DEAR DOCTOR K: I'm a man in my early 50s. Lately, my sex drive isn't what it used to be. Plus, I'm irritable and depressed. Could I have low testosterone?
DEAR READER: Yes, you could, but that's not the only possible explanation of your symptoms. There are some rare diseases that can cause very low blood levels of testosterone and a host of symptoms. I won't be talking about those in this column. Instead, I'm talking about a more common condition.
Testosterone is the major sex hormone in men. As men get older, testosterone production falls. If it falls low enough, it can cause some of the symptoms you described.
Testosterone plays a number of important roles, even after puberty. It affects muscle size and strength, bone growth and strength, sex drive (libido), sperm production and mood.
A cascade of chemical signals leads to the production of testosterone. One part of the brain (the hypothalamus) sends a signal to another part (the pituitary gland). The pituitary gland then relays signals to the testes to produce testosterone.
As blood levels of testosterone rise, a “feedback loop” kicks in: When levels rise high enough, the brain temporarily shuts down the signals that lead to more testosterone. When blood levels fall, the brain turns the signals back on.
During teenage years and early in adult life, this signaling system works fine. However, for reasons we don't understand, as men get older the pituitary gland sends fewer signals and the testes produce less testosterone.
It's similar to what happens during menopause in women, when the brain stops signaling the ovaries to make female hormones. In fact, some experts call this reduction in testosterone, and the symptoms it can cause, “andropause.”