What is free testosterone (and why it matters)?

Free testosterone is the small fraction of your total testosterone that is actually available for your body to use, and it often tells you more about how you feel than the total figure.

Most of the testosterone in your blood is bound to proteins, mainly SHBG and albumin. Testosterone locked tightly to SHBG is held in reserve and cannot act on your tissues. Only the unbound portion, your free testosterone, which is usually just 1 to 2 per cent of the total, plus the loosely albumin-bound part (together called bioavailable testosterone), can do the real work of driving 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 away, so his free level, and how he feels, can be far lower despite a normal total. Total testosterone alone misses this entirely, which is exactly how men end up being told their bloodwork is fine while still feeling flat, and why an age-appropriate reading of the numbers matters.

How it is measured is worth a word, because not all "free testosterone" results are equal. The gold-standard lab method (equilibrium dialysis) is accurate but expensive, so most panels instead calculate free testosterone from your total, SHBG and albumin. That calculated figure is good enough for everyday use and far better than total alone, but it is an estimate, which is one more reason to read it alongside symptoms rather than chase a precise number.

What to do: when you get tested, ask for free testosterone (or bioavailable testosterone) and SHBG alongside total, never total on its own, which is the same advice in what a proper testosterone test includes. If your total looks normal but your symptoms do not match, low free testosterone with high SHBG is one of the most commonly missed explanations. You can run the standard calculator yourself from total, SHBG and albumin as a useful starting point before seeing your doctor.