Free Testosterone Longevity-Medicine Target Ranges

Why Free Testosterone Matters More Than Total Testosterone

Total testosterone includes both protein-bound and unbound fractions, but only the unbound fraction, roughly 1 to 4% of the total, can enter cells and activate androgen receptors. Free testosterone is that unbound fraction. SHBG tightly binds about 44 to 65% of circulating testosterone, and albumin binds another 33 to 54%; neither fraction is meaningfully bioavailable under physiologic conditions.

A man can carry a total testosterone of 600 ng/dL and still be functionally androgen-deficient if his SHBG is elevated enough to suppress free testosterone below 10 pg/mL. Conversely, a man with total testosterone of 400 ng/dL and low SHBG may have a free testosterone of 18 pg/mL and feel completely well. This is why measuring total testosterone alone misses a clinically meaningful subset of patients.

The SHBG Problem

SHBG rises with age, thyroid disease, liver disease, exogenous estrogen, and caloric restriction. When SHBG rises, free testosterone falls even if the testes or ovaries are producing the same amount of hormone. The Framingham Heart Study (N=1,454 men) found that SHBG increases roughly 1.6% per year after age 40, which explains why free testosterone declines faster than total testosterone across the lifespan [1].

Albumin-Bound Versus Free Testosterone

Albumin-bound testosterone is weakly held and may dissociate at the capillary level to contribute some bioactivity. The sum of free plus albumin-bound fractions is called "bioavailable testosterone." Some clinicians track bioavailable testosterone instead of, or alongside, free testosterone. For most clinical decisions, free testosterone and bioavailable testosterone move in parallel, so either metric is acceptable as long as the same method is used consistently for follow-up.

How Free Testosterone Is Measured

Equilibrium Dialysis

Equilibrium dialysis is the reference-standard method. The serum sample is dialyzed across a semipermeable membrane at physiologic pH and temperature; the concentration of testosterone in the dialysate equals the true free fraction. The Endocrine Society's 2018 clinical practice guideline on testosterone therapy explicitly states: "We recommend measuring free testosterone by equilibrium dialysis in men in whom total testosterone is near the lower limit of normal or when alterations in SHBG are suspected" [2].

Equilibrium dialysis is expensive (roughly $150, $250 per draw) and not available at most point-of-care labs. Quest Diagnostics and a handful of reference laboratories offer it.

Calculated Free Testosterone

The Vermeulen formula, which uses total testosterone, SHBG, and albumin (assumed at 4.3 g/dL), produces a calculated free testosterone that correlates well with equilibrium dialysis in most adult populations [3]. The online calculator at issam.ch is widely used in clinical practice. Accuracy degrades when albumin is abnormal, such as in cirrhosis or nephrotic syndrome.

Direct Immunoassay

Many routine panels report "free testosterone" via a direct radioimmunoassay or analog immunoassay. The Endocrine Society guideline explicitly discourages this method, noting it "should not be used because it lacks accuracy" [2]. If your lab report says "free testosterone" without specifying the method, ask. If the answer is analog immunoassay, disregard the result and order a calculated or dialysis-based measurement.

Reference Ranges Versus Longevity-Medicine Targets

Why Reference Ranges Are Insufficient

Published reference ranges for free testosterone are derived from population samples that include older adults, sedentary individuals, men with subclinical metabolic disease, and people with untreated sleep apnea. A range defined as "normal" by the 2.5th, 97.5th percentile of such a population tells you what is statistically common, not what is physiologically optimal.

The TwinsUK study found that free testosterone in the highest quartile (above approximately 14 pg/mL in men aged 40 to 69) was associated with significantly lower all-cause mortality risk compared to the lowest quartile over a median 10.6-year follow-up [4]. That top quartile sits well above the midpoint of most conventional reference ranges.

Longevity-Medicine Targets for Men

Most longevity-medicine practitioners and TRT specialists now target free testosterone in the 15 to 25 pg/mL range for adult men. The clinical rationale:

• The TRAVERSE trial (N=5,204 men, mean age 57, mean follow-up 33 months) established cardiovascular safety for testosterone therapy titrated to mid-to-upper reference ranges, removing a major barrier to optimization-level dosing [5].
• Bone mineral density gains in hypogonadal men plateau when free testosterone exceeds approximately 15 to 18 pg/mL based on data from the Testosterone Trials (TTrials, N=790 men aged 65 and older) [6].
• Muscle protein synthesis in healthy older men responds to free testosterone in a dose-dependent manner up to approximately 20 to 25 pg/mL, after which additional androgen exposure adds marginal lean-mass benefit but increases erythrocytosis risk.

