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Male appearance, genital and sexual development as well as male fertility are mainly regulated by the hormone testosterone that is produced in the testes -- more precisely, in the Leydig cells.
It's often said that boys like action. But one thing's for certain -- when their testosterone hormone kicks into gear, they're in for a hell of a ride. Suddenly, the pituitary gland is sending messages and saying, "Wake up, testes! Start making testosterone!" And they do. In fact, a maturing boy's testosterone levels can rise by 30 times. This is a strong mix, but testosterone is such a powerful chemical that the amount of it in a boy's body is minuscule. Its concentration would be the equivalent of dissolving a single grain of salt into 60 pints (28.3 liters) of water. And if you add up the amount of testosterone inside every man on Earth, you would only end up with this amount -- 76 pounds' (34.5 kg) worth.
In the male newborn, serum testosterone concentrations are comparable to those found in normal adults, decrease then at the end of the 1st week of life, increase again in the 2nd month of life to a new height and fall by the 6th month of life to very low levels such as those found in female babies. Up to the 7th year of life, androgen concentrations in serum are very low, although there is a remarkable increase of free testosterone levels. During this phase, Leydig cells can be stimulated to produce testosterone through the exogenous administration of hCG (human chorionic gonadotropin).
At the age of about 7 years androgen production starts increasing, first due to increased secretion of dehydroepiandrosterone by the adrenal gland (adrenarche). At the age of about 10 years gonadotropin secretion starts, first with pulsatile discharge of LH (luteinizing hormone) by the pituitary only during sleeping hours. This nocturnal LH pulsatility becomes progressively evident, also during the day, in the course of pubertal maturation until the pattern typical for the adult man is reached. FSH (follicle-stimulating hormone) also starts increasing, along with basal LH and testosterone serum concentrations.
In the male testosterone is not only responsible for secondary sex characteristics, libido and potency, but also influences mood and intellectual capacity, erythropoesis, bone metabolism, protein anabolism, fat distribution, induces growth of muscle mass (especially when accompanied by relevant physical exercise), is important for sexual life and vital functions, improves sociability and adjustment, supports brain functions such as spatial cognition by activating the relevant brain areas, influences vocal register through changing larynx size and vocal cord length, assists testicular development, accelerates pubertal development with all its psychic and physical consequences, increases the possibility of coronary atherosclerosis and acute infarction.
Normal testosterone concentrations of young men in peripheral serum range between 12 and 30 nmol/l. Testosterone concentrations in blood follow a circadian rhythm with higher levels in the morning hours and about 25% lower levels in the evening. Levels of total testosterone (serum testosterone) for various symptoms (nmol/L):
15-20
Normal
12-15
Loss of libido
Loss of vigor
10-12
Obesity
08-10
Feeling depressed, disturbed sleep
Lacking concentration
Diabetes mellitus type 2 (also non-obese men)
00-08
Hot flushes
Erectile dysfunction
In the aging man a decline of free serum testosterone concentrations occurs but simultaneously the SHBG (sex hormone binding globulin) levels increase. In the case of a strong decrease of testosterone some men develop symptoms of late onset hypogonadism with clinical consequences such as frailty, changes in body composition, cardiovascular disease, sexual dysfunction and osteoporosis.
The real surge of male hormones occurs around the age of 13 or 14, which sends boys off on a completely different route from girls. For boys, the first signs of puberty are the testicles growing bigger. And in five years they grow from 1 1/2 cubic centimeters to 20 cubic centimeters. That's 13 times larger. Then a boy's penis starts growing and by the age of 18 reaches an average length of 6 1/2 (17 cm) inches. But not long before that boys began setting up sperm-making factories, each producing 300 million sperm a day.
Chefs note: In the brain the hypothalamus issues orders to the pituitary in the form of a substance called GnRH (gonadotropin releasing hormone). From here the orders are passed on by messengers, hormones regulating testicular function: LH and FSH. LH causes the Leydig cells in the testes to produce testosterone. FSH regulates sperm production in the coiled tubules of the testes. Chemically blocking GnRH production (by GnRH analogs or antagonists, or by artificial testosterone and estrogen) suppresses both LH and FSH, prevents sperm production and causes dramatically low levels of testosterone, putting an end not only to sperm but also to libido and potency as well as many other male bodily functions.
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