Free Testosterone Calculation: How to Interpret Your TRT Lab Results

What Is Free Testosterone and Why Does the Calculation Matter?

Free testosterone is the unbound fraction of testosterone that can cross cell membranes and activate androgen receptors directly. Total testosterone tells you how much testosterone is circulating, but it does not tell you how much is biologically available. The Endocrine Society's 2018 Clinical Practice Guideline on male hypogonadism explicitly states: "We recommend measuring total testosterone as the initial diagnostic test; when total testosterone is near the lower limit of normal, we suggest also measuring free testosterone by a reliable method." [1]

Testosterone in the bloodstream exists in three pools. Roughly 44 to 56 percent is tightly bound to sex hormone-binding globulin (SHBG) and is essentially inactive. Another 40 to 50 percent is loosely bound to albumin and can dissociate at the tissue level. The remaining 2 to 3 percent circulates as free, unbound hormone. [2] Conditions that raise SHBG, including aging, hyperthyroidism, and liver disease, can produce a normal total testosterone with a clinically low free testosterone. Conversely, obesity and insulin resistance suppress SHBG, which can produce a low total testosterone with an adequate free fraction.

Because free testosterone concentrations are so low (measured in picograms per milliliter), direct immunoassay methods are frequently unreliable. A 2017 analysis in the Journal of Clinical Endocrinology and Metabolism confirmed that equilibrium dialysis remains the reference standard for direct measurement, but this method is not widely available in routine clinical labs. [3] The calculated free testosterone using the Vermeulen equation is the next best option and is accepted by both the Endocrine Society and the American Urological Association for clinical decision-making.

How the Vermeulen Equation Calculates Free Testosterone

The Vermeulen equation is the most widely validated formula for estimating free testosterone. It requires three inputs: total testosterone (in nmol/L), SHBG (in nmol/L), and albumin (conventionally fixed at 4.3 g/dL or 43 g/L). [4] Online calculators at sites like the Hormonology or ISSAM calculator implement this math automatically; you enter the two measured values and the calculator returns both free testosterone and bioavailable testosterone.

The conversion steps matter. Most U.S. labs report total testosterone in ng/dL. Divide by 28.85 to convert to nmol/L. SHBG is reported in nmol/L by most reference labs, so no conversion is needed. Once you have both values in SI units, the Vermeulen equation iteratively solves for the free fraction using published binding constants for SHBG (Ka = 5.97 × 10^8 L/mol) and albumin (Ka = 3.6 × 10^4 L/mol). [4]

A practical example: a man with a total testosterone of 700 ng/dL (24.3 nmol/L) and an SHBG of 55 nmol/L will calculate to a free testosterone of approximately 10.2 pg/mL, which is below the lower reference limit of 15 pg/mL used by most TRT clinics. The same total testosterone with an SHBG of 25 nmol/L yields a free testosterone near 20 pg/mL, which is solidly therapeutic. This difference explains why two men with identical total testosterone can have completely different symptom profiles and treatment needs.

The HealthRX clinical team uses the following decision framework when reviewing lab results before and during TRT:

1. If total testosterone is below 300 ng/dL on two morning samples, treat based on total T alone.
2. If total testosterone is 300 to 400 ng/dL, calculate free T. If free T is below 15 pg/mL and symptoms are present, treatment is appropriate.
3. If total testosterone exceeds 400 ng/dL but SHBG is above 50 nmol/L, calculate free T before ruling out hypogonadism.
4. On TRT, target total testosterone of 500 to 900 ng/dL and calculated free testosterone of 15 to 25 pg/mL at trough (the day of or day before next injection).

Total Testosterone Reference Ranges

The Endocrine Society defines the lower limit of normal for total testosterone as 300 ng/dL, based on the Third National Health and Nutrition Examination Survey (NHANES III) data in healthy men aged 19 to 40. [1] The upper end of the normal range is approximately 1 to 000 ng/dL in most laboratory reference intervals, though LabCorp lists 916 ng/dL and Quest Diagnostics lists 1 to 197 ng/dL as upper limits, reflecting differences in the assay populations used to establish those ranges. [5]

Timing matters considerably. Testosterone peaks in the early morning between 7 and 10 a.m. and can decline by 20 to 30 percent by afternoon. [6] The Endocrine Society guideline specifically recommends drawing total testosterone between 7 and 11 a.m. to capture peak values and avoid a false diagnosis of hypogonadism from an afternoon draw.