The ceiling of 25 pg/mL is a soft guideline. Some practitioners accept up to 30 pg/mL in men on TRT who are asymptomatic, have a hematocrit below 52%, and have stable PSA. Above 30 pg/mL, the benefit-to-risk ratio narrows.

Longevity-Medicine Targets for Women

Women's free testosterone ranges are roughly 10-fold lower than men's. The conventional reference range in premenopausal women is approximately 0.3 to 5.0 pg/mL depending on cycle phase, with a trough in the follicular phase and a peak near ovulation.

Longevity-medicine targets for women cluster around 1.5 to 3.5 pg/mL, corresponding to the mid-to-upper range of a healthy 25- to 35-year-old woman in the mid-follicular phase. This range is associated in observational data with better lean mass, libido, bone density, and mood without significant androgenic side effects at estradiol-replete states.

The ISSWSH (International Society for the Study of Women's Sexual Health) 2019 position statement on testosterone therapy in women notes that serum total testosterone below the normal premenopausal range is associated with hypoactive sexual desire disorder, and that doses producing levels at or modestly below the upper end of the premenopausal range appear both effective and safe for up to 24 months of follow-up [7].

A free testosterone above 5 pg/mL in women warrants evaluation for polycystic ovary syndrome, adrenal hyperplasia, or exogenous androgen misuse before optimization therapy is started.

A Practical Decision Framework for Target Selection

The table below summarizes how free testosterone targets shift across clinical contexts. Targets are starting points; individual symptom burden, hematocrit, SHBG, estradiol, and PSA (men) modify the final goal.

| Patient Context | Free T Target (men) | Free T Target (women) | |---|---|---| | General population longevity | 15 to 20 pg/mL | 1.5 to 2.5 pg/mL | | Active TRT / HRT optimization | 18 to 25 pg/mL | 2.0 to 3.5 pg/mL | | Erythrocytosis risk (Hct 50 to 52%) | 12 to 18 pg/mL | No change | | Post-prostatectomy, no evidence of disease | Individualize with urology | N/A | | Bone-density priority (osteoporosis) | 15 to 22 pg/mL | 2.0 to 3.5 pg/mL | | Female HSDD without estrogen deficiency | N/A | 1.5 to 3.0 pg/mL |

Age-Related Decline and the Case for Early Monitoring

Free testosterone begins declining meaningfully in men around age 30, approximately 1 to 2% per year [8]. By age 60, the average American man has free testosterone roughly 40 to 50% below his peak. This trajectory is not inevitable; sleep, resistance training, body-fat reduction, and treatment of contributing conditions (hypothyroidism, sleep apnea) all slow the decline.

The European Male Aging Study (EMAS, N=3,369 men aged 40 to 79) defined late-onset hypogonadism by a combination of three sexual symptoms plus total testosterone below 11 nmol/L (317 ng/dL) and free testosterone below 220 pmol/L (approximately 6.3 pg/mL) [9]. That diagnostic threshold is far below the longevity-medicine optimization target, which means a man can be "not hypogonadal" by EMAS criteria while still sitting in a range associated with suboptimal body composition, cognitive performance, and bone density.

When to Start Testing

Annual free testosterone measurement is reasonable starting at age 35 in men with risk factors (obesity, type 2 diabetes, opioid use, sleep apnea) and at age 40 in the general male population. Women benefit from baseline measurement at perimenopause onset or whenever libido, fatigue, or lean-mass loss is clinically prominent.

Morning Draw Protocol

Testosterone follows a diurnal rhythm with a peak between 07:00 and 10:00. A draw obtained at 14:00 may be 20 to 30% lower than a morning draw for the same individual. All baseline and follow-up measurements should be obtained in the morning to allow valid comparisons. Fasting is not required, but heavy alcohol intake the prior evening can suppress free testosterone by up to 23% acutely [10].

Free Testosterone in TRT Dose Titration

Starting TRT and Setting the First Follow-Up Target

When initiating testosterone cypionate or enanthate injections (typical starting dose 100 to 150 mg intramuscular every 7 days), free testosterone should be checked at weeks 6 to 8, which is roughly two to three half-lives after a dose change. This timing lets the level stabilize before adjustment.

