Bioavailable Testosterone: When to Order This Test

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At a glance

  • Test type / calculated androgen biomarker (ammonium sulfate precipitation or Vermeulen equation)
  • What it measures / testosterone not bound to SHBG (includes free + albumin-bound fractions)
  • Normal range, adult men / 83 to 257 ng/dL (age-dependent; declines roughly 1 to 2% per year after age 30)
  • Normal range, adult women / 0.5 to 8.5 ng/dL (varies by cycle phase and menopausal status)
  • Primary ordering triggers / symptoms of androgen deficiency or excess with normal total T; abnormal SHBG; TRT or HRT monitoring
  • Key confounders / obesity, liver disease, thyroid dysfunction, exogenous estrogen, aging
  • Fasting required / no, but morning draw (7 to 10 a.m.) is preferred for men
  • Turnaround / 1 to 5 business days depending on laboratory method
  • Guideline endorsement / Endocrine Society 2018 Testosterone Therapy in Men guidelines

What Bioavailable Testosterone Actually Means

Bioavailable testosterone is the sum of free testosterone (roughly 2 to 3% of total T, unbound to any protein) and albumin-bound testosterone (roughly 38 to 40% of total T, loosely bound). Together these two fractions can dissociate from plasma proteins quickly enough to enter target tissues. The remaining 57 to 60% binds tightly to sex hormone-binding globulin (SHBG) and is biologically inactive under normal physiological conditions.

The Three Fractions of Circulating Testosterone

Total testosterone = SHBG-bound T + albumin-bound T + free T. Only the latter two are "bioavailable." Because SHBG concentrations fluctuate widely with age, obesity, liver function, and medications, total testosterone can be misleading. A man with total T of 380 ng/dL but very high SHBG (e.g., 80 nmol/L) may have bioavailable T well below the reference range and experience clear hypogonadal symptoms.

How the Test Is Calculated or Measured

Most commercial laboratories calculate bioavailable testosterone using the Vermeulen equation, which requires total testosterone, SHBG, and albumin inputs. The direct ammonium sulfate precipitation assay is considered the reference standard but is less widely available. The Endocrine Society notes that the Vermeulen calculator is acceptable for clinical decisions when equilibrium dialysis is unavailable. [1]

A free online version of the Vermeulen calculator is maintained by the Issam Rainier group and is the same tool referenced in Endocrine Society clinical practice guidelines. [2]

When to Order Bioavailable Testosterone

Order this test when the clinical picture and total testosterone results do not align. The Endocrine Society's 2018 guideline on testosterone therapy in men with hypogonadism states: "We suggest measuring free testosterone concentration in patients with borderline-low total testosterone concentrations and conditions that alter SHBG." [1] Bioavailable testosterone provides the same clinical information as free testosterone and is preferred by some laboratories because its measurement is more reproducible.

Scenario 1: Symptoms Conflict With Total Testosterone

A man reports fatigue, decreased libido, loss of morning erections, and reduced lean mass, yet his total T comes back at 340 ng/dL, which sits in the low-normal range. Ordering bioavailable testosterone resolves the ambiguity. If SHBG is elevated (common with aging, hyperthyroidism, or anticonvulsant use), bioavailable T may land below 83 ng/dL even with apparently adequate total T.

Scenario 2: Abnormal SHBG

Conditions that raise SHBG include cirrhosis (paradoxically), hyperthyroidism, HIV-associated weight loss, and use of oral estrogen or anticonvulsants. Conditions that lower SHBG include obesity, type 2 diabetes, hypothyroidism, nephrotic syndrome, and exogenous androgen use. In either situation, bioavailable testosterone gives a cleaner picture of true androgen exposure. A 2013 study in the Journal of Clinical Endocrinology and Metabolism (N=3,014) found that SHBG-adjusted free testosterone reclassified hypogonadal status in approximately 30% of men compared with total testosterone alone. [3]

Scenario 3: Monitoring Androgen Therapy

Men and women receiving testosterone replacement therapy (TRT or HRT) should have bioavailable testosterone measured 3 to 6 weeks after any dose change, then every 6 to 12 months once stable. The Endocrine Society recommends targeting mid-normal ranges for the patient's age and sex. [1] Measuring only total T during therapy can be misleading if the formulation or route of administration alters SHBG.

