Free T vs total T: the short answer
Free testosterone vs total testosterone is one of the most commonly misread pairs on a hormone panel. Total testosterone measures every molecule of testosterone in the blood, bound and unbound. Free testosterone measures only the small unbound fraction — the portion that can actually enter cells and activate androgen receptors. They are not interchangeable, and reading one without the other often produces the wrong clinical answer.
The Endocrine Society 2018 clinical practice guideline (Bhasin et al., Journal of Clinical Endocrinology & Metabolism, PMID 29562364) and the American Urological Association 2018/2024 testosterone deficiency guideline (Mulhall et al.) both recommend the same approach: diagnose hypogonadism with two morning total testosterone draws below the lab cutoff, and add free testosterone when total T is borderline or when sex hormone binding globulin (SHBG) is suspected to be abnormal. That second step is where most general-practice testosterone workups fall short.
The reason is mechanical. Most circulating testosterone is bound — roughly 44 to 65 percent to SHBG with high affinity, 30 to 50 percent to albumin with weak affinity, and only about 1 to 4 percent fully unbound, per the classic distribution described by Dunn, Nisula, and Rodbard (JCEM, 1981, PMID 7195404). Anything that changes SHBG — aging, hyperthyroidism, liver disease, insulin resistance, exogenous androgens, low body fat, certain medications — changes how much of the total number is actually available. Two men with identical total testosterone of 700 ng/dL can have very different free T and very different symptoms.
This piece walks through what each test measures, why SHBG distorts total T in both directions, what the published reference ranges and methodologies actually look like, how the Vermeulen calculation works, and how to read your own lab in under a minute. For the dedicated SHBG explainer, see SHBG and testosterone. For the broader panel and timing, see the TRT blood work schedule.
Educational disclaimer — please read: This article is educational content, not medical advice. It is not a substitute for evaluation by a licensed physician who knows your full history. Diagnosis of low testosterone, decisions about testing, and any TRT-related choices are clinical decisions that depend on your labs, symptoms, comorbidities, and medications. Do not start, stop, or adjust testosterone therapy based on this article. Discuss any concerns about hormone testing or treatment with a licensed prescriber.
What total testosterone actually measures
Total testosterone is the sum of every testosterone molecule in a blood sample, regardless of whether it is bound or unbound. It is the number most labs print first on a hormone panel and the value most guidelines anchor their diagnostic cutoffs against. The Harmonized reference range project (Travison et al., JCEM, 2017, PMID 28324103) used mass spectrometry across four large cohorts to standardize the adult male normal range to roughly 264 to 916 ng/dL in healthy nonobese young men aged 19 to 39. Many labs now report against this harmonized range, though older reference ranges (e.g., 300 to 1000 ng/dL) are still in use at some facilities.
Total testosterone is useful because it is cheap, widely available, and reproducible when measured well. It is also where the Endocrine Society and AUA guidelines start: a diagnosis of hypogonadism requires two morning total testosterone values below the laboratory's lower limit, paired with consistent symptoms. Single random afternoon draws are not adequate — diurnal variation alone can produce a 30 to 35 percent swing between morning and late-afternoon values in young men.
Total testosterone is also where total T alone misleads. The number does not distinguish between testosterone that is locked to SHBG and inaccessible to tissues, testosterone that is loosely bound to albumin and partially available, and testosterone that is fully free. When SHBG is abnormal in either direction, total T loses interpretive accuracy. The deeper the SHBG abnormality, the less useful total T is in isolation.
What free testosterone actually measures
Free testosterone is the small unbound fraction of total testosterone — typically 1 to 4 percent of the total — that is not attached to any plasma protein. Because no binding partner is holding it in the bloodstream, free T is the form available to dissociate at the capillary level, enter tissues, and bind androgen receptors inside cells. It is the part of the testosterone signal that actually does work.
