Selenium is an essential trace element involved in thyroid hormone metabolism, antioxidant defence (via selenoproteins including glutathione peroxidase), and immune function. The hair connection comes through both these mechanisms. Selenium deficiency causes telogen effluvium that resolves with repletion. Selenium toxicity also causes hair loss, often dramatic and concerning. The therapeutic window between deficient and toxic intake is narrower than for most nutrients.

Normal dietary selenium intake (55–200 mcg/day depending on regional soil content) is generally sufficient in most populations. True deficiency is uncommon in areas with selenium-adequate soil but does occur in specific contexts: severe malabsorption, chronic kidney disease, and regions with selenium-deficient soil (some parts of China historically). Toxicity (selenosis) typically requires sustained intake above 800 mcg/day and produces hair loss along with brittle nails, garlic breath odour, and neurological symptoms.

Practical implications: selenium supplementation for hair loss without documented deficiency is not supported by evidence and risks toxicity over time. Daily multivitamins typically contain 55–100 mcg selenium, adequate and safe. Standalone selenium supplements at 200 mcg+ used long-term carry meaningful risk. Brazil nuts contain unusually high selenium (50–100 mcg per nut); regular consumption of 4+ Brazil nuts daily over months can produce selenium toxicity, including hair loss. Testing serum or plasma selenium is reasonable for patients with unexplained hair loss before considering supplementation.