What Works Skin — Independent · Evidence-First · Ad-FreeIssue 014 · 20 April 2026 · Next: 04 Maywhatworksskin.com
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Supplement · Hair & nails · Biotin (B7)

P. 15 · BRIEF

Biotin.

Real for deficiency. Marketing for everyone else.

The supplement category most aggressively over-sold in the last decade. Biotin deficiency is genuinely rare in adults eating an unrestricted diet — and biotin supplementation in non-deficient individuals is one of the most-studied null effects in trichology. It also interferes with thyroid blood tests.

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— § 02

What the literature shows.

Hair shedding (with deficiency)
Case series

Real, occasionally dramatic effect when biotin is genuinely low. Document deficiency before supplementing.

75%
Hair shedding (without deficiency)
Multiple null RCTs

The 2017 Patel review remains definitive: no convincing data for benefit in biotin-replete adults.

10%
Brittle nails
Small case series

Mixed but more positive signal than hair. Reasonable trial in nail-specific complaints.

55%
Thyroid lab interference
Established

High-dose biotin (>5 mg/d) interferes with TSH and T4 immunoassays. Stop 72 hours before thyroid tests.

Caution
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— § 03

Forms and bioavailability.

Biotin 30 mcg/d (RDA)

Absorption · Good

Daily-multivitamin level. Sufficient to prevent deficiency in healthy adults.

Biotin 1 mg/d

Absorption · Good

Mid-range supplement dose. Reasonable trial when deficiency genuinely suspected.

'Hair, skin, nails' formula 5–10 mg

Absorption · Good

Marketing-driven dose. Lab-test interference is the main concern at this strength.

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Bottom line

The marketing-to-evidence ratio in this category is among the worst we cover. Test before supplementing; stop before thyroid testing; do not expect transformative results in healthy adults.

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— § 04

Frequently asked.

How do I know if I'm deficient?

Most healthy adults are not. Deficiency is associated with prolonged raw-egg-white consumption, specific genetic conditions (biotinidase deficiency), severe malnutrition, long-term anti-epileptic use, or after prolonged broth diets. Ask for a serum biotin or 3-hydroxyisovalerate test if uncertain.

Why does it interfere with thyroid tests?

Many immunoassay platforms use streptavidin-biotin chemistry to detect target molecules. High serum biotin competes with the assay's biotin reagents, producing false high T4/T3 and false low TSH — a pattern that mimics hyperthyroidism. Stop biotin 72 hours before any thyroid testing.

How long to trial it?

12 weeks. If hair shedding has not improved measurably (count shed hairs over a defined surface for 60 seconds; standardise the comparison), the supplement is not your bottleneck. Look at iron, ferritin, thyroid status, and androgen-driven shedding instead.

Will it help nails specifically?

Possibly. The case-series literature on biotin and brittle nails is more positive than for hair. Reasonable 12-week trial; not a guaranteed effect.