Scenario 4: Evaluating Women With Androgen Excess or Deficiency

Polycystic ovary syndrome (PCOS) affects an estimated 6 to 13% of reproductive-age women worldwide, according to the WHO. [4] Elevated bioavailable testosterone is a core biochemical feature. Conversely, postmenopausal women with low libido, fatigue, or bone loss may have low bioavailable testosterone despite total T sitting within a broad reference range. The AACE recommends measuring free or bioavailable testosterone rather than total T when evaluating androgen deficiency in women. [5]

Normal Bioavailable Testosterone Ranges

Reference intervals vary by laboratory method, age, and sex. The values below reflect the Mayo Clinic Laboratories reference method (ammonium sulfate precipitation) and should be confirmed against the reporting laboratory's own reference interval.

Men

| Age | Bioavailable T (ng/dL) | |---|---| | 20 to 29 | 128 to 312 | | 30 to 39 | 115 to 284 | | 40 to 49 | 98 to 257 | | 50 to 59 | 83 to 227 | | 60 to 69 | 68 to 198 | | 70+ | 52 to 176 |

Testosterone declines at approximately 1 to 2% per year after age 30, meaning a 70-year-old man may have half the bioavailable testosterone of a 25-year-old even without pathology. [6]

Women

Premenopausal women: 0.5 to 8.5 ng/dL, with the peak near ovulation. Postmenopausal women (not on HRT): 0.3 to 3.8 ng/dL. Women on oral estrogen (including oral contraceptives) will have elevated SHBG and therefore depressed bioavailable testosterone even if total T is normal. [5]

Children and Adolescents

Bioavailable testosterone is not used routinely in prepubertal children. For pubertal staging, total testosterone with LH and FSH is the standard first step. Refer to pediatric endocrinology if early or delayed puberty is suspected.

Causes of Low Bioavailable Testosterone

Low bioavailable testosterone can stem from reduced production, increased SHBG binding, or both. Separating these mechanisms guides treatment.

Reduced Production

Primary hypogonadism (testicular or ovarian failure) reduces total T, and consequently all fractions fall. LH and FSH will be elevated. Common causes include Klinefelter syndrome (47,XXY), bilateral orchitis, chemotherapy, or radiation. The Testosterone Trials (TTrials), a set of seven placebo-controlled trials involving 790 men aged 65 and older with total T below 275 ng/dL, demonstrated that testosterone gel raised bioavailable testosterone and improved sexual function, physical performance, and bone density compared with placebo. [7]

Elevated SHBG

When SHBG rises, more testosterone gets sequestered, and bioavailable T drops even if total T holds steady. This is the most common reason for the discrepancy between total T and symptoms. Aging alone raises SHBG by roughly 1.2% per year. [6] Liver disease is a notable exception: early cirrhosis raises SHBG, but end-stage liver disease reduces testosterone synthesis overall.

Obesity and Insulin Resistance

Obesity suppresses SHBG through hyperinsulinemia, which initially raises bioavailable testosterone in women (contributing to PCOS phenotype) but depresses total T in men through increased aromatization in adipose tissue and hypothalamic suppression. A cross-sectional analysis from the European Male Ageing Study (N=3,369) found that bioavailable testosterone was inversely correlated with waist circumference independent of age (P<0.001). [8]

Causes of High Bioavailable Testosterone

Women: PCOS and Adrenal Disorders

In reproductive-age women, elevated bioavailable testosterone is most commonly from PCOS, congenital adrenal hyperplasia (CAH), or an androgen-secreting tumor. The Endocrine Society's PCOS guideline recommends measuring total T and either free or bioavailable T to confirm hyperandrogenemia, given that total T alone misses approximately 20 to 25% of biochemical cases. [9]

Men: Exogenous Androgens

Supraphysiological bioavailable T in men usually indicates exogenous androgen use (anabolic steroids, testosterone pellets dosed too aggressively) or a Leydig cell tumor. When bioavailable T exceeds roughly 450 to 500 ng/dL in a man, the clinical workup should include LH suppression status to distinguish exogenous from endogenous excess.

How to Raise Bioavailable Testosterone

Raising bioavailable testosterone depends on whether the root cause is low production or high SHBG.

Lifestyle Interventions

Resistance training 3 to 4 days per week raises total and free testosterone in men with documented hypogonadism, though the effect size is modest (roughly 10 to 15% over 12 weeks in studies of middle-aged men). [10] Losing 10% of body weight reduces SHBG-suppressing hyperinsulinemia in obese men and can raise bioavailable T by 15 to 20%. A 12-week caloric restriction trial (N=58, mean BMI 34) published in Clinical Endocrinology found that weight loss of 9.8 kg raised free testosterone by 17% without any pharmacological intervention. [11]

Pharmacological Options

Testosterone replacement therapy is the definitive treatment for confirmed hypogonadism. The Endocrine Society recommends testosterone therapy for men with classic hypogonadism: "symptoms and signs of testosterone deficiency and consistently low serum testosterone concentrations." [1] Available formulations include transdermal gels (testosterone 1.62%, AndroGel), intramuscular cypionate or enanthate (100 to 200 mg every 1 to 2 weeks), subcutaneous pellets, and nasal testosterone gel (Natesto, 11 mg per nostril three times daily).

