Free testosterone
The small fraction of testosterone that is not bound to proteins and is biologically active. It clarifies the picture when the total testosterone is borderline or when SHBG is abnormal — because it is the free hormone that actually acts on the body.
What it measures
Most testosterone in the blood is bound to proteins — tightly to sex hormone-binding globulin (SHBG) and loosely to albumin — and only the small unbound (free) fraction, together with the loosely bound part, is biologically available to act on tissues. Free testosterone measures (or estimates, from total testosterone and SHBG) this active portion. Its value is in situations where the total testosterone can mislead: because SHBG changes with age, obesity, thyroid disease, liver disease and other factors, a 'normal' total testosterone can hide a genuinely low free (active) level — for example an older or obese man with symptoms of low testosterone but a borderline-normal total. In practice, free testosterone is most often calculated from a morning total testosterone and SHBG rather than measured directly, since the direct assays are technically difficult. It is interpreted with symptoms and the other reproductive hormones (LH, FSH), not in isolation.
What a high value can mean
- Androgen excess — in women, an important finding: PCOS is the commonest cause, causing acne, excess hair and irregular periods.
- Testosterone or anabolic steroid use.
- Low SHBG states — insulin resistance, obesity; can raise the free fraction relative to total.
What a low value can mean
- Low testosterone (hypogonadism) — in men, with reduced libido, fatigue, erectile difficulty; free testosterone can reveal this when the total looks borderline.
- High SHBG (ageing, thyroid or liver disease) — locks up testosterone, lowering the free active portion even when total is normal.
- Pituitary or testicular problems — interpreted with LH and FSH.
When to discuss with a doctor
Free testosterone earns its place when the total testosterone doesn't fit the clinical picture. In a man with symptoms of low testosterone (low libido, fatigue, erectile difficulty) but a borderline-normal total, a low free testosterone confirms a genuine deficiency and supports treatment; this is especially relevant with older age or obesity, where SHBG changes distort the total. In a woman with signs of androgen excess (acne, excess hair, irregular periods), a raised free testosterone supports a PCOS assessment. It should be measured on a morning sample and read with SHBG, LH, FSH and the symptoms. Mediora.AI shows free (or calculated) testosterone with total testosterone and SHBG so a 'normal' total isn't read in isolation; the hormonal decision belongs with your doctor or endocrinologist.