biotin
Summary
Biotin (Vitamin B7) is a water-soluble vitamin that acts as an essential cofactor for carboxylase enzymes involved in gluconeogenesis, fatty acid synthesis, and amino acid catabolism. Deficiency is rare but classically caused by excessive raw egg white consumption or long-term antibiotic use.
Detail
Biotin functions as a covalently bound cofactor (via biotinylation) for carboxylase enzymes: pyruvate carboxylase (gluconeogenesis), acetyl-CoA carboxylase (fatty acid synthesis), propionyl-CoA carboxylase (amino acid/odd-chain fatty acid metabolism), and methylcrotonyl-CoA carboxylase (leucine catabolism). It is synthesized by gut flora and obtained from diet (egg yolk, nuts, liver).
Deficiency causes: (1) Excessive consumption of raw egg whites, which contain avidin, a protein that tightly binds biotin and prevents its absorption; (2) Prolonged antibiotic use that disrupts gut flora producing biotin; (3) Biotinidase deficiency, an inherited disorder impairing biotin recycling.
Clinical features of deficiency: alopecia, dermatitis (scaly, red rash periorificially), and neurologic symptoms (e.g., paresthesias, depression, lethargy). This triad is a classic USMLE association.
Clinical/lab significance: High-dose biotin supplementation (common in over-the-counter supplements) can interfere with immunoassays (e.g., troponin, TSH), causing falsely abnormal lab results—an important emerging clinical pearl for Step 2.
Treatment of deficiency is oral biotin supplementation and avoidance of raw egg whites.
Sources
- First Aid for the USMLE Step 1
- Lippincott Biochemistry
- Harrison's Principles of Internal Medicine
- UpToDate: Biotin interference with laboratory tests
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