Free Testosterone: How to Interpret Your Result

Why Free Testosterone Matters More Than Total

Total testosterone includes protein-bound fractions that cannot enter cells. Roughly 44% binds tightly to SHBG, 50% binds loosely to albumin, and only 1-3% circulates unbound [1]. The free fraction plus the albumin-bound fraction together compose "bioavailable testosterone," which is the portion accessible to target tissues like muscle, bone, and brain.

A man can have a total testosterone of 450 ng/dL and still be functionally hypogonadal if his SHBG is elevated enough to sequester most of that hormone. The 2018 Endocrine Society guideline on male hypogonadism states: "We suggest measuring free testosterone using equilibrium dialysis or calculated free testosterone when total testosterone is near the lower limit of normal or when SHBG concentrations are altered" [2].

Conditions that raise SHBG (and thus lower free T without changing total T) include aging, hyperthyroidism, liver disease, anticonvulsant use, and estrogen therapy. Conditions that suppress SHBG (raising free T relative to total T) include obesity, insulin resistance, type 2 diabetes, hypothyroidism, and androgen use [3].

Reference Ranges by Age and Sex

Normal free testosterone declines with age in men at approximately 2-3% per year after age 30 [4]. Women produce far less testosterone overall, and their reference ranges are correspondingly lower.

For men measured by equilibrium dialysis, the Endocrine Society's Framingham-derived reference ranges are approximately:

• Ages 19-29: 9.3-26.5 pg/mL
• Ages 30-39: 8.7-25.1 pg/mL
• Ages 40-49: 6.8-21.5 pg/mL
• Ages 50-59: 7.2-24.0 pg/mL
• Ages 60-69: 6.6-18.1 pg/mL

These values come from the Framingham Heart Study male cohort (N=456 men with no known pituitary, gonadal, or adrenal disease) [5]. The ranges shift significantly depending on which assay your lab uses. Direct analog immunoassays for free testosterone (the cheap, automated versions) are poorly standardized and can differ by 30-50% from dialysis values [6].

For premenopausal women, free testosterone typically ranges from 0.2-5.0 pg/mL, though this varies across the menstrual cycle and by assay methodology. The International Consortium for Harmonization of Clinical Laboratory Results has identified free testosterone measurement as a priority for standardization because of this inter-laboratory variability [7].

How Free Testosterone Is Measured

Three methods exist, and the differences between them matter clinically.

Equilibrium dialysis is the reference method. A sample is dialyzed across a semipermeable membrane; unbound testosterone crosses freely and is then quantified by mass spectrometry. This method is expensive, slow (results in 5-7 days), and available mainly through reference laboratories like Quest (test code 36170) or LabCorp (test code 144103). The 2018 Endocrine Society guideline identifies equilibrium dialysis as "the gold standard for measurement of free testosterone" [2].

Calculated free testosterone uses the Vermeulen equation, which requires total testosterone, SHBG, and albumin inputs. A 2020 validation study in the Journal of Clinical Endocrinology and Metabolism (N=2,105 men) found that the Vermeulen calculation correlated with dialysis at r=0.89, making it acceptable for routine clinical use [8]. Free online calculators (such as the ISSAM calculator) automate this equation.

Direct analog immunoassay uses a labeled testosterone analog that competes for antibody binding sites. These assays are fast and cheap but poorly correlate with dialysis (r=0.43-0.60 in validation studies) and are explicitly not recommended by the Endocrine Society for clinical decision-making [2].

If your lab report says "free testosterone" without specifying the method, it is almost certainly a direct analog immunoassay. Ask your provider to order calculated free T (total T + SHBG) or equilibrium dialysis instead.

Interpreting a Low Free Testosterone Result

A free testosterone below your age-adjusted reference range, confirmed on two separate morning draws, suggests hypogonadism. But one abnormal value is not a diagnosis.

The Endocrine Society requires two morning measurements showing low testosterone before initiating treatment [2]. Testosterone peaks between 6-8 AM and can fall 20-30% by afternoon, so timing matters [9]. Acute illness, sleep deprivation, recent heavy exercise, and opioid use can all transiently suppress testosterone. Dr. Shalender Bhasin, lead author of the 2018 Endocrine Society guideline, has stated: "A single testosterone measurement is insufficient to diagnose hypogonadism because of the substantial day-to-day variability" [2].

Once confirmed, low free testosterone warrants investigation for underlying cause:

• Primary hypogonadism (testicular failure): elevated LH and FSH alongside low T. Causes include Klinefelter syndrome, prior chemotherapy, orchitis, and age-related Leydig cell decline.
• Secondary hypogonadism (pituitary/hypothalamic): low or inappropriately normal LH/FSH with low T. Causes include pituitary adenoma, hyperprolactinemia, obesity, opioid use, and anabolic steroid suppression.

The Testosterone Trials (TTrials, N=788 men aged 65+ with total T <275 ng/dL) demonstrated that testosterone gel improved sexual function, walking distance, and bone density over 12 months compared to placebo, establishing clinical significance for treating confirmed low T in older men [10].

Interpreting a High Free Testosterone Result

Elevated free testosterone in men may result from exogenous testosterone use (including undisclosed supplementation), hCG therapy, or rarely, androgen-secreting tumors. In most clinical settings, high free T in a man on TRT simply means the dose needs reduction.

In women, elevated free testosterone is the biochemical hallmark of hyperandrogenism. The 2023 International Evidence-based Guideline for Polycystic Ovary Syndrome (PCOS) recommends using calculated free testosterone or equilibrium dialysis (not direct immunoassay) as "the most sensitive biochemical marker for hyperandrogenism" in women being evaluated for PCOS [11].

