Sex hormone binding globulin is a liver-produced protein that binds testosterone, DHT, and estradiol with high affinity, rendering them temporarily inactive.
Mechanism — Free testosterone = total T minus protein-bound. High SHBG sequesters T, lowering free T even when total looks adequate. Insulin lowers SHBG; thyroid hormone, estrogens, and aging raise it.
Lab range — Men: 20-60 nmol/L. Women: 30-90 nmol/L. Sweet spot for symptom-free function tends to be middle of range.
Caveats — High SHBG plus 'normal' total T can mean clinically low free T — and symptomatic men get told their labs are fine. Always calculate free T or measure it directly. Boron, weight loss, and metformin can modestly lower SHBG.