GnRH Agonist

A GnRH agonist is a medicine that switches off the testes' or ovaries' own hormone production by overstimulating the signal from the brain until it stops responding. In feminizing hormone therapy it is used as a powerful [anti-androgen](/glossary/anti-androgen). Triptorelin, goserelin and leuprorelin are the common ones.

Where you'll meet it

In the UK, where many NHS gender services use it for adults in preference to cyproterone, partly because of cyproterone's meningioma concern, and in some private and specialist clinics elsewhere. It is given as an injection or an implant under the skin, every month or every few months depending on the product, usually by a GP practice nurse under shared care.

You may also know these medicines under other names. The same drugs are used in prostate cancer, endometriosis and fertility treatment, and as puberty blockers for adolescents. This page is about their use alongside hormones in adults.

How it works

The name stands for gonadotropin-releasing hormone, the brain's signal to the pituitary gland, which in turn tells the testes or ovaries to make hormones. The body normally sends that signal in pulses. A GnRH agonist mimics it so strongly and constantly that the pituitary stops passing it on. The testes, or ovaries, lose their instruction and production falls very low.

For the first week or two, before that happens, hormone levels can briefly rise, known as a flare. Some prescribers cover those first weeks with another anti-androgen; ask whether yours will.

How it's measured

On a GnRH agonist, testosterone on your routine bloods should fall well into the typical female range, commonly cited as under about 50 ng/dL (under about 1.7 nmol/L), often much lower. If it creeps back up, the usual questions are whether a dose was late and whether the interval suits you. Estradiol is watched as closely, because it is now doing all the work. Blood tests and monitoring covers the schedule.

The trade-offs

It is effective and avoids the side effects of the tablet anti-androgens: no potassium checks as with spironolactone, no meningioma concern as with cyproterone. But it is expensive, needs regular injections on time, and funders do not always cover it. In the US it is rarely used for adults because insurers resist it. Supply problems and missed appointments are the most common reason people end up with a gap.

Bones, and why estrogen is not optional

Because it lowers the body's own sex hormones so completely, a GnRH agonist on its own leaves you with almost no sex hormone at all. That brings hot flushes, fatigue, low mood and, over months and years, bone loss. Adequate estrogen alongside it is essential for bone health, not an extra. Long-term health covers bones and when a bone density scan makes sense.

Common questions

It feminizes. It does not. It removes testosterone; estrogen does the feminizing.

It is the same as a puberty blocker for adults. It is the same class of medicine, but an adult taking it alongside estrogen is in a different situation, clinically and legally, from an adolescent taking it alone.

Agonist and antagonist are interchangeable. GnRH antagonists exist too; they block the signal directly with no flare, but are used far less for this purpose.

What to ask

Which product, how often, and who gives the injection? What happens if an injection is late, and how late is too late? Will anything cover the first few weeks? How will my estrogen be checked so my bones are protected? Anti-androgens compares it with the alternatives.

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