Bioavailable Testosterone: What This Lab Test Actually Measures

Why Total Testosterone Is Not the Whole Story

Total testosterone counts every molecule of the hormone in serum, whether it is free, loosely attached to albumin, or locked onto SHBG. Only the first two fractions can cross cell membranes and bind intracellular androgen receptors. A man with a total testosterone of 450 ng/dL and high SHBG may have less biologically active hormone than a man at 350 ng/dL with normal SHBG.

The Endocrine Society's 2018 clinical practice guideline on male hypogonadism recommends measuring free or bioavailable testosterone "when total testosterone is near the lower boundary of the normal range or when SHBG concentrations are expected to be abnormal" [1]. SHBG rises with aging, hyperthyroidism, hepatic cirrhosis, and estrogen use. It falls with obesity, insulin resistance, hypothyroidism, and androgen supplementation [2]. Each of these shifts reshapes the ratio between bound and unbound testosterone without changing the total number.

This distinction matters clinically. A 2017 analysis of 3,177 men in the European Male Ageing Study (EMAS) found that free testosterone predicted sexual symptoms and physical function more accurately than total testosterone did (adjusted OR 1.42 per 1-SD decrease in free T vs. 1.18 for total T) [3]. Bioavailable testosterone captures that same biologically active pool, plus the albumin-bound fraction that dissociates rapidly in capillary beds. Think of it as the hormone supply that tissues can actually draw from.

The Three Testosterone Fractions in Blood

Testosterone circulates in three distinct states. Free testosterone floats unattached to any protein and makes up only 1-3% of the total. Albumin-bound testosterone accounts for roughly 25-50%, held by a low-affinity bond that breaks apart within seconds in tissue capillary beds. SHBG-bound testosterone, the largest fraction at 40-70%, is locked in a high-affinity grip that prevents tissue uptake under normal physiological conditions [4].

Bioavailable testosterone is the sum of the first two: free plus albumin-bound. The term was formalized in clinical endocrinology because researchers recognized that albumin's weak binding effectively makes that fraction "available" at the tissue level [5]. SHBG-bound testosterone, by contrast, stays in circulation. It serves as a reservoir, but under standard conditions it does not deliver hormone to androgen receptors.

The ratio between these fractions is not static. SHBG production in the liver responds to a range of signals. Estradiol and thyroxine increase SHBG synthesis. Insulin and androgens suppress it. A patient on oral estrogen therapy, for instance, can see SHBG double or triple, pulling a large share of testosterone into the inactive SHBG-bound pool even if the gonads or exogenous therapy maintain a stable total T [2].

How Bioavailable Testosterone Is Measured

Direct measurement uses ammonium sulfate precipitation. The lab adds ammonium sulfate to a serum sample, which precipitates SHBG (along with its bound testosterone) out of solution. The testosterone remaining in the supernatant, the bioavailable fraction, is then quantified by immunoassay [6]. This method is accurate but labor-intensive, and most commercial labs have moved away from it.

The more common approach is calculation. The Vermeulen equation (also called the Sodergard or modified Vermeulen method) uses three inputs: total testosterone, SHBG, and albumin concentration. It applies mass-action binding equations to estimate free testosterone, then derives bioavailable T by adding the calculated albumin-bound share [7]. The calculator published by the International Society for the Study of the Aging Male (ISSAM) uses this equation and is freely available online.

A head-to-head validation in the Journal of Clinical Endocrinology & Metabolism showed that calculated free testosterone using the Vermeulen equation correlated with equilibrium dialysis (the gold standard for free T) at r = 0.91 across 355 serum samples spanning male and female patients [7]. The correlation is strong enough for routine clinical use, though the Endocrine Society notes that "calculated free testosterone may overestimate true free testosterone in populations with very low SHBG" [1].

For the most precise result, clinicians can order free testosterone by equilibrium dialysis and multiply total T by the non-SHBG-bound percentage. This is expensive and slow (turnaround of 7-10 days at reference labs like Quest or LabCorp). Most clinical decisions rely on the calculated value instead.

Normal Bioavailable Testosterone Ranges

Reference ranges depend on the assay, the calculation method, and the patient's age and sex. The values below reflect commonly cited ranges from major reference laboratories.