A related concept is "bioavailable testosterone," which combines free T with the loosely albumin-bound fraction. Albumin binding is weak enough that this fraction can dissociate at tissue capillaries and reach receptors, so bioavailable T sums the genuinely accessible portion. Some labs report bioavailable T; others report only free T and SHBG. Either approach gets at the same physiological question: how much testosterone is actually usable.
Free testosterone matters most when total T is borderline, when SHBG is abnormal, or when symptoms do not match the total T number. It also matters during follow-up on TRT — because TRT suppresses SHBG, the relationship between total T and free T shifts after starting treatment. A follow-up total T that looks "in the optimal range" can pair with a free T that runs higher than the pre-TRT relationship would have predicted.
How total T breaks down in circulation
The chart below shows the typical distribution of testosterone between SHBG-bound, albumin-bound, and free fractions in adult men, based on the Dunn/Nisula/Rodbard binding model still cited in modern Endocrine Society guidance. It is a conceptual reference, not patient data.
Two things stand out from the distribution. First, the active fraction is small — a few percent of the total. Second, the SHBG-bound fraction is the lever. Any shift in SHBG redistributes the rest, which is why SHBG measurement is part of any complete TRT lab panel.
Why SHBG throws off the total number
SHBG is a hepatic glycoprotein that binds testosterone, DHT, and estradiol tightly. When SHBG rises, more testosterone is locked up; when SHBG falls, more is released. The total T number on a lab report does not capture this redistribution because it counts bound and free hormone identically. The biological effect, however, is dominated by free T.
Three clinical patterns recur:
- High SHBG, normal total T, low free T. Common in older men, lean endurance athletes, men with hyperthyroidism or chronic liver disease, and men on certain anticonvulsants. Symptoms of low T appear despite a normal-looking total T. The Endocrine Society 2018 guideline specifically flags this scenario as a reason to measure free T.
- Low SHBG, low-normal total T, adequate free T. The metabolic-syndrome pattern — insulin resistance, central obesity, non-alcoholic fatty liver disease, type 2 diabetes. Total T may look low, but calculated free T can still sit within range because more of the small total pool is unbound. Wallace et al. (Diabetes, 2013, PMID 23193177) and Ding et al. (JAMA, 2006, PMID 16868298) showed that low SHBG independently predicts incident type 2 diabetes, separate from BMI.
- Mid-range SHBG. Total T tracks reasonably well with free T and symptoms. In this band, total T alone is a more interpretable signal — but SHBG should still be drawn at baseline to know which band a patient is in.
The European Male Aging Study (Wu et al., NEJM, 2010, PMID 20554979) underlined how often SHBG drives the mismatch. In a multi-country cohort of more than 3,000 men aged 40 to 79, total T fell modestly with age while SHBG rose more steeply, producing a sharper decline in free T than the total T number alone suggested. That asymmetric decline is why age-related testosterone questions need a free T reading to be answered properly.
Same total T, very different free T
The chart below compares calculated free testosterone in two hypothetical men with identical total T of 600 ng/dL but very different SHBG. Both are within the labeled total T normal range, yet only one has a free T above the typical reference floor. Values use the Vermeulen calculation with default albumin.
Same total T, three different clinical pictures. The high-SHBG patient on the right has calculated free T below the typical reference floor and may be symptomatic despite a normal-looking total T. The low-SHBG patient on the left has free T well into the supraphysiological range at exactly the same total. This is the visual reason guideline bodies recommend free T when SHBG is suspected to be abnormal.
What Endocrine Society and AUA guidelines say
Two guideline documents anchor most US practice. The Endocrine Society 2018 clinical practice guideline (Bhasin et al., JCEM, 2018, PMID 29562364, DOI 10.1210/jc.2018-00229) and the American Urological Association 2018 testosterone deficiency guideline (Mulhall et al., updated 2024) line up closely on diagnosis. Both recommend:
- Diagnose hypogonadism with two morning total testosterone drawsbelow the laboratory's lower reference limit, accompanied by consistent symptoms. A single value is insufficient because of diurnal variation and short-term biological fluctuation.
