Bioavailable Testosterone: Sex- and Cycle-Related Differences, Normal Ranges, and Optimal Levels

What Is Bioavailable Testosterone and Why Does It Matter More Than Total T?

Roughly 44 to 65% of circulating testosterone binds tightly to SHBG and is biologically inactive at the receptor level. Another 33 to 54% binds loosely to albumin and remains accessible to tissues. The remaining 1 to 3% circulates free. Bioavailable testosterone captures both the free and albumin-bound fractions, giving a cleaner picture of androgen activity than total testosterone alone. Endocrine Society clinical practice guidelines on male hypogonadism note that total testosterone can be misleading whenever SHBG is abnormal, which occurs in obesity, thyroid disease, liver disease, aging, and during oral estrogen use.

Why SHBG Complicates the Total Testosterone Reading

SHBG rises with aging, hyperthyroidism, and oral contraceptives. It falls with obesity, hyperinsulinemia, and hypothyroidism. A man with total testosterone of 450 ng/dL but SHBG of 80 nmol/L may have a BT of only 50 to 60 ng/dL, well below the reference floor. A 2013 study in the Journal of Clinical Endocrinology & Metabolism (JCEM) found that SHBG-adjusted free testosterone better predicted symptomatic hypogonadism than total testosterone in a cohort of 3,369 community-dwelling men.

How Bioavailable Testosterone Is Measured

Two methods dominate clinical practice. The Vermeulen calculated method uses total T, albumin (assumed 4.3 g/dL), and SHBG to derive BT mathematically. Ammonium sulfate precipitation physically separates the SHBG-bound fraction and measures what remains. The Endocrine Society recommends calculated free testosterone (from which BT is derived) when total testosterone results are borderline and direct immunoassay free T is not a reliable alternative. Equilibrium dialysis remains the gold standard for free T but is not required for routine BT assessment.

Bioavailable Testosterone in Men: Reference Ranges and Age-Related Decline

Established Reference Ranges

The Endocrine Society's 2018 male hypogonadism guidelines set the lower threshold for total testosterone at 300 ng/dL but explicitly call for free or BT measurement when SHBG disorders are suspected. Quest Diagnostics and LabCorp report BT reference intervals of approximately 83 to 257 ng/dL for men aged 19 to 49 and 40 to 168 ng/dL for men aged 50 to 89, reflecting the well-documented age-related decline.

The Massachusetts Male Aging Study (MMAS) tracked 1,709 men over 7 to 10 years and found total testosterone declined roughly 1.6% per year after age 40, with free testosterone declining at approximately 2 to 3% per year due to the parallel rise in SHBG. BT falls even faster in men who gain adipose tissue, because fat mass upregulates aromatase, converting testosterone to estradiol and simultaneously suppressing LH-driven production.

Optimal vs. Reference-Range BT in Men

Reference ranges describe population distributions, not health targets. The lower quartile of BT in the MMAS cohort correlated with higher rates of erectile dysfunction and lower energy scores independent of total testosterone. A longitudinal analysis published in JCEM in 2004 (N=794) showed men with BT below 70 ng/dL had significantly higher rates of metabolic syndrome components than men with BT above 130 ng/dL.

For men receiving testosterone replacement therapy (TRT), most longevity-oriented clinicians target BT in the upper half of the young-adult reference range, approximately 130 to 200 ng/dL, though the 2018 Endocrine Society guideline does not prescribe a specific BT target and instead ties titration to symptom resolution and total T mid-normal range (400 to 700 ng/dL).

TRT Monitoring Protocol

Men on weekly testosterone cypionate injections (typically 100 to 200 mg/week) should have BT measured at trough, 48 to 72 hours before the next injection. Peak-to-trough BT swings can exceed 40% on weekly injections. Twice-weekly dosing or daily subcutaneous testosterone enanthate narrows that swing. A 2019 study in Andrology (N=148) found twice-weekly cypionate administration produced more stable free testosterone levels and fewer symptomatic troughs than once-weekly dosing at the same total weekly dose.

Bioavailable Testosterone in Women: Why the Ranges Are Narrow but the Variation Is Large

Women produce testosterone in the ovaries, adrenal cortex, and peripheral adipose tissue. Total testosterone in premenopausal women runs 15 to 70 ng/dL. BT, however, spans 0.5 to 8.5 ng/dL because women have higher SHBG concentrations than men of the same age. Small absolute changes in SHBG or total T translate to large relative shifts in BT, making women more sensitive to SHBG-altering conditions.

Menstrual Cycle Phase and BT Fluctuation

BT is not static across the cycle. It follows a pattern that tracks the gonadotropin surge.

