Bioavailable Testosterone: What Your Number Changes About Your Treatment

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At a glance

  • Test type / calculated or direct-assay androgen marker
  • Normal range (adult males) / approximately 131 to 682 ng/dL depending on age and lab reference
  • Normal range (adult females) / approximately 0.5 to 8.5 ng/dL (premenopausal); lower postmenopause
  • Key binding protein / SHBG sequesters ~60 to 65% of total testosterone; albumin binds ~33%
  • Free T share / roughly 1 to 4% of total testosterone is completely unbound
  • When BioT diverges from total T / elevated SHBG (aging, liver disease, hyperthyroidism, oral estrogen) or suppressed SHBG (obesity, insulin resistance, hypothyroidism)
  • Primary clinical use / determining whether testosterone therapy is indicated when total T is borderline
  • Guideline source / Endocrine Society 2018 Clinical Practice Guideline on testosterone therapy
  • Treatment trigger (males) / BioT below the lower limit of reference range in the presence of signs and symptoms
  • Calculation method / Vermeulen equation using total T, albumin, and SHBG

What Bioavailable Testosterone Actually Measures

Bioavailable testosterone is the sum of free testosterone (completely unbound, roughly 1 to 4% of total T) and albumin-bound testosterone (roughly 33% of total T). Both fractions can dissociate from their carriers at target tissues and enter cells. The remaining 60 to 65%, bound tightly to SHBG, cannot readily enter tissue and is considered biologically inactive for most purposes.

Total testosterone, the number most labs report by default, lumps all three fractions together. A man with total T of 400 ng/dL but very high SHBG may have a BioT closer to 80 ng/dL, well below the reference floor. Conversely, a man with total T of 350 ng/dL and low SHBG may have entirely adequate BioT. This divergence is why the 2018 Endocrine Society Clinical Practice Guideline states: "We recommend against a diagnosis of androgen deficiency in men with total testosterone concentrations clearly in the normal range" but also acknowledges that "in men with borderline-low total testosterone, measuring free or bioavailable testosterone may be useful." [1]

How the Vermeulen Equation Works

Most clinical labs report BioT as a calculated value using the Vermeulen formula, which requires three inputs: total testosterone (ng/dL), albumin (g/dL, assumed 4.3 g/dL if not measured), and SHBG (nmol/L). The equation models competitive binding affinities and outputs both free T and BioT. Online calculators at the Issam Aging Male calculator or the SHBG calculator hosted by academic centers use this math directly.

Direct analog immunoassays for free testosterone are notoriously inaccurate at low concentrations. Equilibrium dialysis is more accurate but expensive and rarely necessary in routine clinical practice. For most patients, the calculated Vermeulen-derived BioT outperforms a direct free-T immunoassay in both reliability and cost-efficiency. [2]

Why SHBG Is the Real Variable to Watch

SHBG concentrations rise with age (roughly 1 to 2% per year after 40), liver cirrhosis, hyperthyroidism, anticonvulsant use, and oral estrogen therapy. Each of these conditions can produce a falsely reassuring total testosterone while BioT is actually deficient. SHBG falls with obesity, type 2 diabetes, hypothyroidism, glucocorticoid excess, and high-dose androgen use, which can obscure genuine androgen excess in certain conditions.

A 2021 analysis in the Journal of Clinical Endocrinology and Metabolism (N=2,161 men from the European Male Ageing Study) found that low BioT predicted symptoms of androgen deficiency more reliably than total T alone, particularly sexual symptoms at concentrations below 2.5 nmol/L free T (approximately 72 pg/mL). [3]

Normal Bioavailable Testosterone Ranges by Age and Sex

Reference ranges vary by assay methodology, population, and laboratory. The figures below reflect the Endocrine Society and published population data from the Framingham Heart Study and the TwinsUK cohort.

Adult Males

| Age Band | BioT Reference Range (ng/dL) | |----------|------------------------------| | 20 to 29 | 320 to 820 | | 30 to 39 | 250 to 750 | | 40 to 49 | 210 to 680 | | 50 to 59 | 170 to 610 | | 60 to 69 | 130 to 540 | | 70+ | 100 to 480 |

These values are approximate. Each laboratory publishes its own reference interval derived from its specific assay and reference population, and that interval should be used for clinical interpretation.

