What is Free Testosterone Calculator?
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Free testosterone is the biologically active fraction of total testosterone that circulates unbound to carrier proteins and is therefore available to enter cells and exert androgenic effects. Testosterone circulates in three fractions: sex hormone-binding globulin (SHBG)-bound testosterone (approximately 60-80%), which is tightly bound and biologically inactive; albumin-bound testosterone (approximately 19-38%), which is loosely bound and biologically available; and free (unbound) testosterone (approximately 1-3% in men, 0.5-2% in women), which is directly available to target tissues. Bioavailable testosterone is the sum of free and albumin-bound fractions. In clinical practice, total testosterone is often measured first, but it can be misleading when SHBG levels are abnormally high or low. SHBG rises with ageing, hepatic disease, hyperthyroidism, oestrogen therapy, and certain medications, causing total testosterone to appear adequate while free and bioavailable testosterone are actually low. SHBG falls with obesity, hypothyroidism, insulin resistance, and androgen excess, causing total testosterone to underestimate the true androgenic exposure. The Vermeulen formula (1999) estimates free testosterone using the Law of Mass Action, taking into account SHBG, albumin, and total testosterone concentrations. Direct free testosterone measurement by equilibrium dialysis is the gold standard but is expensive and not routinely available. The calculated free testosterone using the Vermeulen formula correlates well (r > 0.9) with dialysis-measured free testosterone across most populations.
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Formula
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Vermeulen formula: Free T = Total T x [1 - (SHBG x Total T) / ((SHBG + Total T) x Kd_SHBG) - (Alb / (Alb + Kd_Alb))]; simplified approximation: Free T (pg/mL) ~ Total T (ng/dL) x 10 / (1 + 0.0001 x SHBG nmol/L); Normal free T men: 9-30 pg/mL; women: 0.3-1.9 pg/mLVariable Legend
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| Symbol | Name | Unit | Description |
|---|---|---|---|
| SHBG | Sex hormone-binding globulin | nmol/L | High-affinity carrier protein; tightly binds testosterone making it biologically inactive |
| Alb | Albumin | g/dL | Low-affinity carrier protein; albumin-bound testosterone is bioavailable and released at tissues |
| Free T | Free testosterone | pg/mL or pmol/L | Unbound biologically active testosterone fraction; typically 1-3% of total T in men |
| BioT | Bioavailable testosterone | pg/mL or nmol/L | Sum of free and albumin-bound testosterone; represents all androgenically available testosterone |
How to Free Testosterone Calculator
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- 1Measure total testosterone, SHBG, and albumin from a morning fasting blood sample (testosterone peaks in the morning).
- 2Enter values into the Vermeulen calculator, using appropriate units (total T in nmol/L or ng/dL; SHBG in nmol/L; albumin in g/dL).
- 3The calculator applies the Law of Mass Action equations to distribute testosterone between its three fractions according to binding affinities.
- 4The free testosterone output (typically in pmol/L or pg/mL) is compared against age-specific and sex-specific reference ranges.
- 5Bioavailable testosterone (free + albumin-bound) is calculated simultaneously and may be reported alongside free testosterone.
- 6Interpret low free testosterone in men (below 9 pg/mL) in the context of symptoms of hypogonadism (fatigue, low libido, erectile dysfunction, loss of muscle mass, mood changes).
- 7In women, elevated free testosterone (above 1.9 pg/mL) in the context of hirsutism, acne, irregular periods, or polycystic ovaries supports a diagnosis of androgen excess or PCOS.
Worked Examples
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Age-related SHBG rise causing functional hypogonadism despite normal total T
This 72-year-old man's total testosterone appears normal, but very high SHBG binds most of it, leaving insufficient free testosterone. Symptoms of hypogonadism in this setting reflect biochemical androgen deficiency that total testosterone alone would have missed.
Low SHBG means more of total T is free — not truly hypogonadal
Obesity suppresses SHBG, which may cause total testosterone to appear lower than it functionally is. This young man's free testosterone is in the normal range, suggesting he does not have true androgen deficiency despite a low-normal total testosterone. Weight loss will typically raise both total T and SHBG.
