Biotin (Vitamin B7) is a water-soluble B vitamin that acts as a cofactor for carboxylase enzymes involved in fatty acid synthesis, amino acid metabolism, and gluconeogenesis. True deficiency is rare because gut bacteria produce some biotin and many foods contain it.
True biotin deficiency is uncommon and presents with hair thinning, brittle nails, or skin issues, though most supplement marketing dramatically overstates biotin's role in these symptoms. More clinically relevant: high-dose biotin supplementation (>5 mg/day, commonly sold for hair and nails) interferes with many immunoassay lab tests, causing falsely elevated or decreased results for thyroid hormones, troponin, and other markers. If biotin is in your stack, notify your provider before lab testing. Direct biotin testing is rarely necessary.
Optimal Biotin: 0.2-1 ng/mL.
Typically re-tested every 26 weeks.