Sex hormone-binding globulin (SHBG)
A liver protein that binds testosterone and estradiol and controls how much is biologically free. It is the reason a 'normal' total testosterone can still mean too little active hormone — SHBG is the modifier that makes the number make sense.
What it measures
SHBG is a protein made by the liver that binds sex hormones tightly in the bloodstream. Only the hormone NOT bound to SHBG (the free fraction) can enter cells and act. So SHBG sets the balance between total and free hormone: a high SHBG locks up testosterone and estradiol, leaving less active; a low SHBG frees more. It is measured together with total testosterone to calculate free or bioavailable testosterone — essential when the total result doesn't match the clinical picture. SHBG itself is driven by metabolism: insulin, obesity and hypothyroidism lower it, while estrogen, thyroid hormone, ageing and liver disease raise it.
What a high value can mean
- Hyperthyroidism — thyroid hormone strongly raises SHBG.
- Liver disease / cirrhosis — altered hepatic synthesis.
- Estrogen (oral contraceptives, pregnancy, HRT, oral estrogen) — raises SHBG markedly.
- Ageing (in men) — SHBG rises, lowering free testosterone even when total looks fine.
- Anorexia / very low body weight. A high SHBG can mask a low free testosterone behind a normal-looking total.
What a low value can mean
- Insulin resistance, type 2 diabetes, metabolic syndrome — the classic driver; low SHBG is an early metabolic flag.
- Obesity — strongly lowers SHBG.
- PCOS — low SHBG raises free androgens, worsening acne and hirsutism.
- Hypothyroidism.
- Androgen or steroid use, acromegaly.
When to discuss with a doctor
SHBG is rarely the target itself — it is the key to reading testosterone correctly. When a man has symptoms of low testosterone but a borderline-normal total, a high SHBG explains it (free testosterone is genuinely low). In women, a low SHBG in PCOS amplifies androgen effects. Because low SHBG tracks insulin resistance, an unexpectedly low value is worth pairing with fasting glucose and HbA1c. Mediora.AI uses SHBG with your total testosterone to show the free/bioavailable estimate, so a 'normal' total isn't read in isolation; the hormonal or metabolic decision belongs with endocrinology.