Free Testosterone: What Your Number Changes About Your Treatment

What Free Testosterone Actually Measures

Free testosterone is the fraction of circulating testosterone that is not chemically bound to a carrier protein. Total testosterone in the blood travels mostly attached to sex hormone-binding globulin (SHBG) and, to a lesser extent, albumin. Only the unbound portion, roughly 1 to 3% of the total, can enter target cells and activate androgen receptors directly.

The Endocrine Society's 2018 clinical practice guideline on testosterone therapy states: "We suggest the measurement of free testosterone concentration in patients in whom total testosterone concentrations are in the normal range but who have conditions that alter SHBG concentrations." [1]

This matters because total testosterone can be completely normal on paper while a patient has every symptom of androgen deficiency. The reverse is also possible: total testosterone can appear low in an obese man whose SHBG is suppressed, leaving free T within range and symptoms explained by other causes.

The SHBG Problem

SHBG is produced by the liver. Conditions that raise SHBG include aging, hyperthyroidism, liver cirrhosis, use of estrogens, and low BMI. Conditions that lower SHBG include obesity, insulin resistance, hypothyroidism, exogenous androgens, and nephrotic syndrome [2].

When SHBG rises, more testosterone gets bound and free T falls even if the testes or ovaries are producing the same amount of total hormone. A 60-year-old man with a total testosterone of 420 ng/dL and an SHBG of 70 nmol/L may have a free T below 50 pg/mL, which sits well below the lower limit of most reference intervals.

How the Lab Actually Measures It

Three methods exist:

• Equilibrium dialysis. The reference standard. Slow, expensive, not available in every commercial lab, but the most accurate.
• Calculated free testosterone. Uses total testosterone, SHBG, and albumin in a validated formula (Vermeulen equation). The Endocrine Society considers this acceptable when equilibrium dialysis is unavailable [1].
• Analog immunoassay (direct free T). Widely available but systematically inaccurate. The Endocrine Society and American Association of Clinical Endocrinology (AACE) both advise against using analog immunoassay for clinical decisions [1][3].

If your lab report says "free testosterone, direct," ask your HealthRX clinician whether a recalculated or dialysis-based result is warranted before any prescribing decision is made.

Normal Free Testosterone Ranges

Reference ranges vary by measurement method, laboratory, and population. The numbers below reflect equilibrium dialysis or validated calculation.

Men

The Endocrine Society's 2018 guideline cites a lower limit of approximately 9 pg/mL for men aged 19 to 39 using the Framingham Heart Study reference population [1]. The same guideline cautions that ranges derived from mixed-age or mixed-health populations can be misleading, and recommends using a reference interval built from healthy, non-obese, non-smoking young men when available.

A 2017 analysis published in the Journal of Clinical Endocrinology and Metabolism (N=9,054 men, ages 19 to 39) established a reference range of 9.0 to 30.0 pg/mL for free testosterone by equilibrium dialysis [4]. Values below 9 pg/mL in a symptomatic man are consistent with hypogonadism.

Age reduces free T independently of SHBG changes. The Massachusetts Male Aging Study showed free testosterone declines approximately 2 to 3% per year after age 40, compared with roughly 1 to 2% per year for total testosterone [5].

Women

Free testosterone ranges in women are narrower and method-dependent. Premenopausal women generally fall between 0.3 and 1.9 pg/mL by equilibrium dialysis. The Endocrine Society's 2014 guideline on androgen therapy in women notes that "there is no level of testosterone that reliably distinguishes women who will respond to therapy from those who will not," but values below the lower reference limit in the context of hypoactive sexual desire disorder remain a clinical consideration [6].

Postmenopausal women not on hormone therapy typically have free T values 10 to 25% below premenopausal norms because both ovarian androgen production and SHBG concentrations shift after menopause [6].

Why "Normal" Does Not Mean "Optimal for You"

A number sitting in the middle of a population range does not guarantee that number is right for a specific patient. Symptoms, functional status, bone density, libido, and cardiovascular risk factors all contribute to the clinical picture. A man with a free T of 11 pg/mL and severe fatigue, low libido, and a T-score of minus 2.1 on DEXA scan is clinically different from a man with the same free T and no complaints.

How a Low Free Testosterone Affects Your Treatment Plan

Low free testosterone in a symptomatic patient is the single most common laboratory finding that initiates TRT at HealthRX. The prescribing pathway is not automatic, but the free T result is weighted heavily.

Diagnostic Criteria Before Starting TRT

The Endocrine Society requires two criteria be met before starting testosterone therapy in men [1]:

1. Consistent symptoms and signs of androgen deficiency (reduced libido, fatigue, loss of muscle mass, morning erections absent, depressed mood).
2. Unequivocally low testosterone on at least two separate morning measurements.

