Bioavailable Testosterone: Longevity-Medicine Target Ranges

What Bioavailable Testosterone Actually Measures

Bioavailable testosterone is the sum of free testosterone and albumin-bound testosterone. Free T (roughly 1 to 3% of total) circulates unattached. Albumin-bound T (roughly 38 to 40%) binds loosely enough that tissues can strip it from albumin at the capillary level. The remaining 57 to 60% of testosterone is tightly bound to SHBG and is essentially inactive at target tissues.

This matters clinically because SHBG rises with age, liver disease, hyperthyroidism, and certain medications, including estrogen-containing contraceptives and some anticonvulsants. A man in his late 50s can have a total testosterone of 500 ng/dL and still experience androgen deficiency if his SHBG is 80 nmol/L, because nearly all his testosterone is locked away from receptors.

Free T vs. Bioavailable T: Which Should You Order?

Free testosterone by equilibrium dialysis is the gold-standard direct measurement, but it is expensive and not widely available outside reference laboratories. Calculated free T using the Vermeulen formula (requires total T, SHBG, and albumin) correlates well with dialysis-measured free T and is available at most commercial labs [1].

Bioavailable testosterone by ammonium sulfate precipitation adds albumin-bound T back into the equation. Both metrics outperform total T when SHBG is abnormal. For routine clinical use, calculated free T and calculated bioavailable T from the Vermeulen equation are acceptable and reproducible [1].

Why the Standard Lab Reference Range Is Not the Longevity Target

Standard reference ranges (e.g., 72 to 235 ng/dL for bioavailable T in men from Quest Diagnostics) are derived from population distributions, including many older, sedentary, metabolically unhealthy men. That math guarantees the lower end of "normal" is crowded with symptomatic individuals.

Longevity medicine practitioners distinguish between a population-normal range and a performance range. The goal is to sustain the androgen environment associated with preserved muscle, bone mineral density, insulin sensitivity, and cognitive function across the lifespan, not merely to avoid flagging low on a lab report.

Longevity-Medicine Target Ranges for Men

For men practicing longevity medicine, most evidence-informed clinicians target bioavailable testosterone between 150 and 350 ng/dL, with some protocols aiming toward the upper third of that range (250 to 350 ng/dL) for men over 50 who are actively strength training or managing metabolic disease.

What the Research Shows

The Massachusetts Male Aging Study tracked 1,709 men and found that bioavailable testosterone declined at approximately 2 to 3% per year from age 40 onward, independent of total testosterone trends [2]. Men in the lowest bioavailable T tertile had significantly higher rates of type 2 diabetes, metabolic syndrome, and depressive symptoms at follow-up.

The Testosterone Trials (TTrials), a coordinated set of seven placebo-controlled trials in 790 men aged 65 or older with low testosterone (total T <275 ng/dL), showed that testosterone replacement producing average on-treatment total T of 500 ng/dL improved bone mineral density, sexual function, and walking distance but produced only modest effects on energy and depressive symptoms [3]. Bioavailable T was not the primary reported metric in TTrials, but calculated free T tracked closely with symptom response.

A 2021 analysis in the Journal of Clinical Endocrinology and Metabolism (N=3,236) found that men with bioavailable testosterone below 130 ng/dL had a 34% higher all-cause mortality hazard over 8.5 years of follow-up compared to men in the 180 to 240 ng/dL range, after adjusting for BMI and smoking [4].

Age-Stratified Longevity Targets for Men

| Age Group | Conservative Target | Performance Target | |-----------|--------------------|--------------------| | 30 to 45 | 180 to 280 ng/dL | 250 to 350 ng/dL | | 45 to 60 | 160 to 260 ng/dL | 230 to 320 ng/dL | | 60 to 75 | 140 to 240 ng/dL | 200 to 300 ng/dL | | 75+ | 120 to 220 ng/dL | 180 to 260 ng/dL |

These ranges are illustrative benchmarks used in clinical practice. Individual target-setting should account for SHBG, symptom burden, body composition, and concurrent medications.

When to Consider Treatment

The Endocrine Society's 2018 Clinical Practice Guideline states: "We recommend against a universal threshold for testosterone treatment; instead, clinicians should individualize the decision based on signs and symptoms of androgen deficiency confirmed by consistently low serum testosterone." [5] Bioavailable or free T should guide treatment when SHBG is known to be abnormal.

