Classical hypogonadism (also referred to as “organic” hypogonadism) is caused by a congenital, structural, or destructive disorder that results in permanent hypothalamic, pituitary, or testicular dysfunction (primary or secondary hypogonadism). Classical causes of hypogonadism include Klinefelter syndrome (KS), cryptorchidism, some types of cancer chemotherapy, radiation to the testes, trauma, torsion, infectious orchitis, HIV infection, anorchia syndrome, and myotonic dystrophy. The prevalence of classical causes of hypogonadism is very low, around 6% of men with low testosterone.

In contrast, non-classical hypogonadism (also referred to as “functional hypogonadism”) is caused by conditions that suppress gonadotropin and testosterone levels but that are potentially reversible with treatment of the underlying etiology. Non-classical causes of hypogonadism include obesity, type 2 diabetes, metabolic syndrome, opioids, renal failure, COPD, HIV/AIDS. With the epidemic of obesity, type 2 diabetes, metabolic syndrome it is not surprising that the vast majority of men with low testosterone – 94% - have functional hypogonadism.

Considering the wide-ranging benefits of testosterone therapy in men with low testosterone, hypogonadism should be treated regardless of the underlying cause.