page_title: Testosterone
section_title: Regulation
hierarchical_section_title: Testosterone / Biochemistry / Biosynthesis / Regulation
caption_attribution_description: Hypothalamic-pituitary-testicular axis
context_page_description: Testosterone is the primary male sex hormone and anabolic steroid. In male humans, testosterone plays a key role in the development of male reproductive tissues such as testes and prostate, as well as promoting secondary sexual characteristics such as increased muscle and bone mass, and the growth of body hair. In addition, testosterone is involved in health and well-being, and the prevention of osteoporosis. Insufficient levels of testosterone in men may lead to abnormalities including frailty and bone loss.
  Testosterone is a steroid from the androstane class containing a keto and hydroxyl groups at positions three and seventeen respectively. It is biosynthesized in several steps from cholesterol and is converted in the liver to inactive metabolites. It exerts its action through binding to and activation of the androgen receptor. In humans and most other vertebrates, testosterone is secreted primarily by the testicles of males and, to a lesser extent, the ovaries of females. On average, in adult males, levels of testosterone are about seven to eight times as great as in adult females.
context_section_description: In males, testosterone is synthesized primarily in Leydig cells. The number of Leydig cells in turn is regulated by luteinizing hormone (LH) and follicle-stimulating hormone (FSH). In addition, the amount of testosterone produced by existing Leydig cells is under the control of LH, which regulates the expression of 17β-hydroxysteroid dehydrogenase.
  The amount of testosterone synthesized is regulated by the hypothalamic–pituitary–testicular axis (see figure to the right). When testosterone levels are low, gonadotropin-releasing hormone (GnRH) is released by the hypothalamus, which in turn stimulates the pituitary gland to release FSH and LH. These latter two hormones stimulate the testis to synthesize testosterone.  Finally, increasing levels of testosterone through a negative feedback loop act on the hypothalamus and pituitary to inhibit the release of GnRH and FSH/LH, respectively.
  Factors affecting testosterone levels may include:
  Age: Testosterone levels gradually reduce as men age. This effect is sometimes referred to as andropause or late-onset hypogonadism.
  Exercise: Resistance training increases testosterone levels,  however, in older men, that increase can be avoided by protein ingestion. Endurance training in men may lead to lower testosterone levels.
  Nutrients: Vitamin A deficiency may lead to sub-optimal plasma testosterone levels. The secosteroid vitamin D in levels of 400–1000 IU/d (10–25 µg/d) raises testosterone levels. Zinc deficiency lowers testosterone levels but over-supplementation has no effect on serum testosterone.
  Weight loss: Reduction in weight may result in an increase in testosterone levels. Fat cells synthesize the enzyme aromatase, which converts testosterone, the male sex hormone, into estradiol, the female sex hormone.  However no clear association between body mass index and testosterone levels has been found.
  Miscellaneous: Sleep: (REM sleep) increases nocturnal testosterone levels. Behavior:  Dominance challenges can, in some cases, stimulate increased testosterone release in men. Drugs: Natural or man-made antiandrogens including spearmint tea reduce testosterone levels. Licorice can decrease the production of testosterone and this effect is greater in females.
caption: Hypothalamic–pituitary–testicular axis
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