Bioavailable Testosterone: Free vs. Total, SHBG, and What Your Labs Actually Mean

What Is Bioavailable Testosterone?

Bioavailable testosterone is the portion of circulating testosterone that can cross cell membranes and bind androgen receptors in target tissues. It equals free testosterone (the truly unbound fraction) plus the testosterone loosely attached to albumin, because albumin releases its testosterone readily once blood flow reaches capillary tissue. The tightly bound fraction attached to sex hormone-binding globulin (SHBG) does not dissociate at the tissue level under physiological conditions and is therefore considered biologically inactive.

In practice, a standard testosterone panel reports only total testosterone. That number can sit comfortably in the normal range of 400 to 600 ng/dL while bioavailable testosterone is critically low, a scenario common in men over 50 whose SHBG has climbed with age. A 2017 analysis published in the Journal of Clinical Endocrinology and Metabolism (N=9,054 men across four cohorts) found that SHBG rose approximately 1.2% per year of age, meaning a 65-year-old man could have 35 to 40% less bioavailable testosterone than a 30-year-old at the exact same total testosterone concentration [1].

The distinction matters clinically. Symptoms of low testosterone, including fatigue, reduced libido, decreased lean mass, and mood changes, correlate more strongly with bioavailable or free testosterone than with total testosterone alone [2].

Free Testosterone vs. Total Testosterone: Why Both Numbers Matter

Total testosterone measures all three fractions in the blood: SHBG-bound (roughly 60 to 70%), albumin-bound (roughly 30 to 40%), and free (approximately 2 to 3%). Free testosterone is the most metabolically active subfraction. Bioavailable testosterone captures the larger, clinically relevant pool by adding the albumin-bound portion back in.

Neither number alone tells the complete story.

A man with a total testosterone of 350 ng/dL and low SHBG of 15 nmol/L may have adequate free testosterone. A man with a total testosterone of 550 ng/dL and high SHBG of 75 nmol/L may have free testosterone well below 5 pg/mL, which sits beneath most laboratory reference ranges of 5, 21 pg/mL [3]. The 2018 Endocrine Society guideline on male hypogonadism states: "In men in whom total testosterone concentrations are near the lower limit of the normal range, or in whom alterations in SHBG are suspected, a free testosterone measurement should be obtained" [4].

Free testosterone can be measured directly by immunoassay, but this method is notoriously inaccurate. Equilibrium dialysis is the gold-standard method. Alternatively, free testosterone can be calculated from total testosterone, albumin, and SHBG using the Vermeulen equation, a validated formula endorsed by the Endocrine Society [4]. Most telehealth platforms that report calculated free testosterone are using this equation, which introduces no extra blood draw but does require accurate SHBG measurement.

HealthRX Clinical Decision Framework: Interpreting a Testosterone Panel

1. Check total testosterone first (fasting morning draw, 8, 10 a.m., two separate occasions for confirmation).
2. If total T is 300 to 450 ng/dL, always order SHBG and albumin before concluding hypogonadism or ruling it out.
3. Calculate bioavailable T using the Vermeulen formula. A bioavailable T <70 ng/dL in a symptomatic man is clinically significant regardless of total T.
4. If total T is <300 ng/dL on two separate morning draws, the Endocrine Society considers this diagnostic of biochemical hypogonadism when accompanied by symptoms [4].
5. Always co-review LH and FSH to distinguish primary (testicular) from secondary (pituitary/hypothalamic) hypogonadism before starting TRT.

SHBG Explained: The Binding Protein That Controls Testosterone Access

SHBG is a glycoprotein produced primarily in the liver. Each SHBG molecule binds one molecule of sex steroid, with a much higher affinity for testosterone and dihydrotestosterone (DHT) than for estradiol. When SHBG is high, more testosterone is trapped and unavailable; when SHBG is low, more testosterone circulates freely but the total may appear artificially low on a standard panel.

SHBG rises predictably with age. It also rises with hyperthyroidism, cirrhosis, and exogenous estrogen use. Anorexia and prolonged calorie restriction push SHBG up. Obesity, type 2 diabetes, hypothyroidism, and androgen use push it down. A 2021 population study in The Journal of Clinical Endocrinology and Metabolism (N=3,127) confirmed that men with metabolic syndrome had SHBG concentrations 22% lower than weight-matched controls without metabolic syndrome [5].

Clinically, SHBG is the single most important modifier of the total testosterone result. Two drugs used in TRT protocols can intentionally lower SHBG: stanozolol (rarely used therapeutically now due to hepatotoxicity) and danazol. More practically, weight loss of 10% body weight has been shown to reduce SHBG and raise free testosterone without any hormonal medication [6].