"Unequivocally low" in the guideline refers primarily to total testosterone below 264 ng/dL, but the same document specifies that free testosterone should be used when SHBG is altered [1]. In practice, a free T below the lower reference limit on two morning samples, paired with clinical symptoms, is sufficient to begin a treatment conversation.

What TRT Options Free T Helps Select

Free testosterone does not just confirm diagnosis. It guides which delivery system makes sense [3]:

• Testosterone cypionate or enanthate (IM injection, 50 to 200 mg every 7 to 14 days). Best choice when total T and free T are both suppressed and no SHBG abnormality exists. Produces wide peaks and troughs; free T at trough should remain above 9 pg/mL.
• Testosterone undecanoate (Aveed, 750 mg IM every 10 weeks). Produces a steadier free T profile. Suitable for men who have large SHBG swings with shorter-acting injectables.
• Topical gels or creams (AndroGel 1.62%, Testim, compounded testosterone). Produce stable daily free T levels. Preferred when trough-to-peak variability on injections is causing symptomatic cycling.
• Clomiphene citrate or enclomiphene (off-label oral, 12.5 to 50 mg daily or every other day). Stimulates endogenous production. Free T response is monitored at 6 to 8 weeks. Preferred in men who want to preserve fertility.

Monitoring Targets on TRT

After initiating or adjusting TRT, free testosterone should be rechecked at 3 and 6 months [1]. The target during therapy is a free T in the mid-to-upper portion of the age-adjusted reference range. Targeting the upper quartile without a clinical reason increases erythrocytosis risk; hematocrit above 54% is a standard dose-reduction threshold [1][3].

A 2019 randomized trial published in the New England Journal of Medicine (the Testosterone Trials, N=790 men aged 65 and older) found that testosterone gel titrated to a free T target of 150 to 350 pg/dL (note: NEJM used different units; equivalent to approximately 15 to 35 pg/mL) improved sexual function and physical performance scores compared with placebo at 12 months [7]. Free T, not total T, was the primary titration variable in that protocol.

How a High Free Testosterone Affects Your Treatment Plan

High free testosterone can result from exogenous testosterone use, polycystic ovary syndrome (PCOS), adrenal tumors, congenital adrenal hyperplasia, or ovarian hyperthecosis in women. The clinical response depends on the cause.

In Men on TRT

A free T above 30 pg/mL on therapy usually means the dose is too high, the injection interval is too short, or the patient applied gel too recently before the blood draw. The clinical consequence depends on how high the number is:

• Free T 30 to 45 pg/mL: reduce dose by 10 to 15% or extend injection interval by 1 to 2 days. Recheck in 6 weeks.
• Free T above 45 pg/mL: hold the next scheduled dose, recheck in 2 weeks, then restart at a 20 to 25% lower dose. Check hematocrit before restarting [1][3].

Aromatization of excess testosterone to estradiol also increases as free T rises. Estradiol above 40 to 42 pg/mL in men on TRT may produce gynecomastia, water retention, or mood instability. Anastrozole 0.25 to 0.5 mg twice weekly is used in some protocols to control estradiol when free T is intentionally maintained at the higher end of normal [3].

In Women

In women not on testosterone therapy, a free T above 1.9 pg/mL (premenopausal) warrants workup for PCOS, late-onset congenital adrenal hyperplasia, or androgen-secreting tumor. The Endocrine Society's 2018 PCOS guideline recommends measuring both total and free testosterone to confirm biochemical hyperandrogenism [8].

Women on compounded testosterone cream for hypoactive sexual desire disorder should have free T checked at 6 weeks after initiation. Supraphysiologic free T in women (free T above 3.5 to 4.0 pg/mL) is associated with acne, clitoral enlargement, and voice changes; dose reduction is indicated promptly [6].

Free Testosterone and GLP-1 / Weight-Loss Therapy

Obesity suppresses SHBG, which raises free T relative to total T in men and may push free T into an apparently acceptable range despite true androgen deficiency at the tissue level. As patients lose weight on semaglutide (Ozempic, Wegovy) or tirzepatide (Mounjaro, Zepbound), SHBG rises. Free T commonly falls 10 to 20% during the first 12 to 16 weeks of significant weight loss even when total testosterone is climbing [9].

This is a counterintuitive lab pattern. A man starting semaglutide 2.4 mg weekly whose free T drops from 13 pg/mL to 10 pg/mL at week 12 is not becoming more androgen deficient. His SHBG normalization is pulling down free T mathematically while tissue androgen exposure may actually be improving. HealthRX clinicians track serial SHBG alongside free T during active weight loss to distinguish true androgen decline from this mathematical artifact.