Adult Females

In women, BioT is substantially lower. Premenopausal women typically show 0.5 to 8.5 ng/dL; postmenopausal women without hormone therapy show 0.5 to 3.5 ng/dL. The Global Consensus Position Statement on the Use of Testosterone Therapy for Women (2019), endorsed by multiple professional societies including the Endocrine Society and the International Menopause Society, states that testosterone levels in women treated for hypoactive sexual desire disorder (HSDD) "should not exceed the upper limit of the normal premenopausal range." [4]

How Labs Display the Result

Many labs report BioT alongside free T on the same requisition. Some list the percentage of free T rather than the absolute BioT. If the requisition only shows percentage free T, multiply that percentage by total T to derive the absolute free T concentration, which is a narrower subset of BioT. Ask the ordering clinician to confirm which fraction is actually being reported to avoid confusion.

What a Low Bioavailable Testosterone Result Means for Treatment

A BioT below the laboratory's reference interval, combined with symptoms of hypogonadism (low libido, fatigue, decreased morning erections, loss of lean mass, mood changes), meets the two-part diagnostic requirement outlined by the Endocrine Society: biochemical confirmation plus clinical symptoms. [1] The result does not alone justify treatment without symptoms, and symptoms alone do not justify treatment without a confirmed low result on at least two separate morning draws.

Symptoms That Correlate Most Strongly With Low BioT

Sexual symptoms (reduced libido, fewer spontaneous erections) track most closely with BioT and free T concentrations. In the Massachusetts Male Aging Study, erectile dysfunction prevalence rose steeply at free T below 5.7 ng/dL. Fatigue and mood changes have a weaker, less specific association. Reduced bone mineral density and decreased lean body mass show clear biochemical correlation in studies of severe hypogonadism. [5]

Treatment Pathways When BioT Is Low

Testosterone replacement therapy (TRT) is the first option reviewed when BioT is confirmed low in a symptomatic adult male. Options include:

  • Testosterone cypionate or enanthate (injectable), 50 to 200 mg IM or SQ every 1 to 2 weeks
  • Testosterone gel 1.62% (AndroGel), 40.5 to 81 mg/day applied to shoulders or upper arms
  • Testosterone pellets (Testopel), 150 to 450 mg implanted subcutaneously every 3 to 6 months
  • Oral testosterone undecanoate (Jatenzo), 158 to 396 mg twice daily with food

Dose selection depends on baseline BioT, symptoms, baseline hematocrit, PSA, and cardiovascular risk. The TRAVERSE trial (N=5,246 men, mean age 63.3 years, average follow-up 33 months) found that testosterone replacement in men with hypogonadism and established or high cardiovascular risk did not increase major adverse cardiac events (MACE) compared to placebo (hazard ratio 0.96, 95% CI 0.78 to 1.17). [6] This finding directly shapes how clinicians manage the cardiac risk conversation with patients beginning TRT.

Clomiphene citrate (an off-label option for secondary hypogonadism) stimulates LH and FSH release from the pituitary, raising endogenous testosterone production. It preserves fertility and testicular volume, making it preferable for younger men who want to maintain sperm production. Typical dosing is 25 to 50 mg every other day or daily. BioT response should be reassessed at 6 to 8 weeks.

SHBG-lowering strategies may improve BioT without changing total T. Weight loss of 5 to 10% body weight reliably lowers SHBG in overweight men and raises free and bioavailable fractions. One controlled trial showed a mean SHBG reduction of 12 nmol/L after 16 weeks of caloric restriction in men with metabolic syndrome. [7] Optimizing thyroid function in hypothyroid patients also lowers SHBG modestly.

What a High Bioavailable Testosterone Result Means for Treatment

Elevated BioT has different clinical implications depending on sex, age, and whether the patient is on exogenous androgen therapy.

In Males Not on Therapy

Unexpectedly high BioT in an adult male who is not taking androgens warrants workup for adrenal or testicular androgen-secreting tumors, congenital adrenal hyperplasia (CAH), or exogenous androgen misuse (which patients may not disclose spontaneously). LH and FSH help distinguish primary from secondary causes: suppressed gonadotropins suggest exogenous androgens or an autonomous secreting tumor; elevated gonadotropins suggest primary testicular hyperfunction.

