hypogonadism

Hypogonadism simply means the gonads are not making enough sex hormone (or, often, enough gametes). In men it usually shows up as low testosterone, with consequences ranging from low libido, fatigue, and erectile problems to loss of muscle and bone, depending on how early in life it starts.

Doctors split it by where the breakdown lies. Primary hypogonadism is a problem in the testis itself, so testosterone is low while LH and FSH are high because the brain is pushing a non-responsive gland; causes include Klinefelter syndrome, mumps orchitis, chemotherapy, or injury. Secondary (central) hypogonadism is a problem upstream in the hypothalamus or pituitary, so testosterone is low and LH and FSH are low or inappropriately normal; causes include pituitary tumors, Kallmann syndrome, or severe illness.

Distinguishing primary from secondary is the key clinical fork, because it points to different causes and treatments and tells you whether fertility might be recoverable. The timing matters too: hypogonadism before puberty prevents normal sexual development and growth-plate closure, while hypogonadism arising in adulthood causes a more gradual loss of established male characteristics.