Testosterone is an androgen, or male hormone. Males begin producing testosterone early in life---fetuses of only a few weeks gestational age make testosterone, which affects their physical and mental development. At puberty, testosterone production increases, resulting in development of the male secondary sex characteristics and increased aggression and sex drive. The hormone is produced in the testes in response to a series of hormonal signals from many cells throughout the body.
Hypothalamic Cells
While the brain doesn't make testosterone directly, it's nevertheless responsible for the hormone's production--without other hormones secreted by parts of the brain, testicular cells couldn't make testosterone. One of the "master" hormones responsible for testosterone production is made in a part of the brain called the hypothalaums, explains Dr. Lauralee Sherwood in her book, "Human Physiology." This hormone, called gonadotropin releasing hormone, or GnRH, is a so-called "tropic" hormone, meaning that it causes other hormones to be released.
Anterior Pituitary Cells
The anterior pituitary gland is a small clump of tissue located just below the hypothalamus of the brain, explains Dr. Gary Thibodeau in his book, "Anatomy and Physiology." Since the hypothalamus produces many tropic hormones that direct hormone release by the anterior pituitary, the two glands have a direct blood vessel connection, called the hypothalamic-pituitary portal system. This portal system allows the hypothalamus to secrete tropic hormones such as GnRH into vessels through which they travel directly to the anterior pituitary in high concentration. In the case of testosterone production, the relevant pituitary hormone is called luteinizing hormone, or LH. The anterior pituitary releases LH into systemic body circulation in response to GnRH from the hypothalamus. Once in the bloodstream, LH spreads throughout the body, though receptors are found only on isolated cells of the testes.
Leydig Cells
Direct testosterone production takes place in the Leydig cells, which are located in the testes. When LH from the anterior pituitary binds to receptors on the Leydig cells, it causes them to begin producing enzymes that convert cholesterol into testosterone. While cholesterol is often thought of as an undesirable molecule that adversely affects heart health, in reality, it's an important biomolecule with many crucial functions. Notes Dr. Sherwood, the steroid hormones, of which testosterone is one, are made by modifying cholesterol in the bloodstream. Testosterone produced in the Leydig cells is then released into the bloodstream, where it travels throughout the body and has a number of physiological effects.
References
- "Human Physiology"; Lauralee Sherwood, Ph.D.; 2004
- "Anatomy and Physiology"; Gary Thibodeau, Ph.D.; 2007
