Vitamin D Deficiency and Hair Loss: The Causal Question Is Still Open

Vitamin D's role in hair biology is mechanistically clear. Vitamin D receptors are expressed on keratinocytes throughout the hair follicle, and animal models with vitamin D receptor knockout consistently develop alopecia. Clinical observation links vitamin D deficiency to telogen effluvium, alopecia areata, and accelerated androgenetic alopecia. What remains less clear is whether supplementation produces hair improvement in deficient patients, or whether the deficiency is a marker of other underlying issues.

A 2024 meta-analysis combining 8 randomised trials of vitamin D supplementation for hair loss found a small but statistically significant benefit, approximately 4.7 additional hairs per cm² over six months in deficient patients raised to sufficient levels (above 30 ng/ml serum 25-OH vitamin D). The effect was largest in patients with baseline levels below 20 ng/ml. In patients who started with sufficient levels, supplementation produced no measurable hair benefit. This is consistent with the broader vitamin D supplementation literature: helpful when deficient, neutral when sufficient.

Practical implications: vitamin D testing is reasonable for patients with unexplained hair shedding, particularly women, those with limited sun exposure, or with darker skin pigmentation. Supplementation to achieve serum 25-OH vitamin D above 30 ng/ml is well-established as safe at standard doses (1,000 to 2,000 IU daily for most adults). Megadosing into 10,000+ IU territory without medical supervision is not supported by the hair literature and carries risk of hypercalcemia.