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What is Testosterone %Free?
Jul 25, 2000

I recently had a total testosterone level and a free testosterone level done to check reason for fatigue,lack of libido and inability to maintain erection. My total testosterone level was done at the local lab reported at high 1330 normal(260-1000ng/dL.) The free testosterone was sent to a Lab in California that reported mine as 127.7 normal(50-210pg/mL. I was concerned about the other test they report which was low the %Free. Mine was 0.96 normal(1.0-2.7%.) Should I be on some type of replacement or are my values very good ? Please explain this %Free test.

Response from Dr. Hellerstein

This is a technical question, but important. Testosterone circulates in the blood in free and bound forms. Most of this hormone, like other hormones, is in the bound form. But the active form (the hormone that can actually be used by and get into tissues of the body)is thought to be the free version, although there still is some uncertainty whether our laboratory measures of free hormone accurately reflect what is available to tissues in the body. In any case, the free version is likely to be the most accurate reflection of hormonal status in your body.

But what determines how much free hormone you have? The answer is two things: how much total hormone is present, and how much of the binding factor (a serum protein) is present. What, in turn, determines how much binding protein is present? This is complicated, and differs between people, as well as being influenced by hormones themselves. The net effect is that the amount of binding protein influences how much of your total testosterone is available to tissues.

And the practical point is that measurements can be made of all these components of the system. The total testosterone in you was on the high side. The percent free, which reflects the binding protein (high binding, low percent free), was low in your case. The free testosterone (total testosterone times percent free)was quite normal. The most reasonable interpretation of all this is that you have a normal hormonal status -- that is, normal amounts of free hormone are available to your tissues. Any symptoms that you have are therefore unlikely to be explained by low testosterone status.

The % free has no independent significance. It is only used to calculate the free testosterone level. So this value does not deserve too much attention; it does not have any diagnostic meaning by itself.

I hope this is helpful.

Marc Hellerstein, M.D., Ph.D.

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