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What formula does the endocrinologist use for free testosterone?

Feb 2, 2012

My endocrinologist uses a formula to determine the level of free testosterone. The lab report shows Total, Free, Bio available values with their corresponding normal ranges. Sometimes the free testosterone on the lab report is shown as below the range. But my endocrinologist uses some calculator and tells me that is within normal range.

What formula is she using? Your book does not address this.

Response from Mr. Vergel

I am not sure what your doctor is using.

In the monitoring of the patient on testosterone replacement therapy-TRT, most physicans utilize the total testosterone. The initial evaluation of a patient might include the use of free testosterone or bioavailable testosterone. In a symptomatic individual, the total testosterone can be normal but the free or bioavailable testosterone abnormal.

Testosterone circulates in three forms. Testosterone circulates in a free or unbound state, tightly bound to sex hormone binding globulin-SHBG, or weakly bound to the blood protein albumin. Bioavailable, non-SHBG, testosterone includes free testosterone and testosterone that is bound to albumin but does not include SHBG -bound testosterone.

Examined changes over time have demonstrated a decrease in the total testosterone and an increase in SHBG levels. Because of this, the total testosterone might be normal, whereas the free or bioavailable testosterone is abnormal. If these alternative methods are used to diagnose hypogonadism, their utility during TRT is limited.

I would caution about the assay methodology used to calculate the free or bioavailable testosterone. The methods used to conduct the measurements vary in their accuracy, standardization, the extent of validation, and the reproducibility of results.

Bioavailable testosterone is measured or calculated in several ways. SHBG bound testosterone can be precipitated with ammonium sulfate and the remaining testosterone is then taken as the bioavailable.

Measures of free testosterone (FT) are controversial. The only standardized and validated method is equilibrium dialysis or by calculating free testosterone levels based on separate measurements of testosterone and SHBG. Other measures of free testosterone are less accurate.


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