On TRT with weekly or twice-weekly testosterone cypionate injections, the trough drawn immediately before the next injection is typically 400 to 700 ng/dL for a patient injecting 100 mg per week. Peak values drawn 24 to 48 hours after injection can reach 900 to 1 to 200 ng/dL. Neither single reading is fully representative of average exposure; some protocols use the midpoint draw (3 to 4 days after injection for weekly dosing) to estimate mean testosterone. A 2021 paper in Andrology found that testosterone cypionate 100 mg weekly produces a geometric mean total testosterone of approximately 650 ng/dL across the dosing interval. [7]

Why You Need a Sensitive Estradiol Assay, Not the Standard Test

Standard estradiol immunoassays are calibrated for female reference ranges, which run from roughly 30 to 400 pg/mL across the menstrual cycle. Men's estradiol concentrations are 10 to 40 pg/mL, a range where standard immunoassays have poor precision and frequent false elevations. [8] The sensitive estradiol assay uses liquid chromatography-tandem mass spectrometry (LC-MS/MS), sometimes called the "ultrasensitive" or "sensitive" assay, and is accurate at concentrations as low as 1 to 2 pg/mL.

The male reference range for estradiol by sensitive assay is 10 to 40 pg/mL. On TRT, estradiol rises because exogenous testosterone is converted to estradiol by the aromatase enzyme, primarily in adipose tissue. Most men tolerate estradiol up to 40 to 50 pg/mL without symptoms. Estradiol above 50 to 60 pg/mL is associated with gynecomastia, water retention, and mood changes in some patients, although individual tolerance varies widely. [9]

Crucially, over-suppressing estradiol with aromatase inhibitors (anastrozole, exemestane) carries its own risks. A 2013 study in the Journal of Bone and Mineral Research found that estradiol concentrations below 10 pg/mL in men are associated with accelerated bone loss and reduced bone mineral density. [10] Men on TRT do not routinely require an aromatase inhibitor unless they are symptomatic and have confirmed elevated estradiol by sensitive assay. The standard estradiol test ordered by mistake will frequently over-estimate estradiol and lead to unnecessary anastrozole prescriptions.

When ordering labs, specify "Estradiol, Sensitive" on the requisition. LabCorp offers test code 140244; Quest Diagnostics offers test code 30289. Both use LC-MS/MS methodology.

CBC Monitoring on TRT: Hematocrit and Polycythemia Risk

Testosterone stimulates erythropoiesis by increasing erythropoietin production in the kidneys and by direct effects on bone marrow progenitor cells. [11] This is a predictable, dose-dependent effect. In the Testosterone Trials (TTrials), a coordinated set of seven placebo-controlled trials in older men with low testosterone, hematocrit increased by an average of 2.9 percentage points in the testosterone group versus 0.6 percentage points in the placebo group over 12 months. [12]

The Endocrine Society guideline recommends checking CBC at baseline, at 3 to 6 months, and then annually. [1] The clinical action thresholds are:

• Hematocrit above 48%: recheck in 4 to 6 weeks.
• Hematocrit 50 to 54%: reduce dose, increase injection frequency to lower peak testosterone, or consider therapeutic phlebotomy.
• Hematocrit above 54%: hold testosterone until hematocrit normalizes, then restart at a lower dose.

Injection frequency affects erythrocytosis risk more than total weekly dose. Twice-weekly or three-times-weekly injections of the same total milligrams produce lower peak testosterone and lower hematocrit than once-weekly injections. [13] Transdermal testosterone (gel or cream) produces a more stable testosterone level and carries a lower erythrocytosis risk than intramuscular or subcutaneous injections, though absorption varies considerably between individuals.

Men with sleep apnea, chronic obstructive pulmonary disease, or a baseline hematocrit above 48% should be monitored more closely, with CBC rechecks at 6 to 8 weeks after dose changes rather than waiting 3 months.

Lipid Panel on TRT: What Changes and When to Retest

The relationship between TRT and lipid parameters is complex and depends on testosterone dose, route of administration, and baseline metabolic health. [14] Oral and some injectable testosterone preparations can suppress HDL cholesterol, but physiologic replacement with transdermal or injectable testosterone cypionate or enanthate has a more modest effect.

A 2016 meta-analysis of 39 randomized controlled trials published in the European Journal of Endocrinology found that testosterone therapy reduced total cholesterol by a mean of 7.9 mg/dL and LDL by 5.8 mg/dL, with a modest reduction in HDL of 1.9 mg/dL. [15] Triglycerides were not significantly changed. These mean effects were statistically significant but clinically small, and they need to be interpreted alongside the individual patient's cardiovascular risk factors.