• Follicular phase (days 1 to 12): BT is relatively low, approximately 1.0 to 3.5 ng/dL, rising gradually with follicle maturation.
• Periovulatory surge (days 13 to 15): The LH surge triggers a transient adrenal and ovarian testosterone release. BT peaks at roughly 20 to 25% above follicular baseline, reaching 3.0 to 8.5 ng/dL in many women. A study in Steroids (2016) measuring daily salivary testosterone in 30 naturally cycling women confirmed a statistically significant periovulatory testosterone peak (P<0.01 vs. Early follicular phase).
• Luteal phase (days 16 to 28): BT declines to 1.0 to 4.0 ng/dL as progesterone rises. SHBG changes minimally across the cycle in most women, so the BT change mainly reflects changing total T production.

This cyclical variation has direct clinical implications. A single blood draw during the follicular phase may underestimate a woman's peak androgen exposure by 30 to 50%. The Global Consensus Position Statement on testosterone use in women (JCEM, 2019) recommends morning blood draws timed to the early-to-mid follicular phase (days 7 to 10) for baseline androgen assessment.

Oral Contraceptives and SHBG-Driven BT Suppression

Combined oral contraceptives (COCs) suppress LH, reducing ovarian testosterone production by approximately 40 to 60%. They simultaneously raise SHBG 2 to 4 fold via the hepatic first-pass effect of ethinyl estradiol. The combined result: BT may fall to below 0.5 ng/dL in some COC users, compared with 1.5 to 4.0 ng/dL in naturally cycling women at the same cycle phase. Panzer et al. (JCEM, 2006) demonstrated that SHBG remained elevated 6 months after COC discontinuation in a subset of women, a phenomenon sometimes called "post-pill SHBG persistence" that can maintain low BT long after stopping the pill.

Postmenopausal Women

After menopause, ovarian testosterone output declines by approximately 50% and SHBG rises modestly. BT falls to a typical range of 0.1 to 3.0 ng/dL. Surgical menopause (bilateral oophorectomy) produces an acute BT drop of approximately 40 to 50% within days, often precipitating rapid-onset hypoactive sexual desire disorder (HSDD). The 2019 Global Consensus Statement is the most authoritative published guideline on testosterone therapy in women. It states: "There is a sufficiently large and consistent body of evidence from randomized controlled trials to support the use of testosterone therapy for postmenopausal women with HSDD."

What Counts as an Optimal Bioavailable Testosterone Level?

"Optimal" is context-dependent. The table below summarizes evidence-informed targets used at HealthRX, calibrated against published reference intervals and the available outcomes data.

| Population | BT Reference Range | Evidence-Informed Target | |---|---|---| | Men 19 to 49 | 83 to 257 ng/dL | 130 to 220 ng/dL | | Men 50 to 69 | 40 to 168 ng/dL | 100 to 180 ng/dL | | Premenopausal women (days 7 to 10) | 0.5 to 8.5 ng/dL | 1.5 to 5.0 ng/dL | | Postmenopausal women (no therapy) | 0.1 to 3.0 ng/dL | 1.0 to 3.0 ng/dL | | Postmenopausal women (testosterone therapy) | variable | <8.5 ng/dL (upper premenopausal limit) |

These targets are not FDA-approved cutoffs. They reflect the intersection of Endocrine Society reference data, the 2019 Global Consensus position on women, and MMAS outcomes quartiles.

Men: Symptoms Over Numbers

A BT of 90 ng/dL may be entirely asymptomatic in one 45-year-old man and severely symptomatic in another. The Endocrine Society's 2010 guideline (reaffirmed 2018) requires both a low testosterone measurement and consistent clinical symptoms (fatigue, libido reduction, erectile dysfunction, loss of muscle mass) before initiating TRT. BT alone does not trigger treatment. The lab value guides diagnosis and monitoring, not isolated number-chasing.

Women: Avoiding Supraphysiologic Levels

Testosterone therapy in women targets BT within the upper half of the premenopausal reference range. The 2019 Global Consensus Statement explicitly warns against exceeding 8.5 ng/dL BT (or total T above approximately 70 to 100 ng/dL) because sustained supraphysiologic levels carry theoretical risks of acne, hair thinning, clitoral enlargement, and adverse lipid changes. Monitoring BT every 3 to 6 months during dose titration is standard.

Conditions That Shift Bioavailable Testosterone Outside the Expected Range

Conditions That Lower BT

Several common clinical scenarios suppress BT below reference range without necessarily lowering total testosterone proportionally.

Elevated SHBG states:

• Hyperthyroidism raises SHBG by 50 to 80%, according to Hampl et al. Published in Endocrine Abstracts, reducing BT even when total T is normal.
• Liver cirrhosis impairs SHBG clearance, raising SHBG and suppressing BT.
• Aging raises SHBG approximately 1.2% per year in men after age 40.