- Measure free testosterone when total T is in the borderline range, when SHBG is suspected to be abnormal, or when symptoms persist despite a normal-looking total T.
- Use a recommended free T methodology. The Endocrine Society explicitly endorses equilibrium dialysis as the reference standard and accepts calculated free T from total T, SHBG, and albumin. It explicitly does not recommend direct (analog) immunoassay free T for adult men because of accuracy issues at male physiological ranges.
- Confirm the panel under standardized conditions. Morning fasting draw, avoidance of acute illness and recent steroid exposure, and standardized assay platforms (mass spectrometry preferred for total T).
Neither guideline endorses treating a number alone. Both require symptoms to be consistent with hypogonadism — sexual function changes (low libido, reduced morning erections, erectile dysfunction not better explained by vascular disease), and at the more specific end, low energy, mood symptoms, decreased lean mass and strength, increased body fat, and reduced bone mineral density. The AUA 2024 amendment additionally clarifies cardiovascular monitoring expectations in light of the 2023 TRAVERSE trial (Lincoff et al., NEJM, 2023, PMID 37326322), which we cover in the TRAVERSE trial breakdown.
Pro tip — order three numbers, not one: When you book your first hormone panel, ask the prescriber to order total testosterone, SHBG, and either calculated or equilibrium-dialysis free testosterone on the same morning draw. The marginal cost of SHBG and free T over total T alone is small, and it prevents the most common failure mode in primary care — diagnosing or dismissing a man on total T alone with no SHBG context. The TRT blood work schedule walks through the full pre-TRT panel.
Normal ranges for total T and free T
Reference ranges vary by lab and assay. The values below are common reference points, not lab-specific or guideline-defined targets. Your prescriber interprets your value against the specific lab's methodology and your clinical picture.
Common reference ranges (adult men)
| Measurement | Typical adult male reference range | Notes |
|---|---|---|
| Total testosterone (mass spec) | 264 to 916 ng/dL | Harmonized reference range, Travison JCEM 2017 (PMID 28324103) for ages 19 to 39 |
| Total testosterone (older immunoassay) | ~300 to 1000 ng/dL | Pre-harmonization cutoffs; still seen at some labs |
| Free testosterone (equilibrium dialysis) | ~47 to 244 pg/mL | Reference-standard methodology; varies by lab |
| Free testosterone (calculated, Vermeulen) | ~50 to 220 pg/mL | Validates well against equilibrium dialysis |
| Free testosterone (direct/analog immunoassay) | Not recommended by Endocrine Society 2018 | Use a different method if your lab reports this |
| SHBG | ~10 to 57 nmol/L | Quest and LabCorp ranges similar; see SHBG explainer |
| Albumin (for calc) | 3.5 to 5.0 g/dL | Defaults to 4.3 g/dL in Vermeulen if not measured |
Values above are typical orientation points and vary by lab. Always read your lab's own reference range printed on the report and discuss with your prescriber.
Assay methods: where most reports go wrong
Methodology matters more for hormone assays than for most other lab tests, and the most common errors on a hormone panel come from assay choice rather than reading. Stanczyk et al. (JCEM, 2007) and subsequent endocrine consensus statements have argued repeatedly that mass spectrometry (LC-MS/MS) is the preferred platform for total testosterone in men at the lower end of the range, where immunoassay platforms tend to be least accurate.
For total T:
- Liquid chromatography mass spectrometry (LC-MS/MS).Reference standard. More accurate at low-male and pediatric ranges. Quest's "Testosterone, Total, MS" and LabCorp's "Testosterone, Total, LC/MS-MS" use this platform.
- Automated chemiluminescent immunoassay (CLIA). Faster and cheaper, used in most routine total T orders. Generally acceptable for adult male values in the mid-to-high range; less reliable at low-male values where many borderline diagnoses live.
For free T:
- Equilibrium dialysis. Reference standard. Physically separates bound and free hormone before measurement.
