Testosterone
The primary male sex hormone, produced mainly by the testes under LH stimulation. Testosterone is the downstream target of GnRH-based therapies for prostate cancer — both GnRH agonists and antagonists suppress testosterone to castrate levels, depriving androgen-dependent cancer cells of their growth signal.
Technical Context
Testosterone is synthesised from cholesterol in testicular Leydig cells through a multistep enzymatic pathway (cholesterol → pregnenolone → DHEA → androstenedione → testosterone). LH controls the rate-limiting step (cholesterol side-chain cleavage). In the prostate, testosterone is converted to the more potent dihydrotestosterone (DHT) by 5α-reductase. Androgen deprivation therapy for prostate cancer aims to reduce testosterone to castrate levels (<50 ng/dL or 1.7 nmol/L). GnRH agonists achieve this in 2-4 weeks (after initial flare), while GnRH antagonists achieve it within 1-3 days. Testosterone monitoring is essential for confirming adequate suppression — failure to achieve castrate levels indicates treatment inadequacy.