Hypogonadism: low testosterone, two locations
Hypogonadism simply means the testes are not producing enough testosterone, sperm, or both. The symptoms are familiar — low energy, low libido, loss of muscle and bone, sometimes infertility. The interesting clinical question is not *whether* testosterone is low but *where* the chain broke. The trick from the axis guide pays off here: read the LH and FSH alongside the testosterone.
- Primary hypogonadism — the testis itself fails. Testosterone is low, so the brake on the brain releases, and LH and FSH rise *high* trying to flog a testis that cannot respond. Low testosterone + high gonadotropins = the problem is at the gonad.
- Secondary (central) hypogonadism — the brain fails to send the order. Without enough GnRH or LH/FSH, the testis is never told to work. Testosterone is low *and* LH/FSH are low or inappropriately normal. The problem is upstream, in the hypothalamus or pituitary.
A clean example of central failure is Kallmann syndrome, where the GnRH-making neurons never migrate into place during development, so the axis never switches on at puberty — classically paired with a reduced sense of smell. At the other end of life, the gradual age-related decline some men experience is sometimes called andropause, though it is far more gradual and variable than menopause in women.
Gynecomastia: a balance, not just an excess
Gynecomastia is the growth of true breast tissue in a male. The key idea is that breast tissue responds to the *ratio* of estrogen to androgen, not to either one alone. Anything that raises estradiol or lowers effective testosterone tips the balance toward breast growth. So gynecomastia is best read as a tug-of-war that estrogen is winning.
This is where aromatase returns. Because fat tissue is rich in aromatase, more body fat converts more testosterone into estradiol, nudging the ratio toward estrogen. The same logic explains the breast tenderness common in newborns and in puberty (transient hormonal swings), and the gynecomastia seen in liver disease (the liver normally clears estrogen) or with certain medications.