What is LH (luteinizing hormone)?

Luteinizing hormone is a hormone produced by the pituitary gland that travels through the bloodstream to stimulate testosterone production in the testes. In males, LH binds to Leydig cells (also called interstitial cells) within the testis, triggering them to manufacture and release testosterone. This signaling pathway is essential for normal sexual function, muscle development, bone density, and other male characteristics.

LH levels matter most when diagnosing the cause of low testosterone. A TRT provider will measure both testosterone and LH together. If testosterone is low but LH is normal or elevated, the problem lies in the testes themselves, a condition called primary hypogonadism. If both testosterone and LH are low, the pituitary or hypothalamus (the gland that controls the pituitary) is not sending the signal, which is secondary hypogonadism. This distinction guides treatment: primary hypogonadism typically requires direct testosterone replacement, while secondary hypogonadism may respond to LH-stimulating medications or hormone replacement directed at the pituitary axis.

Checking LH during the initial workup is standard practice for anyone considering testosterone replacement. An endocrinologist or hormone specialist will order this test as part of a comprehensive hormone panel to confirm the diagnosis and rule out other causes of testosterone deficiency.