STEP-1 (N=1,961), the key semaglutide 2.4 mg trial, produced 14.9% mean body weight loss at 68 weeks versus 2.4% with placebo [10]. Patients in that weight range would be expected to show meaningful SHBG shifts, and HealthRX strongly recommends retesting free T and SHBG at the 12-week and 24-week marks in any patient combining GLP-1 therapy with TRT.

Free Testosterone in Women on HRT

Postmenopausal women prescribed estradiol patches or oral estradiol experience SHBG increases that reduce free T. A woman whose free T was 0.9 pg/mL before starting oral estradiol 1 mg daily may have a free T of 0.4 pg/mL six weeks later, despite her ovaries producing the same amount of androgen. This can worsen libido and energy even as other menopausal symptoms improve [6].

Transdermal Versus Oral Estrogen

Transdermal estradiol patches produce far smaller increases in SHBG than oral estradiol at equivalent doses. The NAMS 2022 Menopause Hormone Therapy Position Statement notes that route of administration significantly affects the androgenic/estrogenic balance in postmenopausal women [11]. A woman experiencing worsening libido or fatigue after switching from a transdermal to an oral estrogen should have free T and SHBG checked before adding testosterone to her regimen.

Adding Testosterone in Women

The Endocrine Society recommends against testosterone therapy in women with any of the following: pregnancy, breastfeeding, androgen-dependent cancer, or free T above the upper limit of normal for premenopausal women [6]. When treatment is appropriate, compounded testosterone cream 0.5 to 2 mg/day transdermal is used, with free T checked at 6 weeks and again at 3 months. The target is a free T in the premenopausal physiologic range, not above it.

Timing, Collection, and Pre-Analytic Variables

Free testosterone is highly sensitive to the time of collection and the patient's physical state before the draw.

Best Practice for Collection

• Draw in the morning, between 7 and 10 a.m. Testosterone follows a diurnal rhythm; levels peak around 8 a.m. And fall 20 to 35% by early afternoon [1].
• Fast for at least 8 hours. Acute caloric intake may blunt the morning peak by 15 to 20%.
• Avoid intense exercise in the 24 hours before the draw. High-intensity resistance training transiently elevates free T for 15 to 30 minutes post-workout, then suppresses it below baseline for up to 24 hours.
• For men on topical testosterone, draw the sample immediately before the next scheduled application, not within 4 hours of applying gel or cream.
• For men on weekly IM injections, draw at trough (6 to 7 days post-injection). For biweekly injections, draw at day 3 to 4 post-injection to capture mid-cycle values.

A 2016 review in the Journal of Clinical Endocrinology and Metabolism (Bhasin et al.) concluded: "Sampling time, assay method, and SHBG concentrations are the three variables most likely to produce a clinically misleading free testosterone result in practice." [4] Getting these variables right before making a prescription change saves patients an unnecessary dose adjustment.

Interpreting Free Testosterone Alongside Other Labs

Free testosterone does not exist in isolation. At HealthRX, free T is always reviewed alongside a panel that includes:

• Total testosterone (to calculate the free fraction and assess SHBG effect)
• SHBG (nmol/L)
• Estradiol (pg/mL)
• LH and FSH (to distinguish primary from secondary hypogonadism)
• Hematocrit (especially on TRT, given erythrocytosis risk)
• PSA (men over 40 or men with prostate cancer risk factors, before and during TRT) [1]
• Prolactin (when LH and FSH are both low, to rule out pituitary adenoma)

Primary vs. Secondary Hypogonadism

Low free T with high LH and FSH points to primary hypogonadism (testicular failure). These men almost always require exogenous TRT because the HPG axis is already maximally stimulated.

Low free T with low or inappropriately normal LH and FSH points to secondary hypogonadism (pituitary or hypothalamic dysfunction). These men may respond to clomiphene citrate, enclomiphene, or HCG monotherapy, which preserve testicular function and fertility [1][3].

The distinction changes everything about the prescription. Free T confirms the deficiency; LH and FSH tell you where the problem originates.

The Estradiol-Free T Ratio in Men

An estradiol-to-free-T ratio above approximately 0.30 (when both are expressed in pg/mL) suggests excessive aromatization. This is most common in obese men, men on high-dose TRT, or men with hepatic dysfunction. Targeting this ratio during TRT optimization, rather than treating estradiol as a standalone number, reduces the risk of over-suppression with aromatase inhibitors [3].