Male hypogonadism (low testosterone)

Male hypogonadism is what many people call low testosterone: a man's body has less testosterone, a hormone, than it should. Even so, a low lab result isn't enough on its own for a clinical diagnosis, the kind that looks at the person and not just the lab report. There also has to be a symptom or sign, such as a lower sex drive.

Nearly all testosterone is made in the testicle, by cells called Leydig cells. But those cells don't start up on their own. They wait for orders from the pituitary, a small gland at the base of the brain, which stimulates them with a chemical messenger called luteinizing hormone, or LH. Think of a workshop that only gets to work when orders come in from the front office: the pituitary is the front office, LH is the orders, and the Leydig cells are the workshop. That whole chain of command, which starts in the hypothalamus, an area of the brain, runs through the pituitary, and ends at the testicle, is called the hypothalamic-pituitary-testicular axis. The problem can be anywhere along that chain, and it can be there from birth, which is called congenital, or show up later in life, which is called acquired.

That's what separates the types. If the workshop itself, meaning the testicle, is what's failing, the hypogonadism is primary: the orders keep coming, so LH comes back high on blood tests. This happens, for example, in Klinefelter syndrome, a genetic condition that has its own card, or in mumps orchitis, when the mumps virus inflames the testicle. If the workshop is fine but the orders aren't getting through the way they should, the hypogonadism is secondary, and LH comes back normal or low, as can happen with obesity or with disorders of the pituitary.