Testosterone increases the production of red blood cells, which thickens the blood. If the blood becomes too thick then there is a small risk that you could have a stroke.

How to look after ourselves

Make sure you get regular blood tests as advised by your endocrinologist. To prevent this, the proportion of red blood
cells in your blood will be monitored. If your blood does become too thick, usually all that is necessary (after ensuring a good fluid intake) is to decrease your testosterone dose or, if you are having injections,
to switch to a testosterone gel as these seem to be less likely to thicken your blood.

We know that smoking also increases the risk of having a stroke or heart attack; in people who smoke and have testosterone treatment this risk is increased even further.  Therefore it is often advised that you stop smoking before you are prescribed testosterone.

Every month the ovaries produce oestrogen, which causes the lining of the womb to thicken, and then progesterone, which reverses this effect. If you take testosterone this will inhibit this cycle of ovaries producing oestrogen and progesterone. However, your body will still produce a small amount of oestrogen as there is a chemical in fat cells that converts testosterone to oestrogen. As there is a reduction in progesterone to reverse the effects of this oestrogen, it may cause the lining of the womb to become too thick; this is called endometrial hyperplasia. 

The concern is that in the longer term this could lead to womb cancer, also called endometrial cancer. One study suggested that the risk of the lining of the womb thickening is about 15% after 2 years of testosterone treatment, although there is now other evidence to suggest that the risk is notably less.

Furthermore, there has reassuringly only been one case of endometrial cancer reported in a transgender man on testosterone treatment, suggesting the risk of this is low.

Severe liver problems can be seen in people using anabolic steroids for bodybuilding. However, these anabolic steroids are different from the testosterone that we use in routine testosterone replacement, and severe liver problems associated with this are rare.

How to look after ourselves

Mild changes in liver function are more common and occur in about 4-7% of transgender men and transmasculine people, but testosterone treatment rarely needs to be altered as a result.  Make sure you're having regular safety monitoring blood tests so your gp can make any adjustments to your dose if need.

Despite the low risk, monitoring of the womb thickness by ultrasound scan every 2 years is recommended. This would no longer be necessary after a hysterectomy. The risk of ovarian cancer appears to be very low; there have only been three cases reported following prolonged testosterone treatment. 

Breast cancer risk appears to be 10 times lower than for cisgender women, and about the same as for cisgender men. Nevertheless, transgender men and transmasculine people would still be advised to perform regular breast self-examination. 

Although the risk of developing cancers that could be related to hormone treatment is low, we still recommend that you have cancer screening in line with national guidelines. For as long as you have a womb this includes the cervical cancer screening programme, which involves having a cervical smear every three years from the age of 25 to 50, and every 5 years from 50 to 65.

There is a large difference in the blood lipids (fat and cholesterol) of cisgender males and cisgender females. This is probably related to the fact that cisgender males are more likely to have heart disease than cisgender females. This has led to some concern that that testosterone treatment in transgender men and transmasculine people may make their blood lipids similar to those of cisgender men, and may in turn increase their risk of heart disease.

Recent findings

Studies have shown that testosterone treatment only changes some blood lipids; it does cause a minor increase in triglycerides and a decrease in blood HDL (good cholesterol), but it causes no change in total cholesterol or LDL cholesterol (bad cholesterol).

More importantly, the changes that occur do not seem to significantly increase the risk of heart disease, with the rate of heart attacks being about one third the expected rate in the cisgender male population.

Most of the studies in transgender men and transmasculine people show that testosterone treatment appears to maintain bone mineralisation (“thickness”).

How to look after ourselves

As Black people and people of colour we are more likely to suffer from Vitamin D defficiency. Firstly consultwith your gp, and they can advice on any vitamin D supplments that may be necessary.

Vitamin D is needed for healthy bones and muscles. Sunlight is our main source of vitamin D.

Everyone in the UK is advised to take a daily supplement containing 10 micrograms (400 international units) of vitamin D during the autumn and winter months (October to early March), when we cannot make vitamin D from sunlight.

Be aware of the difference between public health advice on supplementation and clinical treatment of deficiency. The latter will be at a higher dose and intended to restore optimal vitamin D levels safety but quickly.