Total, free, and bioavailable: the three testosterone numbers every man should know.

A man comes in with the symptoms everyone recognises. Flat mood, thinning motivation, worse recovery, less interest in things that used to interest him. He has had his testosterone checked. It came back normal. He has been told, gently, that this is what forty-five looks like.

Sometimes that is true. Often the test answered a narrower question than the one he was asking.

Three numbers, not one

Testosterone circulates in three states, and they behave differently.

Total testosterone is everything in the blood — the number almost every panel reports. Free testosterone is the small fraction, typically one to three percent, that is unbound and immediately available to tissue. Bioavailable testosterone is the free portion plus the fraction loosely bound to albumin, which detaches easily enough to be usable.

The rest is bound tightly to a carrier protein called sex hormone-binding globulin, and while it is bound it does nothing. It is in the blood, it is counted in the total, and it is not available to the cells that need it.

Total testosterone counts what is present. Free testosterone counts what is usable. Only one of those is what your body experiences.

The protein in the middle

SHBG is the reason those numbers can diverge, and it is not fixed.

It tends to rise with age. It rises with thyroid overactivity, with certain medications including some anticonvulsants and oral oestrogens, and with liver conditions. It tends to fall with insulin resistance and obesity, and with thyroid underactivity.

The consequence is straightforward and frequently missed. If SHBG climbs while production holds steady, total testosterone can sit unchanged year after year while free testosterone quietly falls. Every panel says normal. The man feels the decline accurately, and the standard measurement cannot see it.

The reverse also happens. Low SHBG — common alongside insulin resistance — can leave free testosterone acceptable while the total reads low. Treating the total in that situation would be treating a number rather than a person.

This is why we measure SHBG and albumin alongside total testosterone. With those, bioavailable and free testosterone can be calculated reliably, and the picture usually resolves.

How the sample is taken matters

Testosterone is not a stable quantity you can sample whenever convenient.

It follows a daily rhythm, peaking in the morning and declining through the day, with the pattern flattening somewhat with age. Guidelines generally recommend a morning draw, and a mid-afternoon sample can read substantially lower for reasons that have nothing to do with the man's underlying status.

It also varies day to day. Acute illness, poor sleep, heavy training in the preceding days, and significant stress all move it. A single low reading is a reason to repeat the test, not to start treatment. Where a diagnosis is being considered, two separate morning measurements are the reasonable standard.

What else belongs on the panel

Testosterone on its own does not explain itself. When it is low, the useful question is why.

LH and FSH — the pituitary signals instructing the testes — distinguish between a problem at the testicular level and one at the level of the brain's signalling. Prolactin matters, because elevation there suppresses the axis and occasionally points to something that needs investigating in its own right. Oestradiol matters, because testosterone converts to it and both extremes cause symptoms. Thyroid function and ferritin belong in the picture, because both produce fatigue that is easily attributed to testosterone. And because low testosterone travels with metabolic dysfunction more often than not, fasting insulin, HbA1c and a lipid panel including ApoB are worth having in front of us at the same time.

What symptoms can and cannot tell you

The symptoms attributed to low testosterone are real but strikingly non-specific. Fatigue, low mood, poor concentration, reduced libido and declining strength are equally consistent with poor sleep, depression, thyroid disease, anaemia, chronic stress, alcohol intake, and simply being under-recovered.

That cuts in both directions. It means symptoms alone should not lead to treatment — and it means a normal-looking total testosterone should not close the conversation either. What resolves it is the fuller panel, taken properly, read alongside the person describing the symptoms.

Testosterone therapy is a genuine intervention with genuine considerations — effects on fertility, on red blood cell production, on the prostate, and the practical reality that it is generally ongoing rather than a course. It deserves a proper discussion, and it deserves to follow a proper measurement.

Which is the argument here, and it is a modest one. Before anyone concludes that this is simply what your forties feel like, make sure the question was asked completely.

This article is educational and is not medical advice, diagnosis, or treatment. Laboratory results and therapies require interpretation in the context of your history, symptoms, medications, and examination, and what is appropriate differs from person to person. Nothing here should be used to start, stop, or change any treatment. Please speak with a qualified clinician who knows your case.