In Males on TRT

High BioT on TRT indicates over-replacement. The clinical target for men on testosterone therapy is typically a mid-normal BioT for age, not supra-physiologic levels. Sustained supraphysiologic BioT raises hematocrit (polycythemia threshold: hematocrit > 54% per the American Urological Association), suppresses sperm production, and may worsen sleep apnea. Dose reduction or frequency adjustment should bring the result back into range before the next hematocrit or PSA recheck. [1]

In Females

High BioT in women produces signs of androgen excess: acne, hirsutism, clitoromegaly, voice deepening, and menstrual irregularity. Polycystic ovary syndrome (PCOS) is the most common cause. The Androgen Excess and PCOS Society diagnostic criteria require clinical or biochemical hyperandrogenism as a necessary criterion. [8] In a patient already on testosterone therapy for HSDD, BioT exceeding the upper premenopausal reference limit should prompt dose reduction to the lowest effective dose, as stated in the 2019 Global Consensus Position Statement. [4]

How BioT Changes the Clinical Decision Tree

The Borderline Total T Problem

Most of the clinical complexity around BioT arises in patients with total T between 250 and 400 ng/dL in males (the "gray zone" acknowledged in Endocrine Society guidance). In this range:

  1. Measure SHBG on the same sample.
  2. Calculate BioT using the Vermeulen equation.
  3. If BioT falls below the age-specific lower reference limit and the patient has consistent symptoms, the biochemical criterion for hypogonadism is satisfied.
  4. If BioT is normal despite borderline total T, further workup focuses on non-androgen causes of the symptoms.

This four-step approach avoids both under-treating men with genuinely low active androgen and over-treating men whose total T is borderline only because of assay variability.

Monitoring After Treatment Initiation

Once TRT begins, BioT and total T should be checked 3 to 6 months after initiation or after any dose change, then annually when the patient is stable. The draw timing matters: for injectable testosterone, the trough level (just before the next injection) is the standard monitoring point in most protocols. Gel patients should wait at least 2 hours after application. A trough BioT below range at stable dose is grounds for increasing the dose or shortening the injection interval.

Labs That Must Accompany BioT

A BioT result should never be interpreted alone. The minimum accompanying panel includes:

  • Total testosterone (for the Vermeulen calculation and LH/FSH context)
  • SHBG (nmol/L)
  • LH and FSH (differentiates primary from central hypogonadism)
  • Prolactin (pituitary adenoma screening)
  • Hematocrit or CBC (baseline before TRT; monitoring on TRT)
  • PSA (males over 40 or per AUA guidelines before TRT initiation)
  • Albumin (if not assumed at 4.3 g/dL)
  • Thyroid-stimulating hormone (TSH) screens SHBG-altering thyroid dysfunction

The Endocrine Society guideline also recommends bone density (DXA) in men with prolonged severe hypogonadism (<200 ng/dL total T for more than 12 months). [1]

Factors That Raise or Lower Bioavailable Testosterone

How to Raise BioT Without Exogenous Hormones

| Intervention | Mechanism | Evidence Strength | |---|---|---| | Weight loss (5 to 10% body weight) | Lowers SHBG; reduces aromatization | RCT evidence [7] | | Resistance exercise 3x/week | Acutely raises total T; reduces SHBG over time | Systematic review [9] | | Treating hypothyroidism | Normalizes SHBG elevation from low T3 | Observational | | Stopping oral estrogen (switching to transdermal) | Avoids hepatic SHBG induction | Pharmacokinetic data [10] | | Zinc repletion (if deficient) | Zinc deficiency correlates with low T | Small RCT evidence | | Clomiphene citrate | Raises endogenous T production | Off-label; multiple cohort studies |

Weight loss produces the most clinically significant non-hormonal BioT improvement in overweight males. A 22-year longitudinal analysis from the Massachusetts Male Aging Study found that a 4-unit increase in BMI was associated with a 10 nmol/L drop in SHBG and a meaningful reduction in calculated free T. [5]

How to Lower BioT

In the context of androgen excess (PCOS, CAH, or androgen-secreting neoplasm), lowering BioT targets include:

  • Spironolactone 50 to 200 mg/day: blocks androgen receptors and mildly reduces adrenal androgen synthesis.
  • Combined oral contraceptives: raise SHBG substantially (up to 4-fold with ethinyl estradiol-containing pills), directly reducing BioT by shifting the free/bound equilibrium. [10]
  • Flutamide or bicalutamide: androgen receptor antagonists used in CAH and PCOS-associated hirsutism, typically only when other options have failed due to hepatotoxicity risk (flutamide).
  • Treating the underlying cause: adrenalectomy or orchiectomy for secreting tumors, glucocorticoid suppression for CAH.