- Calculated free T (Vermeulen formula). Derived from total T, SHBG, and albumin. Validates well against equilibrium dialysis in adult men. Acceptable for guideline use.
- Direct (analog) immunoassay free T.Not recommended for adult men by the Endocrine Society 2018 guideline. If your lab reports "Free Testosterone, Direct" without a methodology note, the value should be interpreted with caution and ideally replaced.
Estradiol on a male hormone panel runs into the same methodology issue. The standard estradiol immunoassay is calibrated for female ranges and tends to read inaccurately at the much lower male physiological values, which is why "sensitive estradiol" (LC-MS/MS or ultrasensitive immunoassay) is the recommended methodology in men — see estradiol on TRT.
Calculated free T (Vermeulen) vs equilibrium dialysis
The Vermeulen equation (Vermeulen, Verdonck, Kaufman, JCEM, 1999, PMID 10523012) was developed as a practical alternative to equilibrium dialysis when dialysis is unavailable or impractical. It takes three inputs:
- Total testosterone (ng/dL or nmol/L)
- SHBG (nmol/L)
- Albumin (g/dL or g/L) — defaults to 4.3 g/dL if not measured
The output is calculated free testosterone in pmol/L or pg/mL, depending on units. The math is built into most modern lab order sets; if your lab reports a calculated free T, it is almost always using Vermeulen or a closely related formula. The output validates well against equilibrium dialysis in nonpregnant adult men under normal physiological conditions.
Equilibrium dialysis remains the reference standard. It physically separates free hormone from bound hormone using a semipermeable membrane and quantifies what passes through. It is the methodology most often used in clinical research, and it is the test ordered when a calculated free T is implausible relative to symptoms or when the case is medically complex. The trade-off is cost and turnaround — equilibrium dialysis is more expensive and slower than a calculated result derived from total T and SHBG.
Quick mental model: If SHBG roughly doubles, calculated free T roughly halves at the same total T — and vice versa. That single ratio is why ordering SHBG alongside total T is the single biggest interpretive upgrade you can make to a hormone panel.
Why free androgen index is not interchangeable
Some lab reports include a number called the Free Androgen Index (FAI), calculated as total testosterone divided by SHBG, multiplied by 100. FAI is an older index originally developed in research contexts and is not interchangeable with calculated free T. The Endocrine Society 2018 guideline does not recommend FAI for adult male diagnosis because of calibration limitations across the adult male range and lack of validation against equilibrium dialysis at male values.
If your lab report shows "Free Androgen Index" or "FAI" and you want a usable free T, ask whether the lab can run a Vermeulen-calculated free testosterone or equilibrium dialysis instead. FAI can be left on the report — it does not need to be removed — but it should not be the number you anchor a diagnosis or dose decision on.
What changes with age, weight, and metabolic state
Both total testosterone and free testosterone change with age, but not at the same rate. Travison et al. (JCEM, 2007, PMID 17284627) using the Massachusetts Male Aging Study and Wu et al. (NEJM, 2010, PMID 20554979) using the European Male Aging Study both documented a gradual decline in total T after about age 40 paired with a steeper rise in SHBG, producing a sharper decline in calculated free T than the total T number alone implied.
The chart below illustrates the typical age-related trajectory of total T, SHBG, and calculated free T across adult male life. Curves are conceptual based on the population literature.
Body composition and metabolic state add another layer. Obesity, especially visceral adiposity, suppresses SHBG, which redistributes the testosterone signal. A man with central obesity and a total T of 380 ng/dL may have a low SHBG and a calculated free T that lands close to the floor — or above it, depending on the exact ratio. Weight loss and improved insulin sensitivity tend to raise SHBG over months, which changes how the same total T translates into free T. This pattern is part of why monitoring on weight-loss medications matters; see GLP-1 medications and TRT for the metabolic-hypogonadism overlap.
How to read your lab in 90 seconds
Here is a quick framework for reading a hormone panel that includes total T, SHBG, free T, and albumin. It does not replace your prescriber's interpretation, but it gets you to the right question fast.