For adult males aged 20-49, bioavailable testosterone typically falls between 131 and 682 ng/dL. After age 50, the lower end of the range shifts downward due to rising SHBG and declining testicular output. The Baltimore Longitudinal Study of Aging documented an average decline in bioavailable testosterone of 2-3% per year after age 30, roughly double the rate of total testosterone decline [8].

For premenopausal adult females, bioavailable testosterone ranges from approximately 1.5 to 10 ng/dL. Postmenopausal values are generally lower, though the clinical significance of specific cutoffs in women remains debated. The Endocrine Society's 2019 position statement on testosterone therapy in women noted that "there are no well-accepted blood level thresholds that can be used to classify women as having low testosterone" [9].

Ranges differ across laboratories. A result of 120 ng/dL from one lab may be below the reference floor, while another lab using a different calculation method might call the same serum sample 145 ng/dL. Clinicians should compare serial results from the same lab and the same assay method whenever possible.

When Clinicians Order Bioavailable Testosterone

The test is not part of a standard screening panel. Clinicians order it in specific clinical scenarios where total testosterone alone is unreliable.

High SHBG states. Aging men, patients with hyperthyroidism, those taking anticonvulsants (phenytoin, carbamazepine), and patients on oral estrogen therapy often have elevated SHBG. Their total testosterone may read as normal while bioavailable testosterone is genuinely low. A 2020 study of 1,454 men in the Framingham Heart Study offspring cohort found that 15.4% of men with a "normal" total T (above 300 ng/dL) had a bioavailable testosterone below the age-adjusted reference range [10].

Low SHBG states. Obesity, type 2 diabetes, metabolic syndrome, and nephrotic syndrome suppress SHBG. Here, total testosterone may read as low while bioavailable testosterone remains adequate. The American Association of Clinical Endocrinologists (AACE) 2020 guidelines recommend checking "free or bioavailable testosterone in obese men before diagnosing hypogonadism, because total testosterone can be misleadingly low" [11].

Borderline total testosterone. When total T sits between 200 and 400 ng/dL and the clinical picture is ambiguous, bioavailable or free T helps resolve the diagnosis. The Endocrine Society guideline sets a threshold of total T below 300 ng/dL on two morning samples for diagnosing hypogonadism, but adds that free or bioavailable T should be checked "if total T is between 200 and 400 ng/dL" [1].

Monitoring testosterone therapy. Some clinicians track bioavailable T during TRT, particularly in patients on oral testosterone undecanoate (Jatenzo) or subcutaneous pellets, where SHBG changes can shift the bound/free ratio significantly during a dosing cycle.

Bioavailable Testosterone vs. Free Testosterone

Both tests aim to capture the biologically active fraction. Free testosterone is more specific: it measures only the 1-3% that is completely unbound. Bioavailable testosterone is broader, adding the albumin-bound share. In practice, they track closely because the albumin-bound fraction is a relatively stable proportion of total T.

Free testosterone by equilibrium dialysis is considered the gold standard for assessing androgen status, per the Endocrine Society [1]. But direct free T assays (analog immunoassays) are notoriously inaccurate. The CDC's Hormone Standardization Program (HoSt) found that direct analog free T assays correlated poorly with equilibrium dialysis (r = 0.36-0.64 across platforms), while calculated free and bioavailable T using the Vermeulen equation performed much better [12].

Dr. Shalender Bhasin, a lead author on the Endocrine Society hypogonadism guideline, has stated: "Direct analog assays for free testosterone should not be used for clinical decision-making. Calculated free testosterone or bioavailable testosterone are preferable when equilibrium dialysis is not available" [1]. This guidance means that in most real-world clinical settings, calculated bioavailable testosterone is one of the most reliable options for assessing true androgen exposure.

How to Raise Bioavailable Testosterone

Low bioavailable testosterone can result from gonadal insufficiency, elevated SHBG, or both. Treatment depends on the cause.

Addressing SHBG drivers is the first step. Correcting hyperthyroidism, switching from oral to transdermal estrogen (in women on HRT), and managing liver disease can each lower SHBG and release more testosterone into the bioavailable pool [2]. Weight loss in obese men raises total testosterone by an average of 50-100 ng/dL per 10% body weight reduction, as demonstrated in the MOSH trial (N=153), though the mechanism works partly through improved SHBG balance [13].