Reference ranges for SHBG in adult men run from roughly 10, 57 nmol/L, though many labs narrow this to 16, 55 nmol/L. A value above 60 nmol/L in a symptomatic man with borderline total testosterone should prompt calculation of bioavailable T before any treatment decision.

The HPG Axis: How the Body Regulates Testosterone Production

The hypothalamic-pituitary-gonadal (HPG) axis is the central regulatory loop that controls endogenous testosterone. The hypothalamus releases gonadotropin-releasing hormone (GnRH) in pulses every 60 to 120 minutes. Each pulse drives the anterior pituitary to release luteinizing hormone (LH) and follicle-stimulating hormone (FSH). LH acts on Leydig cells in the testes to produce testosterone; FSH supports Sertoli cells and spermatogenesis.

Testosterone itself completes the loop via negative feedback. When circulating testosterone and estradiol (converted from testosterone by aromatase) are high, GnRH pulse frequency drops, LH and FSH fall, and testicular production decreases. This is a self-correcting system under normal physiological conditions.

Disruption of any node causes predictable pathology. Pituitary adenomas, Kallmann syndrome (absent GnRH neurons), and opioid use all suppress the HPG axis at the top. Primary testicular failure (Klinefelter syndrome, orchitis, chemotherapy damage) knocks out the bottom. Each produces a distinct lab pattern: secondary hypogonadism shows low testosterone with low-normal LH/FSH; primary hypogonadism shows low testosterone with elevated LH/FSH as the pituitary tries to compensate [4].

TRT suppresses the HPG axis within days of the first dose. Exogenous testosterone signals the hypothalamus and pituitary that circulating androgen is adequate or excessive, so GnRH, LH, and FSH drop. Testicular testosterone production stops. Testicular volume decreases an average of 20 to 30% with prolonged TRT, and sperm counts can fall to zero [7]. For men who want to preserve fertility, co-administration of human chorionic gonadotropin (hCG) at 500, 1 to 000 IU two to three times per week can maintain intratesticular testosterone and spermatogenesis by mimicking LH [8].

A 2023 NEJM trial (TRAVERSE, N=5,246) confirmed that TRT with testosterone undecanoate 750 mg IM at 0, 4, then every 10 weeks maintained total testosterone in the eugonadal range in 87% of participants and did not significantly raise major adverse cardiovascular event rates over a median follow-up of 33 months, addressing a major safety concern from an earlier 2010 trial that was stopped early [9].

Aromatization: How Testosterone Converts to Estradiol

Aromatase (CYP19A1) is the enzyme that converts testosterone to estradiol and androstenedione to estrone. It is expressed in adipose tissue, liver, brain, bone, and testes. Men need estradiol. It is required for bone mineral density, libido, cardiovascular protection, and cognitive function. The problem arises when aromatization is excessive.

Aromatase activity scales directly with fat mass. Each kilogram of visceral adipose tissue increases aromatization, which is why obese men often have low testosterone and high estradiol simultaneously. A body mass index above 30 kg/m² is associated with estradiol concentrations above 42.6 pg/mL, the upper limit of the typical male reference range [10].

On TRT, exogenous testosterone provides more substrate for aromatase. Estradiol can rise substantially, sometimes producing gynecomastia, water retention, and mood changes. Anastrozole (0.5 to 1 mg twice weekly) and exemestane (12.5 to 25 mg three times weekly) are both aromatase inhibitors used as adjuncts in TRT protocols when estradiol climbs above 40, 50 pg/mL and causes symptoms. The Endocrine Society cautions against routine aromatase inhibitor use, recommending it only for symptomatic estradiol elevation, because suppressing estradiol too aggressively reduces bone density and libido [4].

The ratio of testosterone to estradiol matters as much as the absolute estradiol number. A man on TRT with testosterone at 700 ng/dL and estradiol at 45 pg/mL may feel fine; the same estradiol with testosterone at 300 ng/dL may produce clear estrogenic symptoms. Monitor both simultaneously at every follow-up visit.

DHT is a second downstream metabolite. 5-alpha reductase converts testosterone to DHT, which is 3, 5 times more potent at the androgen receptor. DHT drives prostate growth, scalp hair loss, and body hair growth. Finasteride (1 to 5 mg daily) blocks 5-alpha reductase and reduces DHT by approximately 65 to 70%, which may reduce scalp hair loss but can also blunt androgenic effects in prostate and other tissues [11].

Measuring Bioavailable Testosterone Accurately: Assay Choice and Timing

The accuracy of testosterone measurement depends heavily on the assay used and the timing of the blood draw. Morning draws between 7 a.m. and 10 a.m. are required because testosterone peaks during early morning hours due to circadian pulsatility. An afternoon draw can underestimate true testosterone by 20 to 30% in young men and 10 to 15% in men over 65, where circadian amplitude is blunted [12].