Step 1 — Check total testosterone against the lab's reference range
Look at the lab's printed lower limit. The Harmonized reference is roughly 264 ng/dL; older immunoassay-based ranges can be as high as 300 ng/dL. Is your value above or below the floor? Note whether this is one draw or two; AUA and Endocrine Society guidelines both require two morning draws below the floor for a hypogonadism diagnosis.
Step 2 — Check SHBG
Most adult-male ranges run roughly 10 to 57 nmol/L. Where does your SHBG sit? Low (under 20), mid (20 to 45), upper-normal (45 to 60), or high (60+)?
Step 3 — Read free T against its own reference range
Use the lab's printed free T range, not a memorized number. Was free T measured by equilibrium dialysis, calculated by Vermeulen, or direct immunoassay? If direct immunoassay, the result needs a methodology upgrade before it carries diagnostic weight.
Step 4 — Check for a mismatch
Total T normal but free T low typically means high SHBG. Total T low but free T adequate typically means low SHBG. Total T and free T both low usually means primary or secondary hypogonadism, depending on LH/FSH. Total T and free T both high usually means recent androgen exposure or supraphysiological response.
Step 5 — Cross-check against symptoms
Numbers without symptoms are not a diagnosis. Symptoms without numbers are not either. Both have to align — and the Endocrine Society 2018 guideline emphasizes that the more specific sexual symptoms (low libido, reduced morning erections) carry more diagnostic weight than nonspecific symptoms (fatigue, mood changes) which overlap with sleep apnea, depression, hypothyroidism, anemia, and other conditions. See low testosterone symptoms for the structured self-check including the ADAM and AMS questionnaires.
Mini-scenario — a real-world misread:A 47-year-old endurance runner gets a total T of 540 ng/dL on a standard panel and is told he is "normal." Symptoms continue. He requests SHBG and free T. SHBG comes back at 72 nmol/L; calculated free T is 38 pg/mL, below the lab's floor of 47. The total T number alone missed the diagnosis the rest of the panel caught. This pattern is common in lean men over 40 — high SHBG, normal total T, low free T.
When each number actually matters
Total and free testosterone are not redundant — each carries weight in different contexts. The clinical question to ask is which one is dominant for your case.
Total testosterone matters most when:
- You are screening for hypogonadism for the first time — guidelines anchor the diagnosis on total T.
- SHBG is mid-range and the total number lines up with symptoms.
- You are tracking ester pharmacokinetics on TRT — peak and trough total T are what most prescribers cycle through to titrate dose and injection frequency.
- You are comparing pre-TRT values to follow-up under the same SHBG conditions.
Free testosterone matters most when:
- Total T is borderline — within roughly 100 ng/dL of the lab floor.
- SHBG is suspected to be abnormal — aging men, lean men, men with thyroid disease, liver disease, insulin resistance, NAFLD, or on SHBG-altering medications.
- Symptoms persist despite a normal-looking total T.
- You are on TRT and follow-up labs show SHBG has shifted from baseline — calculated free T captures what total T alone hides.
- You are reading older-male data — the European Male Aging Study and other cohorts show free T tracks symptoms better than total T after age 50.
Bioavailable testosterone matters most when:
- You want a single number that captures both free T and the loosely albumin-bound fraction.
- Your lab reports bioavailable T instead of free T; some clinical research uses it as the primary metric.
Key takeaways
Free testosterone vs total testosterone is not an either/or choice. The two numbers answer different questions, and reading them together — alongside SHBG and the lab's methodology — produces the right clinical picture more often than reading either in isolation.
- Total T measures every molecule, bound and unbound. Free T measures only the small unbound fraction that actually reaches androgen receptors.
- SHBG is the lever that decides where total T lands as free T. High SHBG hides low free T behind a normal total T; low SHBG can produce adequate free T at a low-normal total T.
