). Insulin resistance improved slightly, but hormonal effects were absent. One small three-month study does not tell the whole story, but a dramatic effect on sex hormones does not appear to be expected at this point.

Furthermore, vitamin D deficiency has been linked in multiple studies to a higher risk of cardiovascular disease, through that same rise in PTH. Whether that association is truly causal has not yet been established. Daily supplementation modestly but measurably reduces upper respiratory tract infections, which is consistent with vitamin D's role as a hormone that regulates immune genes.