Hypogonadism is a medical term which describes a diminished functional activity of the gonads the testes and ovaries in males and females, respectively that may result in diminished sex hormone biosynthesis. In layman's terms, it is sometimes called "interrupted stage 1 puberty". Low androgen (e.g., testosterone) levels are referred to as hypoandrogenism and low estrogen (e.g., estradiol) as hypoestrogenism, and may occur as symptoms of hypogonadism in both sexes, but are generally only diagnosed in males and females respectively. Other hormones produced by the gonads which may be decreased by hypogonadism include progesterone, DHEA, anti-Mllerian hormone, activin, and inhibin. Spermatogenesis and ovulation in males and females, respectively, may be impaired by hypogonadism, which, depending on the degree of severity, may result in partial or complete infertility.

Deficiency of sex hormones can result in defective primary or secondary sexual development, or withdrawal effects (e.g., premature menopause) in adults. Defective egg or sperm development results in infertility. The term hypogonadism is usually applied to permanent rather than transient or reversible defects, and usually implies deficiency of reproductive hormones, with or without fertility defects. The term is less commonly used for infertility without hormone deficiency. There are many possible types of hypogonadism and several ways to categorize them. Hypogonadism is also categorized by endocrinologists by the level of the reproductive system that is defective. Physicians measure gonadotropins (LH and FSH) to distinguish primary from secondary hypogonadism. In primary hypogonadism the LH and/or FSH are usually elevated, meaning the problem is in the testicles, whereas in secondary hypogonadism, both are normal or low, suggesting the problem is in the brain.

Hypogonadism can involve just hormone production or just fertility, but most commonly involves both.

Women with hypogonadism will not begin menstruating and it may affect their height and breast development. Onset in women after puberty causes cessation of menstruation, lowered libido, loss of body hair and hot flashes. In boys it causes impaired muscle and beard development and reduced height. In men it can cause reduced body hair and beard, enlarged breasts, loss of muscle, and sexual difficulties. A brain tumor (central hypogonadism) may involve headaches, impaired vision, milky discharge from the breast and symptoms caused by other hormone problems.[2]

The symptoms of hypogonadotrophic hypogonadism, a subtype of hypogonadism, include late, incomplete or lack of development at puberty, and sometimes short stature or the inability to smell; in females, a lack of breasts and menstrual periods, and in males a lack of sexual development, e.g., facial hair, penis and testes enlargement, deepening voice.

Low testosterone can be identified through a simple blood test performed by a laboratory, ordered by a physician. This test is typically ordered in the morning hours, when levels are highest, as levels can drop by as much as 13% during the day.[3]

Normal total testosterone levels range from 3001000ng/dL (nanograms per decilitre)[4]

Treatment is often prescribed for total testosterone levels below 350ng/dL.[5] If the serum total testosterone level is between 230 and 350ng/dL, repeating the measurement of total testosterone with sex hormone-binding globulin (SHBG) to calculate free testosterone or free testosterone by equilibrium dialysis may be helpful.

Treatment may be necessary even if the patient's total testosterone level is within the "normal" range. The standard range given is based off widely varying ages and, given that testosterone levels naturally decrease as humans age, age-group specific averages should be taken into consideration when discussing treatment between doctor and patient.[6] A twenty-seven-year-old male with a testosterone level of 380ng/dL would be in the "normal" range, but would likely have low testosterone to blame if he experiences some or many of the above symptoms. This score would put him in the bottom 5% of his age-group, but would be a more common score for a man who is 80+ years old.[6] Although, this doesn't automatically mean that a young man with 380ng/dL has the same amount of testosterone of an 80+ year old, since there is usually a big difference in SHBG levels in the bloodstream between young and elderly, resulting in a much higher free testosterone level in the young. In fact, some people with low SHBG and low-normal testosterone experience no symptoms of hypogonadism.

A downturn in the circulation of testosterone should cause the hypothalamus and pituitary gland to trigger a release of brain hormones that stimulate the testicles to ramp up production of testosterone. The specific brain hormones include gonadotropin releasing hormone (abbreviated GnRH), which is released by the hypothalamus, and luteinizing hormone (abbreviated LH), released by the pituitary. They act to control the production of testosterone in synchrony. This response system in the body is a negative feedback loop. When this loop is functioning at its best, the body gets enough testosterone to bind to receptors on the various organs that require it. In the bodies of men as they hit their thirties and forties, testosterone falls approximately 1 to 3 percent each year.[7]

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