What is HCG?

In one line

HCG is a hormone the placenta makes during pregnancy; in a man's body, it signals the testes to make male hormone.

What is HCG?
The very hormone a pregnancy test looks for
HCG is known as the hormone that a pregnancy test detects. But the name also comes up in gym talk about steroids and in diet talk. What is HCG, and what does it do in the body?
In short
A hormone the placenta makes in pregnancy
HCG is a hormone made by the placenta during pregnancy. A hormone is a substance made in one part of the body that travels in the blood and carries a signal to another part. The placenta is the organ that links the mother's womb and the baby during pregnancy.
What the name means
It tells the sex glands to work
HCG is short for 'human chorionic gonadotropin'. The sex glands (gonads) are where the body makes the cells for making babies and the sex hormones: the testes in men and the ovaries in women. As the name says, HCG signals the sex glands to work.
Its job in pregnancy
It helps keep a pregnancy going
In pregnancy, HCG signals the ovary to keep making the hormones that protect the pregnancy. This stops periods and keeps a place where the baby can grow. HCG also passes into urine, so a pregnancy test looks for HCG in urine.
Its job in men
It makes the testes produce male hormone
A man's brain sends the testes a signal hormone that tells them to make testosterone (the main male hormone). This signal hormone is called 'luteinizing hormone (LH)'. HCG does almost the same job as LH, so when it enters a man's body, the testes make testosterone.
HCG as a medicine
A prescription drug
HCG for medical use can be extracted from the urine of pregnant women. The US Food and Drug Administration (FDA) approved HCG as a prescription drug for female infertility and other conditions. In men, it is used for conditions in which the brain's signal is too weak for the testes to work properly.
Why you hear it at the gym
To wake testes stopped by steroids
When people take anabolic steroids (drugs that copy male hormone, used to build muscle), the brain cuts its signal, and the testes almost stop making testosterone and shrink. So some people use HCG without a prescription to make the testes work again. The World Anti-Doping Agency bans HCG in men at all times.
HCG's limit
It wakes the testes, not the brain's signal
HCG skips the brain and stimulates the testes directly. But when the testes make testosterone, the brain cuts its signal even further. So HCG does not help the brain start sending its signal again after steroids are stopped.
Side effects
Headache, low mood, swelling, male breast growth
The HCG drug label lists headache, irritability, depression, tiredness, swelling, and pain at the injection site as side effects. It also lists breast swelling in men. This is not fat but breast gland tissue growing in a man's chest.
A common misconception
Does HCG make you lose weight?
No. The 'HCG diet', which combines HCG with eating very little, is widely known. But the FDA has said that the weight lost comes from eating extremely little, not from the HCG. HCG is not approved as a weight-loss drug.
Summary
A pregnancy hormone and a drug that stimulates the testes
HCG is a hormone the placenta makes in pregnancy; in a man's body, it signals the testes to make testosterone. Weight lost on the HCG diet comes from eating little, not from HCG. Related terms: testosterone, PCT (post-cycle therapy), anabolic steroids.