For total testosterone, liquid chromatography-tandem mass spectrometry (LC-MS/MS) is the gold standard. Direct immunoassays, still common in many commercial labs, have coefficients of variation above 20% at low concentrations, making them unreliable below 300 ng/dL [3]. The CDC's Hormone Standardization Program (HoSt) certifies specific laboratory platforms and publishes a list of certified labs at cdc.gov. HealthRX uses LC-MS/MS-certified reference laboratories for all testosterone panels.

For free testosterone specifically, equilibrium dialysis is the reference method. Calculated free testosterone via the Vermeulen equation is acceptable if SHBG is measured accurately. The direct free testosterone immunoassay (also called analog radioimmunoassay) is unreliable and should not be used to make clinical decisions. A widely cited 2006 paper in Clinical Chemistry by Rosner et al. demonstrated that the direct analog assay underestimates free testosterone in hypogonadal men and overestimates it in men with high SHBG, producing clinically misleading results in precisely the patients where the measurement matters most [13].

SHBG measurement itself is standardized, with most modern immunoassays performing adequately for clinical use. Albumin concentration is rarely the variable that changes meaningfully; most practitioners use a fixed albumin of 4.3 g/dL in the Vermeulen calculation unless frank hypoalbuminemia is documented (as in cirrhosis or severe malnutrition).

When Is Bioavailable Testosterone Too Low? Symptoms and Thresholds

Symptoms of low bioavailable testosterone include reduced libido, erectile dysfunction, fatigue, decreased morning erections, mood changes, reduced muscle mass, increased fat mass, and impaired concentration. No threshold perfectly predicts symptoms; there is significant individual variation.

Population-based reference data from the European Male Ageing Study (EMAS, N=3,369 men aged 40, 79) found that specific sexual symptoms correlated most strongly with total testosterone <11 nmol/L (317 ng/dL) and free testosterone <220 pmol/L (6.4 pg/mL using equilibrium dialysis), suggesting these are clinically meaningful thresholds for the sexual symptom domain [14]. Non-sexual symptoms such as fatigue and low mood had weaker testosterone associations and were less useful as diagnostic criteria.

The Endocrine Society's 2018 guideline recommends initiating TRT when a man has both biochemical hypogonadism (confirmed low testosterone on two morning draws) and at least three consistent symptoms, after ruling out reversible causes such as obesity, opioid use, sleep apnea, and hyperprolactinemia [4]. "Clinicians should prescribe testosterone therapy only to men with classic hypogonadism, in whom testosterone deficiency is confirmed by both symptoms and unequivocally low morning testosterone concentrations" is the exact language from that guideline [4].

Reversible causes deserve treatment before TRT is started. A 2020 randomized trial in JAMA (N=195 obese men with hypogonadism) found that a structured 8-week lifestyle intervention raised total testosterone by a mean of 2.9 nmol/L (84 ng/dL), brought 56% of participants into the normal range, and reduced SHBG by 4.1 nmol/L, all without any hormonal medication [15].

TRT Options and Their Effect on the Testosterone Fractions

Different TRT formulations produce different pharmacokinetic profiles, which in turn affect peak-to-trough variation in free and bioavailable testosterone.

Testosterone cypionate or enanthate, injected intramuscularly at 100 to 200 mg every 1 to 2 weeks, produces supraphysiological peaks in total testosterone (sometimes exceeding 1 to 200 ng/dL at 24 to 48 hours post-injection) followed by troughs that may fall below 300 ng/dL by day 14. These peaks increase aromatization, which can raise estradiol transiently. More frequent dosing at 50 to 100 mg weekly or twice weekly reduces this swing substantially.

Testosterone undecanoate (Aveed, 750 mg IM every 10 weeks after two loading doses) provides much more stable levels and was the formulation used in the TRAVERSE trial [9].

Daily transdermal gels (AndroGel 1.62%, Testim, Natesto intranasal) produce relatively stable testosterone throughout the day but show interpersonal transfer risk and variable skin absorption. Testosterone pellets (Testopel, 75 mg per pellet, typically 6, 12 pellets implanted every 3 to 6 months) provide the flattest pharmacokinetic curve of all delivery systems, minimizing peak aromatization.

Regardless of delivery system, free and bioavailable testosterone fractions should be measured at steady state (at trough for injectables, any time for gels or pellets), alongside estradiol and a complete blood count to monitor hematocrit, which TRT can raise above 54%, increasing thrombosis risk [4].