What is LH (luteinizing hormone)?

Luteinizing hormone is produced by the anterior pituitary gland and acts as a chemical messenger to the testes. When LH levels are normal or elevated, it tells the testicles to manufacture testosterone. This hormone-signaling chain, called the hypothalamic-pituitary-gonadal (HPG) axis, is fundamental to male sexual function and secondary sex characteristics.

In clinical practice, measuring LH is essential for diagnosing the root cause of low testosterone. If a patient has low testosterone but LH is also low or normal, the problem originates in the pituitary gland (called central hypogonadism or secondary hypogonadism). Conversely, if testosterone is low but LH is high, the testes themselves are not responding properly (primary hypogonadism). This distinction determines treatment approach. Providers offering hormone therapy use LH testing alongside testosterone and other markers to design appropriate protocols and monitor whether the pituitary and testicular function are intact or require intervention.