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Testosterone question
Nov 7, 2011

What is difference between serum and free testoterone? My serum came back low 221 (range 241-827ng/dl) Free test came within range 8.13 (range 5-21 nd/dl) %Free test was 3.68 (range 1.5-4.20) these are results i got on my own from lab. my doc just runs free testosterone check with my bloodwork which is within range. isn't serum more important when dealing with low testosterone symptoms? thanks

Response from Mr. Vergel

"Serum" is blood levels...but some people just use it automatically for serum total testosterone. Total testosterone is comprised of free testosterone, albumin bound testosterone and sex hormone binding globulin bound testosterone.

About 2 percent of the testosterone in the body is active. This "free testosterone" is not attached to binding proteins that would prevent it from interacting with its receptor.

About 40 percent of the body's testosterone is attached to albumin. This is a protein that can release the hormone as the need for it arises in the body. Free testosterone and testosterone attached to albumin are referred to as "bioavailable testosterone".

In a healthy young male, about 60 percent of his testosterone is attached to sex hormone binding globulin (SHBG). Hormones bound to SHBG can't be used by the body and lose their anabolic effect. As males grow older or if illness is present, SHBG sweeps up more and more testosterone, lowering free testosterone and its benefits. Total testosterone is the sum of bioavailable testosterone and testosterone bound to SHBG. Measuring just the total testosterone in the blood may not provide the whole picture and let you know how much "active" or usable testosterone you have.

The normal levels of combined bound and free testosterones in male bodies can range anywhere from 300 to 1,100 ng/dL (nanograms per deciliter). Levels will vary with age and individual factors. It is useful to also measure the level of free testosterone as this may be more indicative of how hormone therapy is progressing. Levels of free testosterone can range between 0.3 and 5 percent of the total testosterone count, with about 2 percent considered an optimal level.

The different guidelines based the diagnosis of hypogonadism on total testosterone blood levels AND symptoms (low sex drive, fatigue, lack of mental focus, low mood). Do you have those symptoms? Your total testosterone is low, although your free is below the midrange.

Nelson Vergel



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