Testosterone does not float around the bloodstream on its own. Most of it is bound tightly to sex hormone-binding globulin (SHBG), and a large share is bound loosely to albumin, the most common protein in blood. Only about 1% to 4% of circulating testosterone is unbound, or free.1
You will also see the term bioavailable testosterone. That means free testosterone plus the albumin-bound portion, on the reasoning that albumin lets go easily enough for tissues to use it.1 The idea that only free hormone is active, known as the free hormone hypothesis, is still debated, and experts have questioned some of the binding models used to calculate it.1
In practice, free testosterone matters most when your total testosterone is borderline, or when something has pushed SHBG up or down and made the total misleading.
Why does free testosterone matter?
Because SHBG levels vary a lot between men, two men with the same total testosterone can have quite different amounts of free hormone. Free testosterone helps separate a genuinely low level from a total that has simply been shifted by SHBG.
SHBG tends to rise with age and with conditions such as hyperthyroidism, and it tends to fall with obesity, insulin resistance and type 2 diabetes.1 So an older, lean man can have a respectable total but a lowish free level, while a heavier man can have a low total but a free level that is closer to normal.
Analyses from the European Male Ageing Study found that men with normal total but low free testosterone had higher LH levels and reported more sexual and physical symptoms, which suggests the free number is picking up something real.1
How is free testosterone measured?
The reference method is equilibrium dialysis, which is accurate but slow and expensive. Most clinics instead calculate free testosterone from total testosterone and SHBG, which closely matched dialysis in a careful comparison.2
That comparison, by Vermeulen and colleagues, found calculated free testosterone gave values almost identical to equilibrium dialysis in almost every situation they tested.2 It also found two shortcuts to be unreliable: direct analogue immunoassays, which read only a fraction of the true value, and the free androgen index, which drifts with SHBG levels.2
- Equilibrium dialysis: most accurate, rarely used routinely
- Calculated free testosterone: reliable and practical2
- Direct analogue immunoassay: not a reliable guide2
- Free androgen index: unreliable in men2
When should free testosterone be checked?
The Endocrine Society recommends a free testosterone result when total testosterone is near the lower limit of normal, or when a man has a condition known to alter SHBG.3 It should come from equilibrium dialysis or an accurate formula.
It is not usually the first test. The starting point is a fasting morning total testosterone, repeated to confirm a low result.3 Free testosterone is the tie-breaker when that number sits in a grey zone or looks out of step with how you feel.
What counts as low free testosterone?
In the European Male Ageing Study, late-onset hypogonadism was best defined by at least three sexual symptoms plus total testosterone under 11 nmol/L and free testosterone under 220 pmol/L.4 Individual labs quote their own reference ranges.
The three symptoms that tracked most closely with low levels were fewer morning erections, low sexual desire and erectile dysfunction.4 A low free number without symptoms is a reason for a conversation, not necessarily for treatment. If you have those symptoms, a GP can arrange the right tests.
Common questions
Is free testosterone more important than total testosterone?
Neither wins outright. Guidelines start with total testosterone and add free testosterone when the total is borderline or SHBG is likely to be abnormal.3 Read together, they give a clearer picture than either alone.
Can total testosterone be normal while free testosterone is low?
Yes, usually when SHBG is high, which is more common with age. Men in that situation in the European Male Ageing Study reported more symptoms than men whose levels were both normal.1
Can I raise my free testosterone?
The realistic route is to deal with whatever is shifting your hormones: excess weight, poorly controlled diabetes, a thyroid problem or a medicine. Research on supplements that target free testosterone is still developing, so talk to a doctor before relying on one. Our guide to testosterone after 40 covers the basics.
Read more on this
How testosterone changes after 40 (and what actually helps)
Does testosterone affect sex drive? Separating myth from mechanism
Sources
- Goldman AL, et al. A reappraisal of testosterone's binding in circulation: physiological and clinical implications. Endocr Rev. 2017;38(4):302-324. PubMed
- 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;84(10):3666-3672. PubMed
- 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. PubMed
- Wu FC, et al. Identification of late-onset hypogonadism in middle-aged and elderly men. N Engl J Med. 2010;363(2):123-135. PubMed
Not medical advice. Written by Eric Stephens from the research cited above; not reviewed by a doctor. If something worries you, see a GP.