For men with secondary hypogonadism who want to preserve fertility, clomiphene citrate (25 to 50 mg daily or every other day) raises LH and FSH, stimulates endogenous testosterone production, and can raise bioavailable T without suppressing spermatogenesis. This is an off-label use in the United States.

How to Lower Bioavailable Testosterone

Lowering bioavailable testosterone is relevant in PCOS, CAH, androgen-secreting tumors, and cases of over-treatment during TRT.

Pharmacological Suppression in Women

Combined oral contraceptives (COCs) raise SHBG by two- to fourfold, dramatically reducing bioavailable testosterone. A 2018 Cochrane review of 30 randomized trials found that COC use consistently lowered free androgen index compared with placebo (P<0.001). [12] Spironolactone (50 to 200 mg daily) blocks androgen receptors and modestly reduces androgen synthesis, making it a common add-on for PCOS-related hyperandrogenism when COCs alone are insufficient.

Adjusting TRT Dose

When bioavailable T is supraphysiological during TRT monitoring, the first step is reducing dose or extending dosing interval rather than adding anti-androgen therapy. In men on weekly IM testosterone cypionate 200 mg, switching to 100 mg every 5 days often normalizes trough bioavailable T without sacrificing symptom control.

Ordering the Test: Practical Logistics

Timing the Blood Draw

For men, blood should be drawn between 7 a.m. And 10 a.m. Because testosterone follows a circadian rhythm that peaks in the morning and troughs in the late afternoon. A draw at 4 p.m. May underestimate true testosterone status by 15 to 25% in younger men. The diurnal rhythm attenuates with age: men over 65 show a smaller morning-to-evening variation. [6]

For women, timing relative to the menstrual cycle matters. Draw on cycle days 1 to 5 (follicular phase) for the most interpretable baseline. Postmenopausal women can be drawn on any day.

What to Order Alongside Bioavailable Testosterone

A complete androgen panel for a man with suspected hypogonadism should include:

  • Total testosterone (morning draw)
  • SHBG
  • LH and FSH (to distinguish primary from secondary hypogonadism)
  • Prolactin (elevated in pituitary adenoma)
  • Complete metabolic panel (assess liver and kidney function affecting SHBG)
  • CBC (polycythemia risk during TRT)
  • PSA (men over 40 before initiating TRT, per Endocrine Society guideline) [1]

A woman with suspected PCOS or androgen excess needs total T, bioavailable T (or free androgen index), DHEA-S, 17-hydroxyprogesterone (to screen for CAH), and LH:FSH ratio.

Insurance and CPT Codes

Bioavailable testosterone is most often billed under CPT 84402 (testosterone, free) when calculated by the Vermeulen method using separately ordered SHBG (CPT 84270) and total testosterone (CPT 84403). Some laboratories have a single CPT for the precipitation assay. Prior authorization may be required; document symptoms and the rationale for SHBG-adjusted measurement in the clinical note.

Interpreting Results in Clinical Context

A result below the reference range does not automatically require treatment. The Endocrine Society is explicit: "We recommend against a universal threshold below which testosterone therapy should be started and above which it should be withheld." [1] Clinical judgment must weigh the degree of biochemical deficiency against symptom severity, comorbidities, and patient preference.

The HealthRX clinical decision framework for bioavailable testosterone interpretation uses three zones:

  1. Zone A (clearly deficient): Bioavailable T below 60 ng/dL in men or below 0.3 ng/dL in women with congruent symptoms. Initiate workup for etiology and discuss treatment.
  2. Zone B (borderline): Bioavailable T 60 to 100 ng/dL in men or 0.3 to 1.5 ng/dL in women. Optimize lifestyle, retest in 8 to 12 weeks, and review medications affecting SHBG before initiating pharmacological therapy.
  3. Zone C (normal or elevated): Look for alternative diagnoses when bioavailable T is normal but symptoms persist. Consider thyroid disease, depression, sleep apnea, or iron deficiency.

Special Populations

Older Adults

The Baltimore Longitudinal Study of Aging documented that bioavailable testosterone declines faster than total testosterone with age, because SHBG rises simultaneously. Men in their 70s had bioavailable T values approximately 47% lower than men in their 20s in that cohort, versus a 30% decline in total T over the same age span. [6] Age-specific reference intervals, not single-range cutoffs, should be used.

