Summary
Free testosterone is the small fraction of testosterone (around 2%) that circulates unbound and is immediately available to act on tissues. Because the rest is bound to SHBG and albumin, free testosterone is often a more accurate marker of androgen status than total testosterone — particularly when SHBG levels are abnormal. It is the key measurement for diagnosing testosterone deficiency in borderline cases.
Only free and loosely albumin-bound testosterone (together called ‘bioavailable testosterone’) can enter cells and exert biological effects. SHBG-bound testosterone is effectively locked away.
This matters clinically: conditions that raise SHBG (ageing, hyperthyroidism, liver disease, oestrogen) reduce free testosterone even when total testosterone looks normal — explaining persistent symptoms. Conditions that lower SHBG (obesity, insulin resistance, hypothyroidism) raise free testosterone relative to total.
Free testosterone is either measured directly or, more accurately, calculated from total testosterone, SHBG, and albumin. Calculated free testosterone is preferred by most guidelines, as direct assays can be unreliable.
What It Is
Free testosterone represents the immediately bioavailable androgen fraction. Calculated free testosterone — derived using the Vermeulen equation from total testosterone, SHBG, and albumin — is considered more reliable than direct immunoassay measurement, which suffers from poor accuracy at low concentrations.
The Endocrine Society recommends free testosterone measurement when total testosterone is borderline or when SHBG abnormalities are suspected (obesity, diabetes, thyroid disease, ageing, nephrotic syndrome). Bioavailable testosterone (free plus albumin-bound) is an alternative that some laboratories report.
Reference ranges (men): approximately 0.2–0.62 nmol/L (200–620 pmol/L), varying by assay and laboratory. Calculated free testosterone below the lower limit, in a symptomatic man with low-normal total testosterone, supports a diagnosis of hypogonadism.
Functions
Active androgen delivery
Free testosterone is the fraction that actually enters cells and binds androgen receptors — directly determining testosterone's biological effect on tissues.
Deficiency detection in borderline cases
Reveals true androgen deficiency when total testosterone is low-normal but SHBG is elevated — a common and easily missed scenario.
Libido and sexual function
Symptoms of low libido and erectile dysfunction correlate more closely with free than total testosterone.
Treatment monitoring
Tracks the bioavailable androgen response to testosterone replacement therapy more accurately than total testosterone.
Reference Ranges
Free Testosterone (calculated)
Measured in pmol/L| Status | Range (pmol/L) | Range (nmol/L) | What it means |
|---|---|---|---|
| Low | < 200 (men) | < 0.20 | Low bioavailable androgen — supports hypogonadism even if total testosterone is borderline. |
| Optimal | 200–620 (men) | 0.20–0.62 | Adequate active testosterone for muscle, libido, mood, and bone. |
| High | > 620 (men) | > 0.62 | Elevated — consider supplementation, low SHBG, or androgen excess. |
Reference ranges vary widely by assay and method. Calculated free testosterone is preferred over direct measurement. Female ranges are much lower. Interpret with total testosterone, SHBG, and clinical symptoms.
Symptoms of Imbalance
Free testosterone symptoms mirror those of total testosterone, but free testosterone better explains symptoms when SHBG is abnormal.
- Reduced libido and erectile dysfunction
- Fatigue and reduced vitality
- Loss of muscle mass and strength
- Low mood and reduced motivation
- Increased abdominal fat
- Poor concentration
- Reduced morning erections (men)
- Acne and oily skin
- Hirsutism (excess hair) in women
- Scalp hair thinning
- Irregular periods (women with PCOS)
- Mood changes
Causes of Imbalance
- Hypogonadism (primary or secondary)
- High SHBG (ageing, hyperthyroidism, liver disease, oestrogen therapy)
- Chronic illness and stress
- Obesity (though this often lowers SHBG, total may fall more)
- Opioid or corticosteroid use
- Type 2 diabetes
- Low SHBG (obesity, insulin resistance, hypothyroidism)
- PCOS in women
- Anabolic steroid use
- Androgen-secreting tumours
- Congenital adrenal hyperplasia
FAQs
Total testosterone can be misleading when SHBG is abnormal. For example, an older man with high SHBG may have a ‘normal’ total testosterone but a low free testosterone — and therefore genuine androgen deficiency causing his symptoms. Free testosterone reflects the hormone actually available to tissues, making it more clinically meaningful in these situations.
The most reliable method is calculation using the Vermeulen equation, which combines total testosterone, SHBG, and albumin. This is preferred over direct free testosterone immunoassays, which are inaccurate at the low concentrations found in blood. This is why a complete assessment measures total testosterone, SHBG, and albumin together.
SHBG rises with ageing, hyperthyroidism, liver disease, oestrogen (including oral contraceptives and HRT), and certain medications. Higher SHBG binds more testosterone, reducing the free, active fraction. This is why two men with identical total testosterone can have very different free testosterone levels and very different symptoms.
Yes — this occurs when SHBG is low (as in obesity, insulin resistance, or hypothyroidism). Low SHBG means a greater proportion of testosterone is free and active, so free testosterone can be normal even when total testosterone appears low. This is why both should always be interpreted together.
References
- Vermeulen A, et al. A critical evaluation of simple methods for the estimation of free testosterone in serum. J Clin Endocrinol Metab. 1999;84(10):3666–3672. View source
- Bhasin S, et al. Testosterone therapy in men with hypogonadism: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2018;103(5):1715–1744. View source
- Antonio L, et al. Free testosterone reflects metabolic as well as ovarian disturbances. Eur J Endocrinol. 2018;179(6):D1–D14. View source