Written by: Centre for Men's Health
Reviewed for medical accuracy by Dr Ed Rainbow, BMBS, MRCGP, MSc, men’s health expert
Total testosterone is often seen as the go-to measure in terms of whether a man is suffering from low testosterone or testosterone deficiency. But it is also important to measure free testosterone – the portion of testosterone not bound up by blood proteins and available for use by the body.
Sex hormone binding globulin (SHBG), which is produced mainly by the liver, is the most important blood protein as far as free testosterone is concerned. It does exactly what its name suggests, binding up hormones, in this case testosterone, and making them unavailable for use by body tissues. Sex hormone binding globulin:
- Makes hormones biologically inactive;
- Regulates their availability for use by the body;
- Transports them to target cells;
- Stops them being broken down by the liver and kidneys until they are needed.
Another protein made by the liver and circulating in the blood, albumin, also binds testosterone but less strongly, with bonds that are more easily weakened.
So what is free testosterone?
Free testosterone is the tiny portion (2-5%) that is unbound to proteins like SHBG and albumin and can act on body tissues by binding to androgen receptors.
Why does free testosterone matter?
Free testosterone matters because it is possible to have normal levels of testosterone, but if you have high levels of SHBG, your free testosterone may actually be low.
There is evidence that calculated free testosterone often relates more closely to hypogonadal (low T) signs, symptoms and physical function than total testosterone alone, particularly in older men or when SHBG is abnormal.1,2,3,4
For this reason, for example the British Society of Sexual Medicine guidelines for male adult testosterone deficiency suggest treatment may be recommended for patients with Free T below 0.225 nmol/L even if total testosterone levels are within normal ranges.5
How do we measure free T?
While free testosterone can be measured directly, it can also be calculated reliably using a formula based on levels of total testosterone, SHBG and albumin. The most widely used version of this calculation is the Vermeulen formula, developed by Belgian endocrinologist Alvaro Vermeulen and colleagues.10 Usually the lab will do this for you, but you can do it yourself as well https://www.pctag.uk/testosterone-calculator/ . A simpler way of looking at it is the Free Androgen Index, calculated by dividing your total testosterone level by your SHBG level and then multiplying the result by 100.
A separate measure, bioavailable testosterone, takes account of the fact that the bonds to albumin are comparatively weak and reversible. It is calculated by adding free testosterone to albumin-bound testosterone.
What is the normal range for free testosterone?
Normal reference ranges vary between laboratories depending on the assay method used and the reference population. 0.2 to 0.6 nmol/L is one commonly cited example.6 This variability is one reason why the BSSM uses fixed diagnostic thresholds rather than laboratory-specific ranges when assessing testosterone deficiency.
What impact does age have on free T?
Since SHBG generally increases with age, the normal range for Free T declines as men get older. This can be seen in the chart below. So while total testosterone falls rapidly after 45-50, free testosterone declines far faster and earlier in life.5
Source: Vermeulen A. The male climacterium. Ann Med. 1993;25(6):531-534.¹¹
What affects how much SHBG a man has?
Age is the leading factor in affecting SHBG levels in men. Others that may lead to it being higher include:
- Thyroid disorders;
- Advanced liver diseases like cirrhosis;
- Being underweight;
- Physical inactivity;
- Smoking;
- Low protein intake;
- Medications, including Metformin.
On the other hand SHBG levels may be decreased by:
- Obesity;
- Type 2 Diabetes;
- An underactive thyroid;
- Liver disease, such as non-alcoholic fatty liver disease;
- Steroid use;
- Excess growth hormone;
- High protein or high fat diet;
- Inflammatory cytokines;
- Genetic variation (which can influence SHBG levels either way)
The main reason SHBG rises with age appears to be a direct increase in its production by the liver. Laboratory and human studies suggest that changes in liver signalling proteins, particularly a regulatory factor called HNF-4α, drive greater SHBG synthesis with age.8 At the same time, the metabolic signals that normally suppress SHBG production, including insulin and growth factors such as IGF-1, tend to weaken as men get older. This is compounded by age-related shifts in body composition: as muscle mass decreases and central adiposity changes, the metabolic brakes on SHBG loosen further, allowing levels to rise.8,9
Dr Ed says…
Picture a busy train station with a large taxi rank outside. Hundreds of taxis are lined up, engines running, ready to go. That is your total testosterone. But the rank has been badly designed: there is only one narrow boarding point where passengers can actually get in. The rest of the rank is completely blocked off. Most taxis just sit there, unable to move, never picking up a single passenger. Only the taxis at that one open point get their fare and do their job. Those are your free testosterone. SHBG is what blocks off the rest of the rank. A man can have an impressive queue of testosterone, but if his SHBG is high, very little of it ever reaches the cells that need it.
