Biotin (Vitamin B7)
Biotin is a cofactor for four carboxylase enzymes central to fatty acid, amino acid, and carbohydrate metabolism. It's marketed heavily as a "hair, skin, and nails" supplement — a claim that is largely unsupported outside of true deficiency, and that comes with a real lab-test interference risk at supplement doses.
Daily Adequate Intake (AI)
- Adults: 30 µg/day.
- Pregnancy: 30 µg/day.
- Lactation: 35 µg/day.
- No Tolerable Upper Intake Level — biotin has no known toxicity, but see lab-interference warning below.
Note that a typical "hair, skin, and nails" supplement contains 5,000–10,000 µg — that's 167–333× the AI.
The hair and nails evidence
Biotin deficiency causes hair loss and brittle nails — but true deficiency is rare (raw egg whites contain avidin, which binds biotin; long-term valproate or isotretinoin use can lower biotin). In people who are NOT deficient, RCT evidence for hair or nail growth is essentially absent. The perceived effect in advertising is placebo plus correction of undiagnosed marginal deficiency in a subset of consumers.
The FDA lab interference warning
In 2017 and again in 2019, the FDA warned that biotin at supplement doses (> 1 mg) interferes with many biotin-streptavidin-based lab assays, producing:
- Falsely low troponin — potentially causing missed diagnosis of heart attack.
- Falsely low TSH and falsely high T4/T3 — mimicking hyperthyroidism.
- Falsely elevated hCG — false-positive pregnancy or unreliable oncology monitoring.
- Falsely low PTH, ferritin, various tumour markers.
Stop biotin supplements at least 72 hours before blood work, and tell the phlebotomist and ordering clinician about the supplement.
Food sources
- Beef liver (3 oz): 30.8 µg — the AI in a single serving
- Egg, cooked (1 large): 10 µg
- Salmon (3 oz): 5 µg
- Sunflower seeds (1 oz): 2.6 µg
- Sweet potato (½ cup): 2.4 µg
- Almonds (1 oz): 1.5 µg
- Milk (1 cup): 0.3 µg
A varied omnivorous diet supplies the AI easily. Biotin is also synthesised by gut bacteria, contributing to total availability.