The science behind Testosterone

Testosterone replacement therapy (TRT) is only used in those with low testosterone levels and symptoms (such as low mood, low energy, poor sexual function and weight gain). Both are required for a TRT doctor to make the diagnosis of testosterone deficiency and then to consider TRT.

A single low result isn't enough. Testosterone levels naturally fluctuate, so we require two separate blood tests before any diagnosis is made. Both must confirm deficiency before treatment is considered.

Testosterone levels vary throughout the day. We recommend morning testing for all patients. For men over 40, evidence shows this variation is less pronounced, so tests taken before 2pm are accepted — though morning remains preferred¹.

We look at two things: total testosterone and free testosterone.

Total testosterone is the overall amount in your blood. We use a threshold of below 15 nmol/L — the level the British Society for Sexual Medicine defines as the lower boundary of normal.

Free testosterone is the testosterone your body can actually use. Some men have adequate total testosterone but most of it is bound to proteins in the blood, making it unavailable. In these cases, free testosterone is the more clinically relevant measurement. We apply a free testosterone threshold guided by the International Consultation for Sexual Medicine.

Before any treatment is considered, we screen for conditions that can mimic low testosterone and check that TRT would be safe to start. Nobody should be started on testosterone without this step.

Some TRT regimens can suppress natural sperm production. If preserving fertility matters to you, this will be part of your conversation with your doctor before starting. Treatment options to support fertility alongside TRT are available, and your doctor will discuss these based on your circumstances.

TRT can reduce the testes' natural activity. This typically reverses if treatment is stopped, although prolonged use can occasionally lead to some testicular shrinkage or a reduction in natural testosterone production. Although this may not be a concern for everyone, there are standard treatment additions that can mitigate against these side effects and your doctor will discuss what's right for you.

TRT stimulates red blood cell production, which can benefit energy and physical performance. However, in some men the body can react too well and produce too many red blood cells which can thicken the blood and raise the risk of clots. We monitor this carefully with blood tests at 3 months, 6 months, and every 6 months thereafter. We also have robust preventative pathways in place for those who are at risk.

TRT can be unsafe in certain heart conditions such as severe heart failure, but generally research has shown it to be safe for the heart, including in men who have previously had a heart attack as demonstrated in the landmark TRAVERSE trial⁴ and more recently in this large review of all the evidence.

TRT does not cause prostate cancer. However, if prostate cancer is already present, TRT can accelerate it. We screen for this before treatment begins and continue to check at every monitoring blood test throughout your care.