Free testosterone

Of all the testosterone in your blood, most is attached to protein: tightly to SHBG and loosely to albumin. Only the part circulating free, a few percent, can enter the cell and do its work in muscle, bone, brain and skin. Free testosterone is that part. It is not measured separately; a direct measurement exists but is laborious and rarely needed. Instead it is calculated with a formula from three values you already have: total testosterone, SHBG and albumin, based on how strongly testosterone binds to each of those two proteins. The outcome is an estimate, and a good one, as long as the input is right. Your report shows free testosterone in pmol/L, so in a unit a thousand times smaller than the total. Normal is 210 to 619 pmol/L in a man of 20 to 50 years and 3 to 26 pmol/L in a woman of the same age, where the labs overlap; the range falls with age, and like the total it applies to a morning value.

Total testosterone is usually enough. Free testosterone becomes interesting as soon as SHBG is abnormal, and that is more often than you think. In a man with belly fat SHBG is low, so a low total testosterone still yields a reasonable free part; in an older man or a man with an overactive thyroid SHBG is high, so a normal total still leaves too little free. In both cases free testosterone fits how someone feels better than the total does. In women free testosterone is the better androgen number. In PCOS SHBG is low from insulin resistance, and then free testosterone is raised while the total still looks normal; excess hair growth, acne and an irregular cycle go with that free part. On the pill the opposite happens: SHBG rises strongly and free testosterone falls, which often does the skin good and desire sometimes not. For anyone who wants to stay healthy the gain sits in the combination. Losing weight around the middle raises both the total and SHBG in a man, and so distributes the free part more healthily; in a woman with PCOS the same change lowers free testosterone through a rising SHBG. The free number is therefore the value in which you see the lifestyle change back most honestly.

A low free testosterone has two routes: a low total, or a high SHBG. The first route is the causes of a low testosterone itself: drawn too late, belly fat, lack of sleep, stress, alcohol, overtraining, age, and medicines such as opioids and prednisone. The second route is an SHBG rising from age, the pill, an overactive thyroid or eating too little. With the total and SHBG beside it, you see which of the two is steering the free part, and that decides the lever. A high free testosterone in men is usually testosterone from outside. In women it is usually PCOS: a normal or slightly raised total with a low SHBG, and insulin resistance underneath. A strongly raised value, a multiple of the range, is rare and calls for investigation of ovaries and adrenal glands. The levers are those of total testosterone and SHBG together: losing weight around the middle, strength training, seven to eight hours of sleep, less alcohol, less sugar and fast carbohydrates, and enough food for what you train. In men that lifts the total and normalises SHBG; in women with PCOS the same change lets SHBG rise and free testosterone fall. Count on three to six months. Never read free testosterone apart from the numbers it follows from. An abnormal albumin, in serious illness or malnutrition, carries through into the outcome. And when comparing over time, keep to one and the same lab, because the formula and the range differ per lab.