Primary vs Secondary Hypogonadism
Primary hypogonadism is gonadal failure with raised gonadotropins, while secondary hypogonadism is a low sex steroid with low or inappropriately normal LH and FSH from a pituitary or hypothalamic cause.
The two are separated by one pairing: a low sex steroid read alongside LH and FSH. If the gonad has failed, feedback is lost and gonadotropins rise, giving the hypergonadotropic or primary pattern. If the pituitary or hypothalamus is at fault, the drive is missing and gonadotropins sit low, or unremarkable against a clearly low testosterone, giving the hypogonadotropic or secondary pattern. A mid-range LH with a frankly low testosterone is already abnormal: an intact axis should have raised it.
Primary causes are largely structural: Klinefelter syndrome, present in roughly one in 600 male births, cryptorchidism, orchitis, radiation and alkylating chemotherapy. Secondary causes are more often reversible or iatrogenic - pituitary adenoma, hyperprolactinaemia, chronic opioid therapy, high-dose glucocorticoid, severe obesity, and exogenous androgen, by far the commonest explanation in a man with suppressed gonadotropins. Kallmann syndrome adds congenital GnRH deficiency with anosmia.
The classification decides what can be offered. Fertility can often be restored in secondary hypogonadism because the testis is intact and can be driven with hCG, with or without FSH, or with pulsatile GnRH; in primary hypogonadism there is nothing left to stimulate. It also decides what else to look for: low gonadotropins warrant prolactin, iron studies and pituitary imaging, since the result may be the first sign of a lesion.
Two errors recur. The first is diagnosing from a single testosterone, often drawn in the afternoon or during acute illness, when the standard is two morning fasting samples on a reliable assay. The second is treating secondary hypogonadism with testosterone without asking why the gonadotropins are low: replacement suppresses them further, impairs fertility for its duration, and hides a treatable cause behind a plausible-looking correction.