Bioavailable Testosterone (male-only)

Bioavailable testosterone is the portion of testosterone your tissues can actually use, combining the free hormone plus the fraction loosely bound to albumin. It often reflects your true androgen status more accurately than total testosterone, especially when your SHBG level runs unusually high or low.

Ranges vary by lab, age, and assay method (values run higher in younger men and lower after 50), so compare to the range on your own lab report.

What bioavailable testosterone measures

Bioavailable testosterone adds together your free (unbound) testosterone and the portion loosely attached to albumin, since both can enter cells and activate androgen receptors. The remainder, bound tightly to sex hormone binding globulin (SHBG), stays locked away and biologically inactive. Because roughly 40 to 50 percent of circulating testosterone falls into the usable category, this measure reflects what your body can genuinely put to work.

What high or low levels mean

A low result points to reduced usable testosterone and can line up with fatigue, low libido, reduced muscle mass, or low mood. It is often clearer than total testosterone when your SHBG is abnormal, because a normal total can hide a genuinely low usable fraction when SHBG runs high. A high result suggests androgen excess, which may come from supplementation, certain tumors, or a low SHBG state, and usually prompts a closer look. Keep in mind that most labs do not measure this directly; they calculate it from your total testosterone, SHBG, and albumin using a validated formula such as the Vermeulen equation. Follow up with a clinician if a low value repeats on a morning sample, or if symptoms persist alongside borderline numbers.

Bioavailable testosterone FAQ

Is bioavailable testosterone better than total testosterone?

Neither replaces the other. Bioavailable testosterone can be more informative when your SHBG is high or low, since total testosterone may look normal while the usable fraction is not. Clinicians typically read both together with your symptoms.

What causes low bioavailable testosterone?

Common drivers include aging, which raises SHBG, along with obesity, thyroid or liver disease, and medications like opioids, estrogens, or anticonvulsants. Hypogonadism can also lower it, so repeat testing and a fuller workup help pin down the cause.

How is bioavailable testosterone measured?

Most labs calculate it from your total testosterone, SHBG, and albumin using the Vermeulen equation rather than measuring it directly. Direct assays exist but are less standardized and more costly, so calculated values are the everyday standard.

Read alongside your total testosterone, free testosterone, and sex hormone binding globulin results for the full picture.

Sources

• Mayo Clinic Laboratories. Testosterone, Total and Bioavailable, Serum.