Bioavailable testosterone is the fraction of testosterone not bound tightly to sex hormone binding globulin (SHBG). It represents testosterone that can actually reach and activate androgen receptors in tissue — making it often more clinically meaningful than total testosterone.

Why it matters

In blood, testosterone exists in three forms:

• Tightly bound to SHBG (roughly 60%) — not biologically active

• Loosely bound to albumin (roughly 38%) — bioavailable

• Free or unbound (roughly 2%) — bioavailable

Bioavailable testosterone equals albumin-bound plus free. If SHBG is high or low, total testosterone can mislead you about actual androgen activity. A man with "normal" total testosterone but very high SHBG may still be functionally hypogonadal.

When to measure bioavailable vs. total

Consider bioavailable or free testosterone when:

• Symptoms of low testosterone (fatigue, low libido, muscle loss) despite normal total

• Aging (SHBG rises with age)

• Obesity (SHBG tends to be low)

• Thyroid disease (affects SHBG)

• Liver disease

• Medications affecting SHBG (anticonvulsants, estrogen, opioids)

Reference ranges

Typical adult male bioavailable testosterone: 110–575 ng/dL (varies by lab).

By age:

• 20–29: 280–570 ng/dL

• 40–49: 250–520 ng/dL

• 60–69: 210–470 ng/dL

• 70+: 160–400 ng/dL

Always interpret alongside total testosterone, SHBG, LH, and symptoms.

Calculation vs. direct assay

Most labs calculate bioavailable testosterone from total testosterone, SHBG, and albumin using established formulas (the Vermeulen equation is most common). Direct assays exist but are less standardized and more expensive.

What to do with results

Low bioavailable testosterone with symptoms warrants a full hypogonadism workup. Repeat confirmed low values indicate potential testosterone replacement candidacy, with baseline:

• LH and FSH (identifies primary vs. secondary cause)

• Prolactin

• Iron and ferritin (hemochromatosis screen)

• PSA (before therapy)

• Hematocrit (TRT raises red cell mass)

High bioavailable testosterone in a symptomatic man is unusual and warrants evaluation for androgen excess, supplementation, or lab artifact.

Lifestyle factors

• Resistance training acutely raises testosterone

• Chronic sleep deprivation lowers testosterone

• Obesity converts testosterone to estradiol via aromatase

• Adequate zinc, vitamin D, and healthy fat intake support baseline production

• Chronic stress and elevated cortisol suppress testosterone