Free Testosterone

One caution before you trust any of these cutoffs: the number depends on how it was produced. Free testosterone is reported in ng/dL by most US labs and in pmol/L elsewhere, the two scales don't convert by eye, and the method behind the figure matters even more than the units. A free testosterone from one lab cannot be lined up against one from another the way two totals can.

The drama of this marker lives in a single mismatch: the total reads fine, the symptoms are real, and the explanation is sitting in the withholdings. Picture two men with the same total testosterone of 400 ng/dL. One has an average SHBG and clears a comfortable take-home. The other has a high SHBG that ties up an outsized share, leaving a free fraction near the floor. Same gross, very different amount to spend, and only the second man is likely to feel it.

SHBG is what moves the withholding line, and it isn't fixed. MedlinePlus notes that SHBG controls how much testosterone is active in the body, and its level climbs with age, an overactive thyroid, and estrogen, while it drops with excess weight, an underactive thyroid, and insulin resistance. When SHBG rises, more of the gross moves into the locked column and the take-home falls even though the total hasn't moved. That is the most common reason a free testosterone gets added: the total looked adequate, the story didn't fit, and the available share turned out to be where the shortfall was hiding.

The symptoms people bring to this question, low libido, fatigue, low mood, lost muscle, poor concentration, are common and have many causes, so a low free fraction explains rather than proves. It's a strong clue read with the total and SHBG, not a diagnosis read alone. And not every "free" test works this way: with free PSA the free fraction isn't the active ingredient at all, and a higher free percentage is the reassuring result rather than the worrying one.

Here the marker carries a problem most lab values don't. The free testosterone test that's cheapest and most widely run, the direct or analog immunoassay, is the one specialists trust least. It tends to misread exactly when the answer matters most, when SHBG is unusually high or low, which is the very situation that prompts the test. A direct free testosterone drawn to settle a high-SHBG case can be the least reliable number on the page.

The methods that hold up are different. The Endocrine Society's hypogonadism guideline favors measuring free testosterone by equilibrium dialysis, the laboratory reference approach, or estimating it by calculation from the total testosterone, SHBG, and albumin together. That calculated free testosterone, often produced with the Vermeulen equation, is why a thorough free-T workup tends to order three things at once rather than a single "free T" box.

Free testosterone isn't the only way to name the usable hormone. Bioavailable testosterone is the free fraction plus the loosely albumin-bound share, on the reasoning that the weak albumin bond releases the hormone where tissues call for it. Some labs report bioavailable testosterone in place of, or beside, free testosterone; both are reaching for the same thing, the part that clears the withholdings, just drawing the line in a slightly different place. The tightly SHBG-bound portion sits outside both, counted by the total and reachable by neither.

Free testosterone is one line on a hormone panel, and it's close to meaningless alone. It needs the total it was derived from, the SHBG that set the withholding rate, and often luteinizing hormone to say whether a true shortfall starts in the testes or in the signal above them. In men, some testosterone is converted to estradiol, and sustained stress reflected in cortisol can suppress the whole axis, so the surrounding lines shape how a free result reads. In a woman with acne and irregular cycles, a raised DHEA-S points to the adrenal glands as a source of the androgens behind a high free testosterone. The hormone panel guide walks the report in order.

Like the total it comes from, free testosterone wobbles with the time of day, sleep, and recent illness, and the method adds its own scatter on top. A single value carries less weight than its direction across several consistent morning draws measured the same way. A free testosterone that has slid year over year tells a story no one in-range result can, which is why the trend usually says more than any single figure.