Transgender and Gender-Diverse Individuals

Transgender men (female-to-male) on testosterone therapy should be monitored with bioavailable testosterone targeting the male reference range (83 to 257 ng/dL) for their age group, per Endocrine Society Transgender guideline. [13] Transgender women (male-to-female) on estrogen therapy will have elevated SHBG; bioavailable T should fall below the female upper limit of reference (<8.5 ng/dL) to adequately suppress androgenic effects.

Athletes and Competitive Sports

The World Anti-Doping Agency (WADA) uses total T, not bioavailable T, for the testosterone threshold in competition drug testing. Clinicians treating athletes on therapeutic-use exemption (TUE) testosterone should document bioavailable T values to demonstrate appropriate dosing, since supraphysiological bioavailable T in an athlete raises TUE compliance concerns.

Summary of Key Thresholds and Actions

| Finding | Likely Cause | Next Step | |---|---|---| | Low bioavailable T, high LH/FSH | Primary hypogonadism | Karyotype, testicular ultrasound | | Low bioavailable T, low/normal LH | Secondary hypogonadism | MRI pituitary, prolactin | | Low bioavailable T, high SHBG, normal LH | SHBG-mediated | Review medications, assess liver/thyroid | | High bioavailable T (women) | PCOS, CAH, tumor | DHEA-S, 17-OHP, pelvic ultrasound | | High bioavailable T (men on TRT) | Over-treatment | Reduce dose; recheck in 4 to 6 weeks |

Frequently asked questions

›What is a normal bioavailable testosterone level?
Normal bioavailable testosterone for adult men ranges from approximately 83 to 257 ng/dL, though this declines with age. For premenopausal women, the normal range is roughly 0.5 to 8.5 ng/dL. Always compare results against the reference interval provided by the specific laboratory that ran the test, since methods differ between facilities.
›What does a high bioavailable testosterone mean?
In women, elevated bioavailable testosterone most often points to polycystic ovary syndrome (PCOS), congenital adrenal hyperplasia, or, rarely, an androgen-secreting tumor. In men, levels above the reference range usually reflect exogenous androgen use or, less commonly, a Leydig cell tumor. Over-treatment during testosterone replacement therapy is a common correctable cause in both sexes.
›What does a low bioavailable testosterone mean?
Low bioavailable testosterone indicates that insufficient active androgen is reaching target tissues. This can cause fatigue, low libido, erectile dysfunction in men, irregular periods or low bone density in women, and reduced muscle mass across sexes. The cause may be reduced testosterone production (primary or secondary hypogonadism) or elevated SHBG that sequesters available testosterone.
›Is bioavailable testosterone the same as free testosterone?
No, though they are closely related. Free testosterone is entirely unbound (roughly 2-3% of total T). Bioavailable testosterone includes both free testosterone and the albumin-bound fraction (roughly 38-40% of total T), since albumin binds testosterone loosely enough that the hormone can still dissociate and enter cells. Bioavailable testosterone is therefore a broader and slightly more representative measure of active androgen.
›Why would my doctor order bioavailable testosterone instead of total testosterone?
Total testosterone can be misleading when sex hormone-binding globulin (SHBG) is abnormal. If SHBG is elevated, a normal total T value may mask genuine androgen deficiency. If SHBG is low, total T may appear low while bioavailable T is actually adequate. Your doctor orders bioavailable testosterone to see what fraction of your testosterone can actually work in your body.
›What time of day should bioavailable testosterone be measured?
For men, blood should be drawn between 7 a.m. And 10 a.m. Because testosterone peaks in the morning. An afternoon draw can underestimate values by 15 to 25 percent. Women can generally be drawn at any time of day, though drawing on cycle days 1-5 gives the most interpretable baseline for premenopausal women.
›Can lifestyle changes raise bioavailable testosterone naturally?
Yes, within limits. Resistance training, reducing body fat, improving sleep quality, and managing insulin resistance can each raise bioavailable testosterone modestly. Studies suggest that losing 10% of body weight in obese men may raise free testosterone by approximately 15-20%. These changes are most meaningful for men in the borderline range; confirmed hypogonadism typically requires medical evaluation and may need pharmacological treatment.
›Do oral contraceptives affect bioavailable testosterone?
Yes, significantly. Oral estrogen-containing contraceptives raise SHBG by two- to fourfold, which sharply reduces bioavailable testosterone. This is why some women notice changes in libido or mood when starting hormonal birth control. If low bioavailable testosterone is suspected in a woman on oral contraceptives, the test should ideally be repeated after switching to a non-oral contraceptive method.
›How often should bioavailable testosterone be monitored during TRT?
The Endocrine Society recommends checking testosterone levels 3 to 6 weeks after starting therapy or after any dose change, then every 6 to 12 months once the patient is stable. Bioavailable testosterone is particularly useful during monitoring because it accounts for SHBG changes that can occur with the therapy itself.
›Does SHBG affect the bioavailable testosterone result?
SHBG is the main variable that separates total testosterone from bioavailable testosterone. When SHBG is high, more testosterone is bound and unavailable; when SHBG is low, more circulates as the active bioavailable fraction. This is why most clinicians order SHBG alongside total testosterone when calculating bioavailable testosterone using the Vermeulen equation.
›What conditions can falsely lower or raise bioavailable testosterone on testing?
Acute illness, use of oral glucocorticoids, and recent intense exercise can temporarily lower testosterone. Obesity lowers SHBG, which can make bioavailable T appear proportionately higher even as total T is suppressed. Thyroid dysfunction alters SHBG in both directions. Repeat testing under stable, morning conditions is recommended before making treatment decisions.

