§ Glossary · biology

SHBG

Sex hormone-binding globulin — the liver-produced carrier of testosterone and oestradiol in blood.

SHBG is a glycoprotein synthesised in the liver that binds testosterone, dihydrotestosterone, and oestradiol with high affinity. ~98% of testosterone in serum is bound (mostly to SHBG, some to albumin); only the unbound 'free' fraction and the loosely-albumin-bound fraction are bioactive. Because hepatic SHBG production is suppressed by insulin, low SHBG is one of the earliest readouts of insulin resistance — often before fasting glucose or HbA1c shifts. It also distorts the interpretation of total testosterone: a 'normal' total T with low SHBG can mean borderline-low free T.

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