A Testosterone Bioavailable Free blood test measures the levels of testosterone in the blood that are available for the body to use, focusing specifically on the portions not tightly bound by certain proteins. This type of test typically reports two components: free testosterone (the small fraction not bound to any proteins) and bioavailable testosterone (the sum of free testosterone and testosterone loosely bound to albumin, but not the fraction firmly attached to sex hormone-binding globulin—SHBG).

What the Test Measures

• Free Testosterone: This is the fraction (about 2–4% of total testosterone) circulating unbound in the bloodstream, making it readily accessible to cells for biological activity.

Bioavailable Testosterone: This combines free testosterone plus testosterone that is weakly bound to albumin. Both forms are considered biologically active and available for the body to use, as the albumin-bound testosterone can easily dissociate and enter cells.

Test Usage and Methodology

1. The test is useful for evaluating issues like hypogonadism (low testosterone), infertility, delayed puberty, or symptoms like decreased libido and fatigue, and for monitoring testosterone therapy. Measurement can be done by direct laboratory methods (such as equilibrium dialysis for free testosterone and ammonium sulfate precipitation for bioavailable testosterone) or by mathematical calculation using measured levels of total testosterone, albumin, and

• SHBG.

Clinical Significance

• Abnormal (low or high) levels of bioavailable or free testosterone can cause symptoms and health concerns in both men and women, including changes in sexual function, mood, muscle mass, bone density, energy levels, and fertility.

1. Testing helps distinguish between true testosterone deficiency and cases where the total testosterone might seem abnormal due to altered levels of binding proteins like SHB
2. Abnormal (low or high) levels of bioavailable or free testosterone can cause symptoms and health concerns in both men and women, including changes in sexual function, mood, muscle mass, bone density, energy levels, and fertility.
3. Testing helps distinguish between true testosterone deficiency and cases where the total testosterone might seem abnormal due to altered levels of binding proteins like SHB