GnRH

GnRH is the hypothalamus's “go” signal for reproduction. It is a small peptide hormone that travels the short portal route to the anterior pituitary and tells it to release the two gonadotropins, LH and FSH, which in turn run the ovaries and testes.

What makes GnRH special is its rhythm. It is not released steadily but in brief pulses, roughly every one to three hours. The pituitary reads this pulse pattern like a code: the right pulse frequency keeps LH and FSH flowing normally, which is why the timing of secretion matters as much as the amount.

This pulsatility is also why GnRH-based drugs can do opposite things. Given in matching natural pulses, GnRH stimulates the gonads; but given continuously or as a long-acting analog, it overwhelms the receptors and shuts gonadotropin release down — a paradoxical effect used clinically to suppress sex hormones in prostate cancer, endometriosis, and precocious puberty.

GnRH sits at the top of the hypothalamic-pituitary-gonadal axis. Failure of GnRH neurons — as in Kallmann syndrome — prevents puberty and causes infertility, showing how a single small peptide gates the entire reproductive system.