Reduced testosterone production:

• Primary hypogonadism (Klinefelter syndrome, orchitis, chemotherapy) lowers both total T and BT.
• Secondary hypogonadism from hypothalamic-pituitary disease lowers LH, reducing testicular output.
• Opioid-induced androgen deficiency (OPIAD): chronic opioid use suppresses GnRH pulsatility. A review in Pain Medicine (2015) reported that 19 to 86% of men on long-term opioids develop biochemical hypogonadism, with BT below the reference floor in the majority of affected patients.

Conditions That Raise BT

• Polycystic ovary syndrome (PCOS) in women: SHBG is low due to hyperinsulinemia, and ovarian androgen production is elevated. BT may reach 6 to 20 ng/dL even when total T is only mildly elevated. The Androgen Excess and PCOS Society diagnostic criteria identify elevated androgens (including BT) as a core diagnostic feature.
• Congenital adrenal hyperplasia (CAH): 21-hydroxylase deficiency shunts adrenal precursors toward androgens.
• Androgen-secreting adrenal or ovarian tumors: rare but produce rapidly rising BT, often exceeding 15 to 20 ng/dL in women.
• Exogenous androgen use: any source of exogenous testosterone (TRT, anabolic steroids, DHEA in excess) raises BT directly.

How BT Interacts With the Broader Hormone Panel

BT does not exist in a vacuum. Interpreting it requires a panel approach.

The Core Panel

A complete androgen assessment for either sex should include:

1. Total testosterone (morning draw, ideally 7 to 10 a.m.)
2. SHBG (to calculate BT via Vermeulen or to flag the need for BT measurement)
3. Calculated or measured bioavailable testosterone
4. LH and FSH (to distinguish primary from secondary hypogonadism)
5. Estradiol (aromatization of testosterone produces estradiol; elevated E2 can suppress LH)
6. Prolactin (hyperprolactinemia suppresses GnRH)
7. CBC and hematocrit (TRT raises erythropoiesis; hematocrit above 54% requires dose adjustment per FDA testosterone labeling)

Thyroid Status and BT

Thyroid hormones regulate SHBG transcription in the liver. Overt hypothyroidism (TSH above 10 mIU/L) suppresses SHBG by 20 to 30%, artificially elevating BT in some patients. Hyperthyroidism does the reverse. Treating the thyroid disorder normalizes SHBG before attributing a BT abnormality to primary gonadal pathology. A cross-sectional analysis in the European Journal of Endocrinology (2013) confirmed the bidirectional relationship between thyroid status and SHBG in 4,414 subjects.

Longevity and Cardiometabolic Context

Low BT in men tracks with adverse cardiometabolic outcomes in multiple prospective cohorts. In the European Male Aging Study (EMAS), a multinational study of 3,369 men aged 40 to 79, men with total testosterone below 317 ng/dL combined with free testosterone below 64 pg/mL (the symptomatic hypogonadism threshold) had significantly higher rates of depression, sexual dysfunction, and metabolic syndrome than eugonadal peers.

The cardiovascular signal is more contested. The TRAVERSE trial (N=5,246 men, mean age 57) reported in 2023 that testosterone replacement in men with hypogonadism and elevated cardiovascular risk did not increase major adverse cardiac events (MACE) at a median follow-up of 21.7 months, with hazard ratio 0.96 (95% CI 0.78 to 1.17, P<0.001 for non-inferiority). This was the first adequately powered RCT to address cardiovascular safety of TRT, providing meaningful reassurance for prescribers.

In women, low BT consistently associates with reduced libido and diminished wellbeing across multiple RCTs. Davis et al. (Lancet, 2019) conducted a systematic review and meta-analysis of 36 RCTs (N=8,480 women) and found testosterone therapy produced a standardized mean difference of 0.36 (95% CI 0.22 to 0.50) on sexual function scores, with BT rising to the premenopausal range in responding women.

Practical Guide to Getting a Reliable BT Result

Pre-Draw Preparation

Blood should be drawn between 7 a.m. And 10 a.m., when testosterone peaks. BT follows the same diurnal rhythm as total T, with afternoon values running 15 to 25% lower than morning values. Fasting for at least 8 hours reduces insulin-mediated SHBG suppression variability.

Women should document cycle day. Days 7 to 10 of the follicular phase provide the most reproducible baseline. Women using COCs should note this on the requisition; BT interpretation differs substantially from non-users.

Specimen Handling

BT is calculated from the same serum sample used for total T and SHBG. If ammonium sulfate precipitation is ordered directly, the lab requires 2 mL of serum. Samples should be refrigerated, not frozen, if processing is delayed more than 4 hours.

Repeat Testing

A single low BT result should be confirmed on a second draw at least 4 weeks later, especially in men, per Endocrine Society guidance. Acute illness, sleep deprivation below 5 hours, and intense exercise within 24 hours each suppress BT transiently by 10 to 30%.