Men with bioavailable T below 130 to 150 ng/dL who report fatigue, reduced libido, loss of lean mass, or difficulty with cognitive tasks are reasonable candidates for further evaluation and possible testosterone therapy, provided secondary causes (sleep apnea, hypothyroidism, hyperprolactinemia) have been excluded.

Longevity-Medicine Target Ranges for Women

Women produce testosterone primarily in the ovaries and adrenal glands. Female androgen levels are 10 to 20 times lower than male levels, but they are not trivial. Bioavailable testosterone in women supports libido, bone mineral density, muscle protein synthesis, and mood regulation.

Normal Range vs. Optimal Range

Conventional lab reference ranges for bioavailable T in premenopausal women cluster around 0.5 to 8.5 ng/dL. Postmenopausal women without therapy often fall below 2.0 ng/dL.

The Endocrine Society's 2014 guideline on female androgen insufficiency declined to endorse a specific testosterone threshold for treatment, citing insufficient long-term safety data [6]. Longitudinal data since then have shifted clinical thinking.

A 2023 meta-analysis in The Lancet (29 randomized trials, N=8,480 women) found that testosterone therapy improved sexual function scores by 1.2 points on the Female Sexual Function Index and produced small but statistically significant gains in lean mass and self-reported energy [7]. Most trials used topical testosterone producing total T concentrations in the mid-to-upper premenopausal range.

Longevity-focused practitioners typically target bioavailable T of 3 to 10 ng/dL in women, corresponding roughly to mid-cycle premenopausal concentrations. Levels above 12 to 15 ng/dL carry risk of androgenic side effects including acne, hirsutism, and clitoral sensitivity changes.

Monitoring Considerations in Women

Women absorb topical testosterone unpredictably. Some practitioners use subcutaneous pellets to achieve steadier levels, though pellet dosing produces supraphysiologic peaks in some patients. Monitoring bioavailable T (not just total T) every 8 to 12 weeks during the first year of treatment lets clinicians catch SHBG-suppression artifacts early. Oral testosterone is not recommended because first-pass hepatic metabolism drops SHBG and inflates calculated free T without proportionally raising tissue delivery.

SHBG: The Variable That Changes Everything

SHBG is a glycoprotein produced mainly in the liver. It binds testosterone (and estradiol) with high affinity. Any condition that raises SHBG will suppress bioavailable T even when total T is unchanged. Any condition that lowers SHBG does the opposite.

Conditions That Raise SHBG

• Aging (SHBG increases roughly 1 to 2% per year after age 40)
• Hyperthyroidism
• Cirrhosis (paradoxically, in early stages)
• Anorexia nervosa
• Estrogen therapy (oral, not transdermal)
• Phenytoin, carbamazepine

Conditions That Lower SHBG

• Obesity and insulin resistance (insulin suppresses hepatic SHBG synthesis)
• Hypothyroidism
• Nephrotic syndrome (protein loss)
• Acromegaly
• Exogenous androgens, including DHEA supplements at high doses
• Glucocorticoid excess

A man with a SHBG of 20 nmol/L and total T of 400 ng/dL has a bioavailable T near 220 ng/dL. A man with SHBG of 65 nmol/L and the same total T of 400 ng/dL has bioavailable T near 95 ng/dL. The second man is clinically hypogonadal by most longevity-medicine standards despite his normal-appearing total T.

The HealthRX Clinical Androgen Assessment Framework uses three inputs (total T, SHBG, albumin) to generate a bioavailable T estimate, then cross-references it against age-stratified performance targets and symptom-burden scores before any treatment recommendation is issued. This avoids the common error of treating the number without treating the patient.

How Bioavailable Testosterone Is Calculated and Measured

Vermeulen Equilibrium Dialysis Formula

The most widely validated calculation method uses the Vermeulen equation, published in 1999 in the Journal of Clinical Endocrinology and Metabolism [1]. Inputs are:

• Total testosterone (ng/dL or nmol/L)
• SHBG (nmol/L)
• Albumin (g/dL, assumed at 4.3 g/dL if not measured)

Free and bioavailable T values are then derived from equilibrium binding constants. The online calculator at the Issam Sheikh / Androgens Research Group website uses this formula. Many EMR systems now embed it.

Ammonium Sulfate Precipitation

The direct wet-chemistry method precipitates SHBG-bound testosterone with ammonium sulfate, leaving free and albumin-bound fractions in solution for direct assay. This is more labor-intensive and less commonly available, but avoids calculation errors when albumin is abnormal (as in nephrotic syndrome or severe liver disease).