Low SHBG from insulin resistance amplifies androgen exposure
In this woman with PCOS, insulin resistance has suppressed SHBG, making more testosterone biologically available. Although total testosterone is within the 'normal' female range, the calculated free testosterone is elevated, confirming biochemical hyperandrogenism. This supports the PCOS diagnosis and justifies treatment of insulin resistance.
Oestrogen raises SHBG, achieving castrate-range free testosterone
Oestrogen therapy for prostate cancer markedly raises SHBG, sequestering circulating testosterone and achieving castrate-level free testosterone despite a normal total testosterone. Free testosterone measurement confirms adequate androgen suppression for tumour control.
Real-World Applications
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Diagnosing biochemical hypogonadism in older men with high SHBG and symptoms despite normal total testosterone, representing an important application area for the Testosterone Free in professional and analytical contexts where accurate testosterone free calculations directly support informed decision-making, strategic planning, and performance optimization
Confirming biochemical hyperandrogenism in PCOS when total testosterone is borderline and SHBG is low due to insulin resistance, representing an important application area for the Testosterone Free in professional and analytical contexts where accurate testosterone free calculations directly support informed decision-making, strategic planning, and performance optimization
Monitoring androgen suppression in transgender women on feminising hormone therapy, representing an important application area for the Testosterone Free in professional and analytical contexts where accurate testosterone free calculations directly support informed decision-making, strategic planning, and performance optimization
Assessing testosterone replacement therapy adequacy in men with hypogonadism, representing an important application area for the Testosterone Free in professional and analytical contexts where accurate testosterone free calculations directly support informed decision-making, strategic planning, and performance optimization
Evaluating androgen excess in women with hirsutism, acne, or menstrual irregularity when total testosterone is within the reference range, representing an important application area for the Testosterone Free in professional and analytical contexts where accurate testosterone free calculations directly support informed decision-making, strategic planning, and performance optimization
Special Cases
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Transgender women (male-to-female) on oestrogen
{'title': 'Transgender women (male-to-female) on oestrogen', 'body': 'Oestrogen therapy in transgender women markedly raises SHBG, reducing free testosterone even when total testosterone is not fully suppressed. Free testosterone targets for gender-affirming therapy aim to achieve female reference range values. Measuring free testosterone alongside SHBG and total testosterone is important for optimising hormonal treatment.'}
Athletes and anabolic steroid users
{'title': 'Athletes and anabolic steroid users', 'body': 'Exogenous androgen use suppresses endogenous LH and FSH, causing testicular atrophy and very low endogenous testosterone production. When exogenous androgens are stopped, a prolonged recovery period with low free testosterone (hypogonadotropic hypogonadism) occurs. SHBG may be very low during exogenous androgen use but rebounds markedly on cessation.'}
Congenital adrenal hyperplasia (CAH)
In the Testosterone Free, this scenario requires additional caution when interpreting testosterone free results. The standard formula may not fully account for all factors present in this edge case, and supplementary analysis or expert consultation may be warranted. Professional best practice involves documenting assumptions, running sensitivity analyses, and cross-referencing results with alternative methods when testosterone free calculations fall into non-standard territory.
Hypothyroidism and SHBG
In the Testosterone Free, this scenario requires additional caution when interpreting testosterone free results. The standard formula may not fully account for all factors present in this edge case, and supplementary analysis or expert consultation may be warranted. Professional best practice involves documenting assumptions, running sensitivity analyses, and cross-referencing results with alternative methods when testosterone free calculations fall into non-standard territory.
Free Testosterone Reference Ranges
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| Population | Free Testosterone (pg/mL) | Clinical Interpretation |
|---|---|---|
| Men 20-40 years | 9.0 - 30.0 | Normal adult male range |
| Men 41-60 years | 7.0 - 26.0 | Normal; decline with age expected |
| Men >60 years | 6.0 - 22.0 | Reduced; interpret with symptoms |
| Women (reproductive age) | 0.3 - 1.9 | Normal female range |
| Women (postmenopausal) | 0.1 - 1.2 | Reduced after menopause |
| Men — hypogonadism threshold | < 6.0 - 9.0 | Biochemical androgen deficiency if symptomatic |
Frequently Asked Questions
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Why is free testosterone more informative than total testosterone?