Resistance training raises bioavailable testosterone acutely and, with sustained training, helps maintain androgen levels over time. A 12-week RCT of progressive resistance exercise in 44 older men (mean age 65) showed a 16% increase in bioavailable testosterone compared to controls [14].

Testosterone replacement therapy directly increases bioavailable T. The T-Trials, a coordinated set of seven placebo-controlled trials enrolling 790 men aged 65 and older with low testosterone (total T <275 ng/dL), showed that transdermal testosterone gel raised mean bioavailable T from 55 ng/dL to 192 ng/dL at 12 months [15]. Benefits included improved sexual function, physical activity, and bone mineral density. The 2018 Endocrine Society guideline recommends TRT for men with "unequivocally low serum testosterone and consistent signs and symptoms of testosterone deficiency" [1].

How to Lower Bioavailable Testosterone

Elevated bioavailable testosterone in women is most commonly seen with polycystic ovary syndrome (PCOS). The Endocrine Society's 2013 PCOS guideline recommends combined oral contraceptives (COCs) as first-line pharmacotherapy because ethinyl estradiol raises SHBG while the progestin component offers direct anti-androgenic activity [16].

Spironolactone at 50-200 mg/day is a second-line option that blocks the androgen receptor directly. Metformin, while primarily an insulin sensitizer, modestly reduces bioavailable testosterone in PCOS patients by improving insulin-mediated SHBG suppression. A meta-analysis of 12 RCTs (N=608) found metformin reduced bioavailable testosterone by 11% on average in women with PCOS [17].

In men, elevated bioavailable testosterone is rarely a clinical problem unless it is iatrogenic (over-replacement with TRT). Dose reduction guided by trough levels corrects this. For men with androgen-dependent prostate cancer, GnRH agonists or antagonists suppress gonadal testosterone production to castrate levels, reducing bioavailable T to near zero.

Factors That Shift SHBG and Change Bioavailable T

Because SHBG is the gatekeeper, anything that moves SHBG moves bioavailable testosterone in the opposite direction. High SHBG lowers bioavailable T. Low SHBG raises it.

Conditions that raise SHBG include aging, hyperthyroidism, hepatic cirrhosis, anorexia nervosa, HIV (especially with lipodystrophy), and medications like anticonvulsants (phenytoin, carbamazepine), oral estrogens, and some statins [2]. The EMAS cohort data showed that each decade of aging after 40 was associated with a 20-25% increase in SHBG concentration [3].

Conditions that lower SHBG include obesity (BMI above 30 reduces SHBG by roughly 30% compared to normal-weight individuals), type 2 diabetes, hypothyroidism, nephrotic syndrome, acromegaly, and use of exogenous androgens, glucocorticoids, or progestins [2]. Insulin is a direct suppressor of hepatic SHBG production, which explains why insulin-resistant states consistently drive SHBG down and push bioavailable testosterone up.

This relationship has diagnostic consequences. An obese man with a total T of 260 ng/dL and a low SHBG may have a perfectly normal bioavailable T once calculated. Starting TRT based on total T alone would be premature. Conversely, a lean 60-year-old man on phenytoin with a total T of 400 ng/dL and high SHBG may genuinely be hypogonadal by bioavailable T criteria.

Preparing for the Test

Blood should be drawn in the morning, ideally before 10 AM. Testosterone follows a circadian rhythm, peaking between 6 AM and 8 AM and declining through the day. Afternoon draws can yield values 20-30% lower than morning values in young men, and 10-15% lower in men over 60 [18].

The Endocrine Society recommends fasting for morning testosterone samples because food intake acutely lowers testosterone by 10-25%, an effect mediated by postprandial insulin release and its suppressive effect on GnRH pulsatility [1]. Two low results on separate morning draws are required before diagnosing hypogonadism. Labs should include total testosterone, SHBG, and albumin at minimum so that bioavailable T can be calculated accurately.

Acute illness, sleep deprivation, intense exercise within 24 hours, and opioid use can all temporarily suppress testosterone. Clinicians should defer testing until these confounders resolve. For patients on exogenous testosterone, the draw should be timed to the expected trough of the dosing interval (for injections, immediately before the next dose; for gels, at least 2 hours after application).