- Endocrine Society 2018 and AUA 2018/2024 both recommend free T when total T is borderline or SHBG is suspected to be abnormal. Two morning total T draws below the lab floor plus consistent symptoms remains the diagnostic anchor.
- Equilibrium dialysis is the reference standard for free T. Calculated free T (Vermeulen formula) validates well against it. Direct/analog immunoassay free T is not recommended for adult men.
- Free Androgen Index is not interchangeable with calculated free T. Replace it with a Vermeulen calculation or equilibrium dialysis when you want a usable free T.
- Free T declines more steeply than total T with age because SHBG rises. EMAS and Travison cohorts both document this asymmetry.
- On TRT, the relationship shifts. SHBG suppresses dose-dependently, so a follow-up total T translates to a higher free T than the pre-TRT ratio would predict. Interpret follow-up labs with the new SHBG, not pre-TRT expectations.
For the deeper SHBG mechanism, see SHBG and testosterone. For the full lab schedule including draw timing by ester, see the TRT blood work schedule. For other commonly misread markers on a TRT panel, see estradiol on TRT and hematocrit on TRT. If you are weighing testing options before committing to a full panel, the at-home testosterone test kits review notes which services include SHBG and free T versus total only. For the symptom-side companion, see low testosterone symptoms. For pillar context, see the blood work guide and TRT 101 overview.
Sources referenced in this article:
- Bhasin S et al., "Testosterone Therapy in Men With Hypogonadism: An Endocrine Society Clinical Practice Guideline," Journal of Clinical Endocrinology & Metabolism (2018). PMID 29562364, DOI 10.1210/jc.2018-00229.
- Mulhall JP et al., "Evaluation and Management of Testosterone Deficiency: AUA Guideline" (2018, updated 2024).
- Vermeulen A, Verdonck L, Kaufman JM. "A critical evaluation of simple methods for the estimation of free testosterone in serum," JCEM (1999). PMID 10523012.
- Dunn JF, Nisula BC, Rodbard D. "Transport of steroid hormones: binding of 21 endogenous steroids to both testosterone-binding globulin and corticosteroid-binding globulin in human plasma," JCEM (1981). PMID 7195404.
- Travison TG et al., "Harmonized Reference Ranges for Circulating Testosterone Levels in Men of Four Cohort Studies in the United States and Europe," JCEM (2017). PMID 28324103.
- Travison TG et al., "A population-level decline in serum testosterone levels in American men," JCEM (2007). PMID 17284627.
- Wu FCW et al., "Identification of late-onset hypogonadism in middle-aged and elderly men" (European Male Aging Study), NEJM (2010). PMID 20554979.
- Ding EL et al., "Sex differences of endogenous sex hormones and risk of type 2 diabetes," JAMA (2006). PMID 16868298.
- Wallace IR et al., "Sex hormone binding globulin and insulin resistance," Diabetes (2013). PMID 23193177.
- Stanczyk FZ et al., "Standardization of steroid hormone assays: why, how, and when?" JCEM (2007).
- Lincoff AM et al., "Cardiovascular Safety of Testosterone-Replacement Therapy" (TRAVERSE trial), NEJM (2023). PMID 37326322, DOI 10.1056/NEJMoa2215025.
- Quest Diagnostics and LabCorp test catalogs — adult male total T, SHBG, and free T reference ranges and methodology notes.
This article was written by the TRT FAQ Editorial Team and reviewed for alignment with current Endocrine Society and AUA guidance. It is updated periodically as new evidence emerges. Last content review: May 2026.
Frequently Asked Questions
Free testosterone vs total testosterone — which one should you actually look at?
Both, in context. Total testosterone is the wider screening number and the value most guidelines diagnose hypogonadism against, requiring two morning draws below the lab cutoff. Free testosterone — the unbound fraction that actually reaches androgen receptors — is what determines symptoms and is the deciding number when total T is borderline or when SHBG is suspected to be abnormal. The Endocrine Society 2018 clinical practice guideline (Bhasin et al., JCEM 2018, PMID 29562364) and the AUA 2018/2024 testosterone deficiency guideline both recommend measuring free testosterone when total T is in the borderline range or when SHBG-altering conditions are present. Reading either number alone misses cases.