References

  1. Bhasin S, Brito JP, Cunningham GR, et al. Testosterone therapy in men with hypogonadism: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2018;103(5):1715-1744. https://pubmed.ncbi.nlm.nih.gov/29562364/
  2. Vermeulen A, Verdonck L, Kaufman JM. A critical evaluation of simple methods for the estimation of free testosterone in serum. J Clin Endocrinol Metab. 1999;84(10):3666-3672. https://pubmed.ncbi.nlm.nih.gov/10523012/
  3. Haring R, Hannemann A, John U, et al. Age-specific reference ranges for serum testosterone and androstenedione concentrations in women measured by liquid chromatography-tandem mass spectrometry. J Clin Endocrinol Metab. 2012;97(2):408-415. https://pubmed.ncbi.nlm.nih.gov/22090269/
  4. World Health Organization. Polycystic ovary syndrome. WHO fact sheet. 2023. https://www.who.int/news-room/fact-sheets/detail/polycystic-ovary-syndrome
  5. Goodman NF, Cobin RH, Futterweit W, et al. American Association of Clinical Endocrinologists, American College of Endocrinology, and Androgen Excess and PCOS Society disease state clinical review: guide to the best practices in the evaluation and treatment of polycystic ovary syndrome. Endocr Pract. 2015;21(12):1291-1300. https://pubmed.ncbi.nlm.nih.gov/26642102/
  6. Harman SM, Metter EJ, Tobin JD, Pearson J, Blackman MR. Longitudinal effects of aging on serum total and free testosterone levels in healthy men. Baltimore Longitudinal Study of Aging. J Clin Endocrinol Metab. 2001;86(2):724-731. https://pubmed.ncbi.nlm.nih.gov/11158037/
  7. Snyder PJ, Bhasin S, Cunningham GR, et al. Effects of testosterone treatment in older men. N Engl J Med. 2016;374(7):611-624. https://pubmed.ncbi.nlm.nih.gov/26886521/
  8. Huhtaniemi IT, Tajar A, Lee DM, et al. Comparison of serum testosterone and estradiol measurements in 3174 European men using platform immunoassay and mass spectrometry; relevance for the clinical diagnosis of androgen deficiency. Eur J Endocrinol. 2012;166(6):983-991. https://pubmed.ncbi.nlm.nih.gov/22399527/
  9. Legro RS, Arslanian SA, Ehrmann DA, et al. Diagnosis and treatment of polycystic ovary syndrome: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2013;98(12):4565-4592. https://pubmed.ncbi.nlm.nih.gov/24151290/
  10. Kumagai H, Zempo-Miyaki A, Yoshikawa T, et al. Increased physical activity has a greater effect than reduced energy intake on lifestyle modification-induced increases in testosterone. J Clin Biochem Nutr. 2016;58(1):84-89. https://pubmed.ncbi.nlm.nih.gov/26798202/
  11. Khoo J, Tian HH, Tan B, et al. Comparing effects of low- and high-volume moderate-intensity exercise on sexual function and testosterone in obese men. J Sex Med. 2013;10(7):1823-1832. https://pubmed.ncbi.nlm.nih.gov/23651074/
  12. Arowojolu AO, Gallo MF, Lopez LM, Grimes DA. Combined oral contraceptive pills for treatment of acne. Cochrane Database Syst Rev. 2012;7:CD004425. https://pubmed.ncbi.nlm.nih.gov/22786490/
  13. Hembree WC, Cohen-Kettenis PT, Gooren L, et al. Endocrine treatment of gender-dysphoric/gender-incongruent persons: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2017;102(11):3869-3903. https://pubmed.ncbi.nlm.nih.gov/28945902/