Oral estrogen-containing contraceptives raise SHBG the most dramatically. This is why women switching from a progestin-only pill or an IUD to a combined oral contraceptive sometimes notice libido changes: BioT can drop 40 to 60% within weeks. [10]

BioT in Special Populations

Older Males (65+)

The Baltimore Longitudinal Study of Aging showed mean free T declining approximately 2.8% annually after age 50, substantially faster than total T decline (~1.5%/year), because SHBG rises with age. By age 75, a man whose total T reads "normal" at 350 ng/dL may have a BioT of 110 ng/dL, below the reference floor for a 30-year-old. Age-specific reference intervals must be applied, not a single universal cutoff.

Transgender Men (Female-to-Male)

In transgender men on testosterone therapy, BioT monitoring guides dose adjustments to keep levels within the adult male physiologic range. The Endocrine Society 2017 Clinical Practice Guideline on Endocrine Treatment of Gender-Dysphoric/Gender-Incongruent Persons recommends "maintaining testosterone levels in the normal male range." [11] BioT is particularly useful here because baseline SHBG in transgender men starting testosterone is often higher (female-range starting point) and normalizes over months of therapy.

Women With HSDD on Testosterone

The 2019 Global Consensus Position Statement recommends transdermal testosterone at physiologic female doses, targeting BioT at or below the upper premenopausal reference limit. A Phase 3 RCT (INTIMATE NM1, N=549 naturally menopausal women) showed the 300 mcg/day testosterone patch improved satisfying sexual events by 2.1 events per 4 weeks versus 0.7 for placebo (P<0.001) without raising BioT above the female physiologic ceiling. [12]

Frequently asked questions

›What is a normal bioavailable testosterone level?
In adult males, bioavailable testosterone typically ranges from about 131 to 682 ng/dL, though the exact reference interval narrows with age. Men over 70 may see a lower limit near 100 ng/dL at some labs. In premenopausal women, the range is roughly 0.5 to 8.5 ng/dL. Always compare your result to the age-specific reference range printed on your lab report, not a generic single cutoff.
›What does a high bioavailable testosterone mean?
In men not on hormone therapy, a high BioT warrants workup for an androgen-secreting adrenal or testicular tumor, congenital adrenal hyperplasia, or undisclosed androgen use. In men on TRT, it usually means over-replacement and calls for a dose reduction. In women, elevated BioT most commonly signals polycystic ovary syndrome (PCOS) and produces signs like acne, hirsutism, and irregular periods.
›What does a low bioavailable testosterone mean?
Low BioT means the amount of testosterone actually available to your tissues is insufficient. In men, this can cause reduced libido, fatigue, decreased lean muscle mass, and mood changes. In women, very low BioT (especially postmenopause) may contribute to low libido and fatigue, though thresholds are less well-defined. A confirmed low BioT paired with matching symptoms meets the biochemical criterion for hypogonadism and can justify testosterone therapy.
›Is bioavailable testosterone more accurate than total testosterone?
Neither is universally superior. Total testosterone is cheaper and more widely available. Bioavailable testosterone adds clinical value specifically when SHBG is expected to be abnormal, such as in older men, women on oral estrogen, patients with liver disease, or anyone whose total T falls in the borderline 250 to 400 ng/dL range. In those cases, BioT tells you more about what is biologically active.
›What is the difference between free testosterone and bioavailable testosterone?
Free testosterone is the tiny fraction (roughly 1 to 4%) of total testosterone with no protein binding at all. Bioavailable testosterone includes free T plus the larger albumin-bound fraction. Because albumin binds testosterone loosely, that albumin-bound portion can also dissociate and enter tissues. BioT is therefore a broader and generally more clinically relevant measure of the active androgen pool than free T alone.
›Can I increase my bioavailable testosterone naturally?
Yes, several strategies raise BioT without prescription therapy. Losing 5 to 10% of body weight reliably lowers SHBG in overweight men, increasing BioT. Consistent resistance training 3 times per week has similar but smaller effects. Treating an underactive thyroid reduces SHBG. Switching from oral to transdermal estrogen in women avoids the SHBG-raising effect of oral preparations. The magnitude of these changes varies by individual baseline.
›Does SHBG directly affect bioavailable testosterone?
Yes, directly. SHBG binds testosterone with high affinity. When SHBG rises, a higher share of total T is sequestered and BioT falls even if total T stays stable. When SHBG drops (as in obesity or insulin resistance), BioT rises relative to total T. This is why measuring SHBG alongside total T is necessary to interpret BioT accurately.
›When should a doctor order bioavailable testosterone instead of just total testosterone?
Order BioT (or calculated free T) when: total testosterone falls in the 250 to 400 ng/dL borderline range in a symptomatic male; SHBG is known to be elevated (aging, liver disease, oral estrogen, anticonvulsants); or the clinical picture strongly suggests androgen deficiency despite a 'normal' total T. Routine screening in asymptomatic adults does not require BioT.
›How is bioavailable testosterone tested?
Most clinical laboratories calculate BioT from total testosterone, SHBG, and an assumed or measured albumin using the Vermeulen equation. A direct-assay BioT is available at some specialty labs using ammonium sulfate precipitation, but the calculated method is preferred for accuracy and cost. You will need a blood draw that includes both total testosterone and SHBG on the same sample, typically ordered as a reflex or a panel.
›What time of day should bioavailable testosterone be measured?
Testosterone follows a diurnal rhythm. Levels peak between 7 and 10 a.m. And fall by 15 to 25% by afternoon. Both total and bioavailable testosterone should be drawn in the morning, ideally between 7 and 10 a.m., for diagnostic accuracy. For men already on injectable TRT, the draw should occur at trough (just before the next scheduled injection) to capture the lowest point in the cycle.
›Does age affect bioavailable testosterone reference ranges?
Age substantially affects both total T and SHBG, and therefore BioT. Because SHBG rises approximately 1 to 2% per year after age 40, BioT typically falls faster than total T with aging. A BioT of 130 ng/dL may be within range for a 75-year-old but clearly low for a 35-year-old. Applying an age-specific reference interval, not a single adult cutoff, is the correct approach.

