Testosterone (Total and Free)

What it is

Testosterone is the principal androgen. Most circulating testosterone is bound to sex-hormone-binding globulin (SHBG); only the free fraction (~2–3%) is biologically active. Standard panels report total testosterone; free testosterone or calculated free can be more informative in conditions affecting SHBG.

Reference ranges (adult men)

• Total testosterone: typically 300–1000 ng/dL.
• Free testosterone: roughly 6–25 ng/dL (assay-dependent).

For diagnosis of hypogonadism, guidelines typically require:

• Two morning fasting samples below the lower reference limit, plus
• Compatible symptoms (low libido, erectile dysfunction, fatigue, depressed mood, decreased muscle mass).

Why it matters for longevity

• Symptomatic hypogonadism reduces quality of life and may contribute to sarcopenia and osteoporosis.
• Population-level testosterone has fallen over decades for reasons that are not fully understood (obesity, sedentary behaviour, environmental factors).
• Treating biochemically low + symptomatic men has clearer benefit than treating low-normal levels in asymptomatic men.

What modifies testosterone

• Down: obesity, poor sleep, alcohol excess, opioids, chronic inflammation, anabolic steroid use (suppression).
• Up: weight loss in obesity, adequate sleep, resistance training, treatment of hypogonadism.

Don’t over-test

Single isolated total-testosterone results can be misleading (diurnal variation, illness, SHBG effects). Confirm with repeat morning fasting sample plus symptoms before acting.

Related entries

TRT, Sarcopenia, Sleep optimization.