'Low testosterone' is really three numbers, not one. Knowing which one matters is what separates a real diagnosis from both a missed one and a needless prescription — and it's where a lot of confusion (and over-treatment) lives.
Total testosterone is everything in the blood. Most of it is bound and inactive: roughly ~44% clings tightly to SHBG (sex-hormone-binding globulin) and ~54% loosely to albumin. Only about 2% circulates free. The free fraction — plus the loosely-bound albumin portion (together 'bioavailable') — is what actually reaches tissue and does the work.
SHBG rises with age, hyperthyroidism, and low insulin (fasting, leanness); it falls with obesity, insulin resistance, and hypothyroidism. Because it changes how much of your total is bound, it distorts the total-T reading in both directions.
High SHBG (often in older or very lean men) can make total T look normal while free T is genuinely low. Low SHBG (typical in obesity) drags total T down even when the free fraction is less affected. So when total T is borderline, or you have an SHBG-altering condition, the honest next step is to measure or calculate free T (equilibrium dialysis, or a validated calculation from total T, SHBG and albumin) rather than treat the total in isolation. Evidence ●●●●○
Why 'just check total T' isn't enough
In an obese man, a low total T is often mostly an SHBG artefact that improves with weight loss; in a lean older man, a normal-looking total can hide low free T. The number that tracks symptoms best is free / bioavailable testosterone — which is why borderline cases need it before any label is applied.
The Testosterone Guide