The TRAVERSE trial (N=5,246), published in the New England Journal of Medicine in 2023, found no significant increase in major adverse cardiovascular events (MACE) in men aged 45 to 80 with hypogonadism and elevated cardiovascular risk who received testosterone undecanoate versus placebo over approximately 22 months, with a hazard ratio of 0.96 (95% CI 0.78 to 1.17). [16] This trial largely addressed the cardiovascular safety questions raised by earlier observational studies.

For lipid monitoring on TRT, the HealthRX protocol recommends:

• Fasting lipid panel at baseline before starting TRT.
• Repeat fasting lipid panel at 3 months after initiation.
• Annual fasting lipid panel thereafter if stable, or sooner if dose is changed.

Men with pre-existing dyslipidemia or statin therapy should have lipids checked at 3 months after any dose adjustment. Total testosterone above 1 to 000 ng/dL sustained over months may suppress HDL more noticeably; this is one clinical reason to target mid-normal rather than supraphysiologic levels.

Other Labs to Monitor on TRT

PSA (Prostate-Specific Antigen). Testosterone does not cause prostate cancer, but it can accelerate growth of pre-existing prostate cancer. [1] PSA should be drawn at baseline and at 3 to 6 months. A PSA rise of more than 1.4 ng/mL above baseline within the first 12 months of TRT warrants urology referral per Endocrine Society guidance. Men over 40 should have a baseline PSA before starting TRT.

LH and FSH. Luteinizing hormone and follicle-stimulating hormone are useful at baseline to classify hypogonadism as primary (high LH/FSH) or secondary (low or inappropriately normal LH/FSH). On TRT, exogenous testosterone suppresses LH to near-zero, confirming HPG axis suppression. These are not routinely monitored after baseline unless fertility or HCG co-treatment is being managed.

Comprehensive Metabolic Panel. Liver function tests, kidney function, and electrolytes give a baseline metabolic picture. Oral testosterone preparations have greater hepatotoxicity potential than injectable or transdermal forms. Injectable testosterone cypionate and transdermal testosterone have minimal hepatic impact, but a baseline CMP is standard practice. [1]

SHBG. Monitoring SHBG every 6 to 12 months helps interpret changes in free testosterone without running a full calculation panel. SHBG tends to decrease on TRT because testosterone inhibits hepatic SHBG production. Men who start TRT with high SHBG (above 50 nmol/L) often see their SHBG normalize into the 25 to 40 nmol/L range within 6 months, which changes their free testosterone substantially even if total testosterone is unchanged.

Thyroid (TSH). Thyroid dysfunction is common in men presenting with fatigue, low libido, and weight gain, the same symptom profile as hypogonadism. A TSH at baseline rules out hypothyroidism as the primary or contributing cause. [17] Hypothyroidism also raises SHBG, which can lower free testosterone even when total testosterone is normal.

How to Order the Right Tests and Read Your Results

The full baseline panel before starting TRT should include: total testosterone (morning draw), calculated free testosterone (requires SHBG and albumin), estradiol sensitive (LC-MS/MS), LH, FSH, PSA, CBC with differential, comprehensive metabolic panel, fasting lipid panel, SHBG, TSH, and prolactin (to rule out pituitary adenoma as a cause of secondary hypogonadism). [1]

At the 3-month monitoring visit, the minimum panel is: total testosterone (trough timing for injections), calculated free testosterone, estradiol sensitive, CBC (hematocrit focus), and PSA. Fasting lipids at 3 months complete the early safety picture.

At 6 months and annually: full panel including total testosterone, free testosterone (calculated), estradiol sensitive, CBC, PSA, fasting lipids, CMP, and SHBG.

Two morning samples of low total testosterone, drawn at least one week apart, are required before a formal diagnosis of hypogonadism. [1] A single low reading is not sufficient for diagnosis. This is a common oversight in direct-to-consumer testosterone programs that skip confirmatory testing.

The Endocrine Society guideline states: "We recommend confirming an initially low serum testosterone concentration with a repeat measurement of total testosterone concentration to account for the episodic secretion of testosterone and variability in laboratory assay results." [1]

For men already on TRT who want to assess where they stand, draw labs on the morning of the day of or one day before their next scheduled injection for trough values. This is the most reproducible timepoint and the one most TRT monitoring protocols use as the primary decision point for dose adjustment.