Low energy, less interest in sex, a gym routine that no longer seems to pay off. Many men in their thirties and forties wonder whether testosterone is behind changes like these. A blood test is a reasonable place to start the conversation, as long as it is done at the right time and read in context. This guide explains the difference between total testosterone, free testosterone and SHBG, and what each adds.
What testosterone does
LML describes testosterone as a vital hormone in both men and women, involved in libido, fertility, muscle and bone strength, and mental health. In men it is produced mainly by the testes, under the control of two pituitary hormones, LH and FSH.
Levels change with age, but more gently than many people think. The NHS explains that testosterone falls steadily by about 1% a year from around the age of 30 to 40, and that this decline is unlikely to cause problems in itself (NHS, The male menopause). A true deficiency developing later in life, called late-onset hypogonadism, is described by the NHS as uncommon and more frequent in men who are obese or have type 2 diabetes.
Total testosterone, free testosterone and SHBG
Total testosterone
Most testosterone in the blood is bound to proteins. Total testosterone measures all of it, bound and unbound. The Endocrine Society guideline recommends a fasting morning total testosterone, measured with a reliable assay, as the first test, and repeating it to confirm a low result (Bhasin et al., 2018).
SHBG
Sex hormone binding globulin (SHBG) is the main protein that carries testosterone. LML notes that SHBG regulates how much sex hormone is available to the body, and that it is affected by thyroid function, the liver and body weight. When SHBG is high, more testosterone is bound and less is free to act; when it is low, the opposite happens.
Free testosterone
Free testosterone is the small fraction available to tissues. Measuring it directly is technically demanding, so laboratories usually calculate it from total testosterone, SHBG and albumin. A 2024 review notes that age and obesity change both total testosterone and SHBG, which is why free testosterone is used as a second-line measure when total testosterone is borderline or SHBG may be altered, while also pointing out that calculators are approximations (Nyamaah et al., 2024). The Endocrine Society guideline makes the same recommendation (Bhasin et al., 2018).
Which test in the catalogue?
- Testosterone Check: total testosterone only. A simple starting point.
- Testosterone Plus Profile: total testosterone, SHBG and albumin, with calculated free testosterone. LML describes it as giving a much more precise picture than total testosterone alone.
- Sex Hormone Binding Globulin: SHBG on its own, if your doctor wants to look at it separately.
- Male Hormone Profile: adds LH, FSH, oestradiol, prolactin, DHEA sulphate and free androgen index, for a broader look at the hormones that control testosterone.
Timing the sample
Testosterone is highest in the morning. LML advises taking the sample between 7am and 10am, and the Endocrine Society recommends a fasting morning sample (Bhasin et al., 2018). Illness, poor sleep and intense training in the days before can also affect the result, so try to test on an ordinary day.