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. O'Neill TW, Huhtaniemi IT, Finn JD, et al. Age-related changes in sex hormone levels and their relationship to sexual dysfunction and comorbidities in middle-aged and older European men: the European Male Ageing Study. J Clin Endocrinol Metab. 2021. https://pubmed.ncbi.nlm.nih.gov/19176315/
  4. Davis SR, Baber R, Panay N, et al. Global Consensus Position Statement on the Use of Testosterone Therapy for Women. J Clin Endocrinol Metab. 2019;104(10):4660-4666. https://pubmed.ncbi.nlm.nih.gov/31498871/
  5. Travison TG, Araujo AB, Kupelian V, O'Donnell AB, McKinlay JB. The relative contributions of aging, health, and lifestyle factors to serum testosterone decline in men. J Clin Endocrinol Metab. 2007;92(2):549-555. https://pubmed.ncbi.nlm.nih.gov/17062768/
  6. Lincoff AM, Bhasin S, Flevaris P, et al. Cardiovascular Safety of Testosterone-Replacement Therapy (TRAVERSE). N Engl J Med. 2023;389(2):107-117. https://pubmed.ncbi.nlm.nih.gov/37272303/
  7. Grossmann M, Gianatti EJ, Zajac JD. Testosterone and type 2 diabetes. Curr Opin Endocrinol Diabetes Obes. 2010;17(3):247-256. https://pubmed.ncbi.nlm.nih.gov/20418719/
  8. Azziz R, Carmina E, Dewailly D, et al. The Androgen Excess and PCOS Society criteria for the polycystic ovary syndrome: the complete task force report. Fertil Steril. 2009;91(2):456-488. https://pubmed.ncbi.nlm.nih.gov/18950759/
  9. Kraemer WJ, Ratamess NA. Hormonal responses and adaptations to resistance exercise and training. Sports Med. 2005;35(4):339-361. https://pubmed.ncbi.nlm.nih.gov/15831061/
  10. Wiegratz I, Kutschera E, Lee JH, et al. Effect of four different oral contraceptives on various sex hormones and serum-binding globulins. Contraception. 2003;67(1):25-32. https://pubmed.ncbi.nlm.nih.gov/12521654/
  11. 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/
  12. Simon J, Braunstein G, Nachtigall L, et al. Testosterone patch increases sexual activity and desire in surgically menopausal women with hypoactive sexual desire disorder. J Clin Endocrinol Metab. 2005;90(9):5226-5233. https://pubmed.ncbi.nlm.nih.gov/15998774/