Total testosterone measures all testosterone in the blood, including tightly SHBG-bound fractions that cannot enter cells or exert effects. Free testosterone measures only the biologically active fraction. When SHBG is abnormal — common with ageing, obesity, thyroid disease, and liver disease — total testosterone can be profoundly misleading about the actual androgenic status of the patient.
How accurate is calculated free testosterone?
The Vermeulen calculated free testosterone correlates strongly with gold-standard equilibrium dialysis (r > 0.9 in most studies). However, it may underperform in patients with abnormal albumin, albumin variants, very high or very low SHBG, or in certain ethnic groups where binding constants may differ. For clinical decisions with significant consequences, equilibrium dialysis measurement is preferred.
Can women with PCOS have normal total testosterone but abnormal free testosterone?
Yes, frequently. Insulin resistance in PCOS suppresses SHBG, amplifying the proportion of testosterone that is free and biologically active. A woman may have total testosterone in the 'normal' female range yet have elevated free testosterone due to the low SHBG. This is biochemical hyperandrogenism even when total testosterone is not frankly elevated, and it supports the hyperandrogenism criterion of PCOS.
What factors significantly impact free testosterone levels?
Free testosterone levels are primarily influenced by total testosterone concentration and the levels of sex hormone-binding globulin (SHBG) and albumin. Conditions that increase SHBG, such as aging, hyperthyroidism, and certain medications (e.g., oral estrogens), will generally decrease free testosterone. Conversely, conditions that decrease SHBG, like obesity, hypothyroidism, and insulin resistance, can lead to elevated free testosterone, even with normal total testosterone. These dynamic interactions highlight why free testosterone provides a more nuanced clinical picture.
What are typical reference ranges for free testosterone, and what symptoms might indicate abnormal levels?
For adult men, typical reference ranges for free testosterone are approximately 50-200 pg/mL (or 0.17-0.69 nmol/L), while for premenopausal women, they are significantly lower, around 0.3-1.9 pg/mL (or 0.001-0.007 nmol/L). Low free testosterone in men can manifest as reduced libido, fatigue, and muscle weakness. In women, elevated levels may lead to symptoms like hirsutism, acne, and menstrual irregularities.
Common Mistakes to Avoid
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- !Relying on total testosterone alone to diagnose or exclude hypogonadism when SHBG is likely to be abnormal (ageing, obesity, liver disease, thyroid disease).
- !Not timing the blood sample in the morning — afternoon testosterone measurements are systematically lower due to diurnal variation.
- !Using the free androgen index (FAI) as a substitute for calculated free testosterone — FAI is a ratio, not a direct measure of free testosterone concentration.
- !Diagnosing hypogonadism from a single testosterone measurement — two morning measurements on separate days are recommended before making the diagnosis.
- !Applying male reference ranges to interpret testosterone in women — female free testosterone reference ranges are an order of magnitude lower.
Pro Tip
In any patient where you suspect the SHBG may be abnormal — obese patients (low SHBG), elderly patients (high SHBG), those on oestrogens or liver disease — always measure SHBG alongside total testosterone. Without SHBG, calculated free testosterone cannot be computed and the total testosterone result can be profoundly misleading in either direction.
Did you know?
The sex hormone-binding globulin (SHBG) molecule evolved from a protein ancestrally related to the same family as androgen-binding protein (ABP) found in the Sertoli cells of the testis. SHBG and ABP are encoded by the same gene — their tissue-specific expression patterns are regulated by different promoters, reflecting evolution's elegant solution to creating two tissue-specific hormone transport systems from a single genetic blueprint.
References
- ›Vermeulen A, Verdonck L, Kaufman JM. A critical evaluation of simple methods for the estimation of free testosterone in serum. J Clin Endocrinol Metab 1999
- ›Bhasin S et al. Testosterone Therapy in Men With Hypogonadism (Endocrine Society 2018)
- ›Rosner W et al. Position Statement: Utility, limitations and pitfalls in measuring testosterone. J Clin Endocrinol Metab 2007
- ›NICE CG156 — Fertility Problems: Assessment and Treatment 2013 (updated 2023)
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