Kallmann syndrome

Kallmann syndrome is a congenital condition in which puberty fails to start because the brain never properly turns on its reproductive hormone, paired with a curious clue: a poor or absent sense of smell. That odd combination is the fingerprint that distinguishes it from other causes of delayed puberty.

The root problem is a developmental one. Early in fetal life the neurons that will make GnRH migrate into the brain alongside the developing olfactory (smell) nerves. In Kallmann syndrome these neurons fail to migrate correctly, so the hypothalamus cannot release GnRH and the olfactory structures are underdeveloped. Without GnRH there is no LH or FSH, hence no gonadal stimulation, producing hypogonadotropic hypogonadism: low sex hormones with low (not high) gonadotropins.

Clinically, affected people present with absent or incomplete puberty and reduced or absent smell, and the diagnosis is supported by finding low testosterone (or estrogen) together with low LH and FSH. Importantly, because the testes themselves are intact, treatment works well: hormone therapy induces sexual development, and GnRH or gonadotropin therapy can restore fertility, making this one of the more treatable forms of inherited hypogonadism.