What is free testosterone (and why it matters)?

Free testosterone is the small fraction of your total testosterone that's actually available for your body to use. It often tells you more about how you feel than the total number on your blood test.

Here's the picture. Most of the testosterone in your blood is bound to proteins — mainly SHBG (sex hormone-binding globulin) and albumin. Testosterone bound tightly to SHBG is locked away and can't act on your tissues. Only the unbound portion — your free testosterone, usually just 1–2% of the total — plus the loosely albumin-bound part (together called bioavailable) can actually do the job: drive libido, energy, muscle, and mood.

This is why two men with the same total testosterone can feel completely different. If one has high SHBG, more of his testosterone is locked up, so his free level — and how he feels — can be much lower despite a normal total. Total testosterone alone can miss this entirely, which is how men end up being told their bloodwork is fine while still feeling flat.

What to do: when you get tested, ask for free testosterone (or bioavailable testosterone) and SHBG alongside total — not total on its own. If your total looks normal but your symptoms don't match, low free testosterone with high SHBG is one of the most commonly missed explanations. You can estimate free testosterone from total, SHBG, and albumin with a standard calculator, which is a useful starting point before seeing your doctor.