Conclusions
Total testosterone is a key blood test marker, but measuring free testosterone gives a fuller picture of a man’s hormonal health and is essential in diagnosing low testosterone/testosterone deficiency and deciding whether he should have TRT.
References:
1. Kim JH, et al. Usefulness of routine assessment of free testosterone for the diagnosis of functional male hypogonadism. Andrology. 2022. https://pubmed.ncbi.nlm.nih.gov/35243960/
2. Li J, et al. Decreased testosterone secretion index and free testosterone level with multiple symptoms for late-onset hypogonadism identification: a nationwide multicenter study with 5,980 aging males in China. Aging Male. 2020. https://pubmed.ncbi.nlm.nih.gov/33234733/
3. Antonio L, Wu FCW, O’Neill TW, Pye SR, Ahern TB, Laurent MR, et al.; European Male Ageing Study Group. Low free testosterone is associated with hypogonadal signs and symptoms in men with normal total testosterone. J Clin Endocrinol Metab. 2016;101:2647-57. https://pubmed.ncbi.nlm.nih.gov/26909800/
4. Snyder PJ, Kopperdahl DL, Stephens-Shields AJ, et al. Calculated free testosterone is associated with lower-extremity physical function. J Gerontol A Biol Sci Med Sci. 2025. https://pmc.ncbi.nlm.nih.gov/articles/PMC12206171
5. Hackett G, Kirby M, Rees RW, Jones HT, Muneer A, Livingston M, Ossei-Gerning N, David J, Foster J, Kalra P, Ramachandran S. The British Society for Sexual Medicine Guidelines on Male Adult Testosterone Deficiency, with Statements for Practice. World J Mens Health. 2023. DOI: 10.5534/wjmh.221027. https://bssm.org.uk/wp-content/uploads/2023/08/WJMH22-1027-4-Copy.pdf
6. Punjani N, et al. Use of calculated free testosterone in men: advantages and limitations. Current Opinion in Endocrinology, Diabetes and Obesity. 2024. https://pubmed.ncbi.nlm.nih.gov/39445719/
7. Marriott RJ, Murray K, Hankey GJ, Manning L, Dwivedi G, Wu FCW, Yeap BB. Longitudinal changes in serum testosterone and sex hormone-binding globulin in men aged 40-69 years from the UK Biobank. Clin Endocrinol (Oxf). 2022 Apr;96(4):589-598. DOI: 10.1111/cen.14648. https://pubmed.ncbi.nlm.nih.gov/34873743/
8. Xu P, Zeng R, Wan Q, Xie Y, Liu X, An S, Jiang J, Yang J, Zhou Y, Shen X. Ageing-related increases in serum sex hormone-binding globulin levels in men might be related to increased synthesis. Exp Gerontol. 2023 Aug;179:112249. DOI: 10.1016/j.exger.2023.112249. [Note: includes both laboratory/animal and human data.] https://pubmed.ncbi.nlm.nih.gov/37392803/
9. Aribas E, Kavousi M, Laven JSE, Ikram MA, Roeters van Lennep JE. Ageing, cardiovascular risk, and SHBG levels in men and women from the general population. J Clin Endocrinol Metab. 2021 Sep 27;106(10):2890-2900. DOI: 10.1210/clinem/dgab470. https://pmc.ncbi.nlm.nih.gov/articles/PMC8475196/
10. 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):3666-72. DOI: 10.1210/jcem.84.10.6079. https://pubmed.ncbi.nlm.nih.gov/10523012/
11. Vermeulen A. The male climacterium. Ann Med. 1993;25(6):531-534.