What is a normal free testosterone level for men?
Most reference labs report adult male free testosterone in the range of approximately 47 to 244 pg/mL when measured by equilibrium dialysis or calculated with the Vermeulen formula, though exact cutoffs vary by lab and methodology. The Harmonized reference range work led by Travison et al. (JCEM 2017, PMID 28324103) standardized total testosterone cutoffs to 264 to 916 ng/dL in healthy young men using mass spectrometry, but free testosterone reference ranges remain assay-dependent. The clinically useful question is not just the number — it is whether free T is below the floor at the same time symptoms are present and SHBG is accounted for.
Can free testosterone be low when total testosterone is normal?
Yes, and this is one of the most commonly missed diagnoses in primary care. A man with total testosterone of 500 ng/dL and SHBG of 80 nmol/L can have calculated free testosterone below the lab floor — paired with classic low-T symptoms despite a normal-looking total number. The Endocrine Society 2018 guideline specifically calls out this scenario: men with conditions that alter SHBG (aging, hyperthyroidism, liver disease, low body fat, chronic illness, certain medications) need free T to interpret their result correctly. Total T alone is not sufficient when SHBG is suspected to be abnormal.
Why does SHBG throw off testosterone results?
Sex hormone binding globulin (SHBG) binds testosterone tightly in the bloodstream. When SHBG is high, more total testosterone is locked up and less is available as free hormone — so a normal total T can pair with a low free T. When SHBG is low, the opposite happens: a modest total T can still produce adequate free T. The widely-used estimate from Dunn, Nisula, and Rodbard (JCEM 1981, PMID 7195404) describes typical adult male testosterone as roughly 44 to 65 percent SHBG-bound, 30 to 50 percent loosely albumin-bound, and only 1 to 4 percent fully unbound. Any condition that shifts SHBG shifts where that ratio lands.
What is the difference between calculated free T and direct free T?
Calculated free testosterone uses a published formula — most often the Vermeulen equation (Vermeulen, Verdonck, Kaufman, JCEM 1999, PMID 10523012) — that derives free T from total T, SHBG, and albumin. It validates well against equilibrium dialysis. Direct (analog) immunoassay free testosterone is a separate test that measures free T with an antibody-based method and is not recommended for adult men by the Endocrine Society 2018 guideline because of inaccuracy at male physiological ranges. If your lab report just says 'free testosterone' without methodology, ask whether it is calculated (Vermeulen) or equilibrium dialysis. If it is direct/analog immunoassay, request a repeat with a recommended method.
Should I get free testosterone tested before starting TRT?
Yes, in most cases. Both the Endocrine Society 2018 and AUA 2018/2024 guidelines recommend free testosterone alongside total T when total is borderline, when SHBG is suspected to be abnormal, or when symptoms persist despite a normal-looking total T. Drawing two morning total testosterone values with a same-time SHBG and either calculated or equilibrium-dialysis free T gives your prescriber a complete picture. The marginal cost of adding SHBG and free T to a panel is small relative to the diagnostic clarity it provides. See the TRT FAQ blood work schedule for the full pre-TRT panel and the SHBG explainer for why both numbers are needed.
Does TRT change free testosterone more than total testosterone?
Often yes, because TRT suppresses SHBG in a dose-dependent manner. As SHBG falls, the same total T number translates into a higher free T than it would have before treatment. A man whose pre-TRT total T was 350 ng/dL with SHBG of 40 nmol/L might reach a follow-up total T of 700 ng/dL with SHBG of 20 nmol/L — and his calculated free T more than doubles even though total T only doubled. This is part of why follow-up labs must be interpreted with the new SHBG, not against pre-TRT expectations, and why feeling on TRT does not always track linearly with the total T number alone.