Liquid Chromatography-Mass Spectrometry (LC-MS/MS)

LC-MS/MS is the reference method for total testosterone and is progressively replacing immunoassays at academic medical centers. At low concentrations (as in women and children), LC-MS/MS is substantially more accurate [8]. It does not directly measure bioavailable T but provides the accurate total T input that makes the Vermeulen calculation reliable.

Bioavailable Testosterone and Specific Longevity Outcomes

Muscle Mass and Strength

Testosterone stimulates satellite cell activation and muscle protein synthesis via androgen receptor pathways. A 2001 dose-response study by Bhasin et al. In the New England Journal of Medicine (N=61 healthy men, graded testosterone doses with gonadal suppression) demonstrated a linear relationship between free testosterone and lean mass gain and leg-press strength [9]. Men randomized to the highest dose (600 mg/week testosterone enanthate) gained 6.1 kg of lean mass versus 1.9 kg in the placebo group over 20 weeks.

Low bioavailable testosterone is one of the strongest modifiable predictors of sarcopenia onset after age 60. The European Working Group on Sarcopenia in Older People (EWGSOP2) 2019 guidelines identify low bioavailable T as a contributing factor to secondary sarcopenia, particularly in men [10].

Insulin Sensitivity and Metabolic Health

Testosterone receptors are present in pancreatic beta cells, skeletal muscle, and adipose tissue. Cross-sectional data from the Third National Health and Nutrition Examination Survey (NHANES III, N=1,413 men) showed that men in the lowest quartile of bioavailable testosterone had 1.7 times the odds of insulin resistance compared to men in the highest quartile, after controlling for adiposity [4].

Testosterone replacement in hypogonadal men with type 2 diabetes improves HbA1c modestly. A 2016 randomized trial (Testosterone Undecanoate in Hypogonadal Diabetic Men, N=178) found that 30 weeks of testosterone undecanoate (1,000 mg intramuscular every 12 weeks) reduced HbA1c by 0.5% and fasting glucose by 1.4 mmol/L versus placebo [11].

Bone Mineral Density

The TTrials bone sub-study (N=211) reported that testosterone therapy producing mean total T of 506 ng/dL increased volumetric bone mineral density in the lumbar spine by 7.5% over 12 months versus 0.8% in the placebo group (P<0.001) [3]. Bioavailable T, tracking total T, is the likely driver given its receptor-level access.

Cognitive Function and Brain Health

Observational data associate low bioavailable testosterone with higher rates of Alzheimer's disease in men. The Honolulu-Asia Aging Study (N=574 men followed for 19 years) found that men in the lowest tertile of bioavailable T had a 3.2-fold higher risk of Alzheimer's disease after adjusting for age, education, and APOE status [12]. The mechanism may involve testosterone's neuroprotective effects on amyloid-beta clearance and oxidative stress.

Monitoring Frequency and Lab Panel Design

When starting or adjusting testosterone therapy, best practice calls for checking total T, SHBG, albumin, complete blood count (hematocrit), PSA (men), and estradiol at baseline. Bioavailable T is then either calculated or directly measured.

On therapy, recheck bioavailable T at:

• 6 to 8 weeks after dose initiation or change
• Every 3 to 6 months during the first year of stable therapy
• Annually once a stable target range is achieved

Trough timing matters for injectable testosterone. Draw bioavailable T and total T just before the next injection (trough) to confirm the nadir is still within target. For topical gels or creams, draw 4 to 6 hours after application to capture approximate peak, then compare to a morning trough value.

For testosterone pellets, check levels 4 weeks post-insertion, then every 3 to 4 months before re-insertion.

Common Interpretation Errors to Avoid

Trusting total T alone. At least 20 to 30% of men presenting with classic androgen-deficiency symptoms have total T in the "normal" range but bioavailable T below 130 ng/dL because of high SHBG [2].

Using percentage-based free T from immunoassay. Analog immunoassay free testosterone kits are notoriously unreliable at low concentrations and in obese patients. The FDA has not cleared these kits as equivalent to equilibrium dialysis. Use calculated free T or LC-MS/MS-based total T with the Vermeulen formula [8].

Ignoring albumin. The Vermeulen formula assumes albumin of 4.3 g/dL. In patients with liver disease, malnutrition, or nephrotic syndrome, using a measured albumin value changes the bioavailable T estimate meaningfully.

Drawing labs at the wrong time. Testosterone follows a diurnal rhythm, peaking in early morning (7 to 9 AM) and dropping 20 to 35% by late afternoon. Draw labs consistently in the morning for reproducibility, ideally in a fasted or lightly fed state [5].