SHBG (Sex Hormone-Binding Globulin): What High and Low Mean

SHBG is a glycoprotein synthesized by the liver. It binds testosterone with high affinity (~60% of circulating testosterone) and estradiol with lower affinity (~30%). The remaining hormone is bound loosely to albumin or unbound (free).

SHBG concentration is regulated almost entirely by the liver, which receives signals from:

• Insulin (downregulates SHBG): high insulin or insulin resistance lowers SHBG.
• Thyroid hormone (upregulates): hyperthyroidism raises SHBG, hypothyroidism lowers it.
• Estrogen, oral, not transdermal (upregulates): first-pass hepatic exposure drives SHBG up.
• Androgens (downregulate): exogenous testosterone or anabolic steroids suppress SHBG.
• Hepatic disease (variable): chronic hepatitis and cirrhosis often raise SHBG; severe failure can lower it.

Because SHBG sits at the intersection of metabolism, thyroid, and reproductive endocrinology, the level is informative far beyond hormone interpretation.

Reference ranges vary by assay and demographic. Typical values:

• Adult men: 10–57 nmol/L.
• Adult women (premenopausal): 18–144 nmol/L.
• Postmenopausal women: 17–125 nmol/L.
• Children: 30–170 nmol/L (declines through puberty).

SHBG rises slowly with age in men. In women, oral contraceptives and hormone replacement therapy substantially elevate SHBG (often to 200+ nmol/L). Pregnancy raises SHBG dramatically, up to 5–10× baseline.

Because SHBG is so context-dependent, a "normal" number means different things in different patients. The clinical question is rarely "is SHBG normal?", it is "does SHBG match the rest of the picture?"

Most causes of high SHBG are explainable from medications or thyroid status. The differential:

• Hyperthyroidism: SHBG is one of the most sensitive markers of thyroid hormone action. A high SHBG with low TSH is highly specific for hyperthyroidism.
• Oral estrogen: combined oral contraceptives, oral hormone replacement therapy, transgender hormone therapy with oral estrogen. Transdermal estrogen affects SHBG much less.
• Pregnancy: placental estrogen drives SHBG up enormously.
• Aging: gradual rise, especially in men over 60.
• Chronic liver disease: hepatitis, early cirrhosis often raise SHBG.
• Anorexia nervosa, severe caloric restriction: counterintuitively raises SHBG (low insulin signaling).
• Anticonvulsants: phenytoin, carbamazepine, phenobarbital induce hepatic SHBG synthesis.
• HIV infection (untreated): can raise SHBG.

The clinical importance: in a man with high SHBG, total testosterone tends to look misleadingly normal or low because more is bound up. Always pair with calculated free testosterone.

This is where SHBG becomes most clinically valuable. Low SHBG is a strong, well-validated marker of insulin resistance, independent of body weight. The differential:

• Insulin resistance, prediabetes, type 2 diabetes: by far the most common cause. Several large prospective cohorts (Framingham, multiple Nurses' Health Studies) show low SHBG predicts incident diabetes years before glycemic markers shift.
• Obesity, especially central obesity: drives insulin resistance.
• Polycystic ovary syndrome: low SHBG amplifies free testosterone, worsening hyperandrogenism. A core feature of PCOS metabolic phenotype.
• Hypothyroidism: opposite of hyperthyroidism; usually mild SHBG reduction.
• Cushing's syndrome: chronic glucocorticoid exposure.
• Exogenous androgens: testosterone replacement, anabolic steroids suppress SHBG.
• Acromegaly: GH/IGF-1 axis suppresses SHBG.
• Nephrotic syndrome: protein loss.

The clinical meaning: a low SHBG in someone with normal weight and no obvious cause should prompt a metabolic workup, fasting glucose, HbA1c, fasting insulin, lipid panel. Even with normal HbA1c, low SHBG raises diabetes risk meaningfully.

SHBG has been quietly accumulating evidence as a metabolic biomarker for two decades, but rarely shows up on standard panels. Key observations:

• Low SHBG predicts type 2 diabetes independently of BMI, insulin, and glucose. The signal is strongest in women.
• Low SHBG predicts non-alcoholic fatty liver disease before liver enzymes elevate.
• Low SHBG correlates with metabolic syndrome severity.
• Mendelian randomization analyses suggest the relationship is partly causal, SHBG itself appears to influence insulin signaling, not just reflect it.

SHBG also tracks well across time within an individual. Big shifts deserve attention. A man whose SHBG dropped from 40 to 20 nmol/L over five years is moving in a metabolically concerning direction even if his total testosterone is unchanged.

• Low SHBG in a non-obese adult: order HbA1c, fasting glucose, fasting insulin, lipid panel, ALT.
• Low SHBG with hyperandrogenism in a woman: strongly supports PCOS; check LH/FSH, DHEA-S, free testosterone.
• High SHBG with low TSH: confirm hyperthyroidism with free T4 and free T3.
• Discordant testosterone and symptoms: total testosterone in range but symptoms persistent → check SHBG and free testosterone.
• Persistent unexplained high SHBG: consider chronic liver disease (ALT, AST, GGT), eating disorder, medication effect.

• Total testosterone and free testosterone: primary use case for SHBG is correctly interpreting these.
• Estradiol: affected by SHBG, especially in women on hormone therapy.
• TSH, free T4, free T3: thyroid status drives SHBG up or down.
• HbA1c, fasting insulin, fasting glucose: investigate the metabolic story behind low SHBG.
• ALT / AST / GGT: investigate the hepatic story behind high SHBG.