The carrier protein that regulates how much testosterone and estradiol are actually bioavailable.
SHBG (sex hormone binding globulin) is a hepatic glycoprotein that binds testosterone, DHT, and estradiol in circulation. Bound hormone is biologically inactive; only free hormone signals at receptors. Because testosterone's binding affinity for SHBG is higher than for albumin, SHBG is the primary regulator of free testosterone availability.
Too-low SHBG (<20 nmol/L in men) often signals insulin resistance, chronic inflammation, or obesity — and can produce misleadingly "normal" total T with very high free T. Too-high SHBG (>60 nmol/L) suppresses bioavailable hormone and is associated with longevity in some studies but can impair symptoms of androgen sufficiency.
SHBG is influenced by: insulin sensitivity (low insulin raises SHBG, high insulin lowers it), thyroid status (hyperthyroidism raises it), alcohol intake, liver function, age (slowly rises with age), body fat (higher = lower SHBG), and pharmacologic agents — oral estrogens raise it, testosterone suppresses it.
On TRT, SHBG typically drops 30–50% from baseline as exogenous T suppresses hepatic synthesis. This is expected and not intrinsically problematic — but it shifts the free/total T ratio higher, which can affect dosing decisions. Track SHBG alongside total + free T to understand the full picture.
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Start tracking →Informational only — not medical advice. Reference ranges vary by lab and individual context. Work with a licensed provider to interpret your specific results.