The target at the first check is 15 to 20 pg/mL (morning, trough draw for weekly injections, or mid-cycle draw for biweekly injections). If the level is below 12 pg/mL and symptoms persist, a dose increase of 10 to 20 mg/week is appropriate. If the level exceeds 28 pg/mL on the first check, reduce the dose before the next follow-up.

Testosterone Pellets and Creams

Pellet implants produce a steady-state free testosterone that peaks at 4 to 6 weeks and then gradually declines over 3 to 6 months. Free testosterone should be checked at the 4-week mark for the first pellet insertion and at 8 to 10 weeks for subsequent insertions to account for inter-individual absorption variation.

Transdermal creams and gels produce highly variable free testosterone levels because scrotal skin absorbs testosterone 5-fold more efficiently than arm or thigh skin. Standardizing the application site and measuring free testosterone 4 to 6 hours post-application (peak for transdermal preparations) gives the most clinically actionable result.

Estradiol Co-Management

Testosterone aromatizes to estradiol, particularly in adipose tissue. When free testosterone rises with TRT, estradiol typically rises in parallel. Estradiol in men should stay within roughly 20 to 40 pg/mL; above that range, gynecomastia and water retention may occur. If an aromatase inhibitor is added to control estradiol, free testosterone may rise further, occasionally requiring a corresponding dose reduction of the testosterone itself.

Free Testosterone and Metabolic Health

Low free testosterone is both a consequence and an amplifier of metabolic dysfunction. Visceral adiposity increases aromatase activity (converting testosterone to estradiol) and increases SHBG indirectly through hepatic insulin resistance. The resulting low free testosterone then reduces lean mass and promotes further fat accumulation.

A 2013 meta-analysis published in the European Journal of Endocrinology (20 studies, N=6,427 men) found that low free testosterone was associated with a 1.47-fold increased risk of metabolic syndrome independent of total testosterone, BMI, and age [11]. This independent association underscores why free testosterone adds predictive information beyond what total testosterone provides.

Weight loss of 10% body weight raises free testosterone by approximately 15 to 25% in overweight men, primarily by reducing SHBG and estradiol. In patients on semaglutide (Ozempic, Wegovy) or tirzepatide (Mounjaro, Zepbound), free testosterone should be rechecked after significant weight loss because TRT doses that were appropriate at a higher body weight may produce supratherapeutic levels after weight loss reduces SHBG.

Special Populations

Men With Obesity (BMI >30)

SHBG is typically low in men with obesity, so total testosterone may underestimate androgen status. Conversely, elevated aromatase in adipose tissue increases estradiol and may suppress LH, reducing testicular testosterone output. Free testosterone is the most clinically useful single metric in this population.

Older Men (Age >65)

The TTrials found that testosterone therapy in men aged 65 and older with free testosterone below 9 pg/mL improved sexual function, physical activity, and bone mineral density, with the largest gains in the subgroup with the lowest baseline free testosterone [6]. Men in this age group often need free testosterone targets on the lower end of the longevity range (12 to 18 pg/mL) to balance benefits against erythrocytosis risk.

Women on Oral Estrogen

Oral estrogen substantially increases hepatic SHBG production. A woman on oral conjugated equine estrogen 0.625 mg daily may have SHBG double that of a woman on transdermal estradiol at equivalent clinical effect. This SHBG elevation can suppress free testosterone even when endogenous testosterone production is normal. Switching from oral to transdermal estrogen often corrects low free testosterone without requiring exogenous testosterone supplementation.

Interpreting Results: A Step-by-Step Approach

1. Confirm the measurement method. If the lab used analog immunoassay, repeat with calculated or dialysis-based method before acting on the result.
2. Confirm morning draw. If the draw time was after noon, repeat in the morning before concluding the patient is deficient.
3. Check SHBG. A high SHBG explains low free testosterone and shifts the intervention toward SHBG reduction strategies (weight loss, thyroid optimization, switching from oral to transdermal estrogen, or low-dose oral testosterone which suppresses hepatic SHBG).
4. Check estradiol (men). Elevated estradiol lowers LH and suppresses endogenous testosterone production.
5. Apply the clinical context. Asymptomatic patients with free testosterone of 13 pg/mL do not necessarily require treatment; symptomatic patients with the same value almost always do.