Common causes of elevated free testosterone in women include:

• PCOS (responsible for 70-80% of female hyperandrogenism cases)
• Congenital adrenal hyperplasia (late-onset 21-hydroxylase deficiency)
• Ovarian hyperthecosis
• Androgen-secreting ovarian or adrenal tumors (rare; suspect when total T exceeds 200 ng/dL in a woman)
• Exogenous androgen or DHEA supplementation

A rapidly rising testosterone in a woman (doubling over weeks to months) raises concern for a virilizing tumor and warrants imaging.

The SHBG Connection: When Total and Free Diverge

SHBG is the variable that most commonly explains a mismatch between total and free testosterone. Understanding this relationship is clinically useful.

A 2021 analysis from the European Male Ageing Study (EMAS, N=3,369 men aged 40-79) found that SHBG increased by approximately 1.3% per year of age, independent of BMI changes [12]. This means a 60-year-old man may have the same total testosterone as a 35-year-old but significantly less bioavailable hormone.

Obesity drives SHBG in the opposite direction. Insulin resistance suppresses hepatic SHBG production, effectively "unmasking" more free testosterone. A man with a BMI of 35 and a total testosterone of 280 ng/dL may have a normal or even high-normal free T because his SHBG is suppressed to 12-18 nmol/L. Treating that man with testosterone based only on his total T reading would be inappropriate.

Medications also shift SHBG significantly. Oral estrogens (including combined oral contraceptives) raise SHBG 2-4 fold. This is why women on the pill frequently show low total testosterone but may still have adequate free T for their tissue needs. Conversely, drugs like danazol, insulin, and some progestins lower SHBG [3].

How to Raise Low Free Testosterone

Interventions fall into three categories: lifestyle optimization, pharmaceutical treatment, and managing SHBG.

Lifestyle interventions with evidence:

Resistance training acutely raises testosterone post-exercise and chronically improves body composition, reducing insulin resistance and the SHBG-suppressing effects of obesity [13]. Weight loss in obese men increases total and free testosterone proportionally. A 2022 meta-analysis (14 RCTs, N=1,067 men with obesity) found that weight loss of at least 5% body weight increased total testosterone by a mean of 84.6 ng/dL (95% CI: 55.4-113.8) [14].

Sleep optimization matters. Restricting sleep to 5 hours per night for one week reduced daytime testosterone by 10-15% in young healthy men (N=10) [9]. Seven to nine hours produces optimal hormonal output.

Pharmaceutical options:

• Testosterone replacement therapy (injections, gels, patches, pellets) directly raises both total and free T and remains the primary treatment for confirmed hypogonadism [2].
• Clomiphene citrate (25-50 mg daily, off-label) stimulates endogenous production by blocking hypothalamic estrogen feedback, preserving fertility. A retrospective analysis (N=86 hypogonadal men) showed clomiphene increased total T from 228 to 612 ng/dL over 12 months [15].
• Enclomiphene (the trans-isomer of clomiphene) is under FDA review as a dedicated hypogonadism treatment.

SHBG management:

If free T is low primarily because SHBG is elevated, addressing the SHBG driver may be more appropriate than adding exogenous testosterone. Switching from oral to transdermal estrogen in women reduces SHBG. Treating hyperthyroidism normalizes SHBG. Boron supplementation (6 mg daily) reduced SHBG by approximately 10% in one small trial (N=13), though larger studies are needed [16].

How to Lower Elevated Free Testosterone

In women with PCOS or hyperandrogenism, the primary strategies involve raising SHBG (to bind more testosterone) and reducing androgen production.

Combined oral contraceptives raise SHBG and suppress ovarian androgen production simultaneously. A Cochrane review (26 RCTs) confirmed that COCs reduce free testosterone in women with PCOS, with anti-androgenic progestins (cyproterone acetate, drospirenone) providing additional benefit [17].

Spironolactone (50-200 mg daily) blocks the androgen receptor directly and modestly reduces adrenal androgen production. It takes 3-6 months to see clinical improvement in acne or hirsutism.

Metformin reduces insulin levels, which indirectly allows SHBG to rise. The effect on testosterone is modest (approximately 10-15% reduction) but additive with other therapies.

In men, elevated free T from exogenous sources is managed by dose reduction. In the rare case of an androgen-secreting tumor, surgical resection is definitive.

When to Retest and How to Optimize Your Draw

Timing and preparation affect testosterone values enough to change clinical interpretation.

Draw blood between 6-10 AM, fasting if possible. Testosterone has a circadian rhythm with morning peaks, and glucose ingestion acutely suppresses testosterone by up to 25% [18]. If your first result was borderline, a poorly timed redraw could falsely normalize or falsely confirm it.

Avoid heavy resistance exercise in the 24 hours before testing (acute post-exercise spikes can confound results). Alcohol binge drinking in the prior 48 hours suppresses testosterone transiently.

If you are on TRT:

• Injections (cypionate/enanthate): Draw at trough, typically the morning before your next injection.
• Daily gels: Draw 2-6 hours after application for peak, or pre-application for trough.
• Pellets: Draw at 4-6 weeks post-insertion to confirm therapeutic levels.

The goal for most men on TRT is a free testosterone in the mid-normal range for age (not the upper extreme), balanced against hematocrit, PSA, and symptom resolution [2].

Units and Conversions

Labs report free testosterone in different units, creating confusion. The most common:

• pg/mL (picograms per milliliter): the standard unit in most U.S. commercial labs
• pmol/L (picomoles per liter): used in European and some academic labs
• ng/dL (nanograms per deciliter): occasionally used; 1 ng/dL = 10 pg/mL

Conversion: 1 pg/mL = 3.47 pmol/L. So a result of 15 pg/mL equals approximately 52 pmol/L.

Always compare your result to the reference range printed on your specific lab report, not to numbers found online, because the assay method determines the range.