The Tavistock & Portman GIC guidance also sets out monitoring requirements and other interventions before initiating prescribing and then during ongoing treatment.

Trans men should be monitored by ultrasound for the effects of testosterone on the uterus every two years, or until hysterectomy. Testosterone may be converted in the body to estradiol and may cause a thickening of the endometrium, with an increased risk of malignancy. Irregular bleeding may need a biopsy to check for any neoplasm.¹²

Testosterone supplementation, especially if injected, can also increase the risk of polycythaemia (a high level of red blood cells), which is associated with increased stroke risk. Risk is considered proportional to serum levels, hence the aim of keeping testosterone levels within the normal range for physiological adult males.

Other effects of testosterone include exacerbating obstructive sleep apnoea.

Among trans women, a significant concern is the increased risk of deep vein thrombosis (DVT). While the rate is less than 3 per cent, most cases (80 per cent) occur within the first two years of starting hormonal treatment. Many of these will be in young adults, making the comparative risk 20 times greater than with untreated people of a similar age group. The Tavistock & Portman GIC advises that this effect may depend on the oestrogens used.¹²

While women aged 50-70 who use HRT have a 3.2-4.0 per 1,000 risk of developing breast cancer, the clinic’s ongoing data suggests that the risk of breast cancer for trans women using feminising hormone therapy is very low. It may be close to the background breast cancer risk in males.

Other secondary outcomes for trans women include:

• a reduction in prostate cancer risk among those who have not had prostatectomy compared to the male population
• heart attack risk reduced to two thirds that of the male population
• no increased risk in stroke (although it is recommended to minimise the dose of oestrogens where possible)
• suppressed spermatogenesis and a reduction in fertility levels (prompting consideration of sperm banking).

Oestrogen therapy is associated with a slight increase in mortality, especially in suicide rates and vascular disease. However, some of the relative risk reduction in cardiovascular events under UK guidelines compared to other countries may arise from clinics insisting that smokers stop smoking before starting on oestrogen therapy. Ethinylestradiol also seems to be more likely to cause vascular disease which may be mitigated by selecting other types of oestrogen.