Bioavailable testosterone is the portion of testosterone in your blood that your body can actually use. It includes both free testosterone (the 2–3% that floats unbound in your bloodstream) and testosterone loosely attached to a protein called albumin. When a lab result shows high bioavailable testosterone, it means you have more of this usable testosterone than the typical reference range, which for men ages 20 to 69 is roughly 110 to 400 ng/dL, and for premenopausal women around 1 to 8 ng/dL.
Why Bioavailable Testosterone Matters More Than Total
Your body produces testosterone that travels through the bloodstream in three forms. About 43–45% binds tightly to a protein called sex hormone binding globulin (SHBG). That portion is locked up and essentially inactive: SHBG grips testosterone so tightly that cells can’t pull it away and use it. Another 53–55% binds loosely to albumin, a common blood protein. The remaining 2–3% circulates completely unbound as free testosterone.
Bioavailable testosterone combines those last two categories: the free portion plus the albumin-bound portion. Because albumin releases testosterone quickly and easily, that loosely bound fraction acts as a readily available reservoir your tissues can tap into. SHBG-bound testosterone, by contrast, stays locked away. This is why two people can have identical total testosterone levels but very different amounts of testosterone their bodies can actually use. Someone with high SHBG will have less bioavailable testosterone, while someone with low SHBG will have more, even if their total numbers match.
How Labs Measure It
Most labs don’t measure bioavailable testosterone directly from a blood draw. Instead, they measure your total testosterone and SHBG levels, then plug those numbers into a mathematical formula (often called the Vermeulen calculation) to estimate the bioavailable fraction. Some labs also factor in your albumin level, though albumin stays fairly stable in healthy people. The calculation essentially works out how much testosterone is left over after accounting for the amount locked up by SHBG.
This calculated approach is the standard in most clinical settings. It’s accurate enough for diagnosis in the vast majority of cases, though results can vary slightly between labs depending on which instruments and formulas they use.
What Causes High Levels in Men
The most common reason for genuinely high bioavailable testosterone in men is exogenous testosterone, meaning testosterone replacement therapy, supplements, or anabolic steroids. When testosterone is introduced from outside the body, circulating levels can climb well above the natural range.
Low SHBG is another frequent driver. Even if your body produces a normal total amount of testosterone, having less SHBG means less of it gets locked away, leaving a larger usable fraction. Conditions that lower SHBG include obesity, insulin resistance, type 2 diabetes, and certain thyroid disorders. In these cases, your total testosterone might look normal on paper, but the bioavailable portion reads high.
Less commonly, tumors in the adrenal glands or testes can produce excess testosterone. These cases usually involve testosterone levels far above normal and come with other noticeable symptoms.
What Causes High Levels in Women
Polycystic ovary syndrome (PCOS) is by far the most common cause of elevated bioavailable testosterone in women. Among women with PCOS, 65–75% show excess androgen levels. The ovaries and, to a lesser extent, the adrenal glands overproduce testosterone and related hormones.
Other causes include adrenal gland disorders that ramp up hormone production, and rarely, androgen-producing tumors of the ovaries or adrenal glands. Tumors tend to cause rapid, severe symptoms and are distinct from the gradual onset seen with PCOS. In postmenopausal women, slowly progressing symptoms of excess androgens can also develop, sometimes reaching more pronounced levels.
Symptoms in Men
When bioavailable testosterone runs high in men, particularly from external testosterone use, the effects can be paradoxical. Rather than boosting fertility, excess testosterone actually suppresses sperm production and can shrink the testes, because the brain responds to high levels by dialing down its signals to produce more. Other physical effects include persistent acne, fluid retention (especially swelling in the legs and feet), weight gain partly driven by increased appetite, and elevated blood pressure and cholesterol.
Behavioral and mood changes are also reported: irritability, mood swings, impaired sleep, and in some cases euphoria alternating with poor judgment. There’s a popular belief that high testosterone causes aggression, though the evidence for that link is weaker than most people assume. One effect that is well supported is a higher risk of blood clots, since elevated testosterone raises hematocrit, the concentration of red blood cells in your blood. In adolescents, excess testosterone can cause growth plates in bones to close prematurely, stunting final adult height.
Interestingly, testosterone also interacts with brain chemistry in ways that influence mood regulation. It promotes the release of dopamine in reward-related brain pathways and affects serotonin activity. This helps explain why both very low and very high levels can disrupt emotional balance, with research linking bioavailable testosterone specifically to depressive symptoms in adult men.
Symptoms in Women
The most recognizable sign of high bioavailable testosterone in women is hirsutism: coarse, dark hair growing in typically male-pattern areas like the face, chest, and back. This happens because testosterone is converted into a more potent form in the skin’s oil glands and hair follicles, stimulating thicker hair growth.
Acne is another common symptom, driven by that same conversion process in the sebaceous glands. Many women also experience thinning hair on the scalp in a pattern resembling male-pattern hair loss, which can feel especially distressing. Menstrual irregularities are frequent too: cycles may become infrequent or stop altogether. In more severe or tumor-driven cases, virilization can occur, including deepening of the voice, increased muscle mass, and enlargement of the clitoris. These more dramatic changes are relatively rare and typically point to a more serious underlying cause than PCOS.
Cardiovascular and Prostate Concerns
For years, there was concern that high testosterone levels might increase the risk of heart attacks or prostate cancer. The TRAVERSE trial, a large and long-running study specifically designed to answer these questions, found no increase in major cardiovascular events among men using testosterone therapy compared to a placebo group. It also found similar rates of prostate cancer between both groups, and testosterone therapy did not worsen urinary symptoms related to prostate enlargement.
That said, testosterone therapy does consistently raise hematocrit levels. Thicker blood increases the risk of clots, which is why regular blood monitoring is part of standard care for men on testosterone therapy. The overall cardiovascular picture is more reassuring than earlier fears suggested, but the clotting risk remains a real consideration when testosterone levels stay elevated over time.
When Bioavailable Testosterone Is More Useful Than Total
Bioavailable testosterone gives a clearer clinical picture in several specific situations. If you have unusually high or low SHBG, your total testosterone can be misleading. For example, aging naturally raises SHBG levels in men, which means total testosterone might look acceptable while bioavailable testosterone has dropped significantly. Obesity does the opposite, suppressing SHBG and potentially inflating the bioavailable fraction. Liver disease, thyroid conditions, and certain medications (especially estrogen-containing therapies) also shift SHBG levels enough to make total testosterone an unreliable marker on its own.
For women, bioavailable testosterone can be especially informative because female testosterone levels are so much lower overall. Small shifts in SHBG can meaningfully change how much testosterone is actually active in the body, making the bioavailable measurement more sensitive to clinically relevant changes. Reference ranges for a 30-year-old woman place bioavailable testosterone between roughly 1.1 and 7.6 ng/dL, and these values decline gradually with age, though SHBG stays relatively stable across the reproductive years.