What are the Androgens?
The male sex hormones are collectively known as androgens. The most abundant of these hormones is testosterone and it is therefore considered as the main male sex hormone. However there are several other male hormones that are secreted by the testes (testicles). All androgens are steroids hormones.
Male sex hormones are also present in females but in much smaller concentrations. These is due to androgens secreted by the adrenal glands in both genders and ovaries in females. Male sex hormones are responsible for the male sexual characteristics – a gruff voice, chest and facial hair and larger muscle mass.
Types of Androgens
There are several different types of androgens secreted by testes in males and adrenal glands in males and females.
- Testosterone is the most abundant of these steroid hormones but is not the most potent androgen.
- Dihydrotestosterone (DHT) is the most active form of androgens and most if not all of testosterone is eventually converted into DHT.
- Androstenedione (andro) is produced by the testes, adrenal cortex and a small amount by the ovaries.
- Dehydroepiandrosterone (DHEA) is another prominent androgen secreted by the adrenal cortex.
Production of Male Sex Hormones
Androgens are steroids hormones synthesized from cholesterol or acetyl coenyzme A (acetyl-CoA). It is produced at two sites in males (testes/adrenals) and females (ovaries/adrenals).
Androgen production and secretion is regulated primarily by luteininzing hormone (LH) and to a lesser degree follicle-stimulating hormone (FSH) from the anterior pituitary. LH and FSH secretion is in turn regulated by the hormone gonadotropin-releasing hormone (GnRH) released by the hypothalamus. In the fetus, human chorionic gonadotropin (HCG) which is the main hormone of pregnancy and also stimulates testosterone production and secretion.
The testes are the main source of testosterone in males but it does not produce any testosterone in male children. This is because the cells responsible for testosterone production known as the Leydig cells are practically non-existent in children. However, in the newborn male infant, these cells are present and secrete large quantities of testosterone for the first few months of life. It then becomes active once again after puberty and continues to secrete testosterone in high quantities.
The cortex of the adrenal glands also secrete androgens. However in males this quantity of adrenal androgens is fairly insignificant when compared to the testicular androgens. Even in females the quantity of adrenal androgens are small and normal secretion is not significant enough to cause virilization (masculinization in females).
The ovaries produces very small quantities of androgens. It is fairly insignificant but can be excessive if rare tumors like an arrhenblastoma forms which then produces large amounts of androgens. Virilization, however, is more likely due to an adrenal tumor in women.
Functions of Androgens
The functions of androgens can be classified by either its androgenic effects or anabolic effects.
The androgenic effects are those seen with masculinity that occur after puberty. The anabolic effects the changes in bone, muscle and height which accounts for the musculoskeletal difference between males and females.
Testosterone secretion by the ridges increases significantly when the male fetus reaches the 7th week of life. This testosterone secretion continues by the fetal testes once it develops. It contributes to the fetal development of the testes and scrotum. Around the 7th month of gestation, the testes descends due to the increased secretion of testosterone.
Testosterone in the male after puberty contributes to the enlargement of the penis, testes and scrotum (primary secondary sexual characteristics) and the development of the secondary male sexual characteristics such as a gruff voice, body and facial hair and larger muscle mass.
Testosterone cause the larynx (voice box) to enlarge with a slight thickening of the mucous membrane lining. This gives men the characteristic deep and gruff voice.
Testosterone causes the skin to become thicker and increases the activity of the sebaceous glands. These glands produce more sebum and increases the chance of developing acne.
Testosterone contributes to the development of body hair after puberty in both males and females. However the body hair growth is more widespread in males due to the higher testosterone levels. This includes hair over the pubis (pubic hair ~ pubarche) which may extend up the abdomen, chest, face, armpits (axillary hair) and in some men even on the back.
High levels of testosterone decreases the growth of hair on the top of the head. However this is also dependent on genetic factors.
Testosterone has a pronounced effect on the muscle mass and increases muscle bulk. This effect of testosterone is the reason that synthetic androgens are used by some athletes to gain a competitive edge.
Testosterone causes thickening of the bone by increasing bone matrix formation. The pelvis experiences some of the major effects of testosterone in that it becomes narrower, longer, stronger and funnel-shaped. This differentiates the narrower male pelvis from the wider female pelvis.
Metabolism and Excretion of Androgens
Almost all of the testosterone travels in the blood stream by binding to the blood protein albumin or a specialized protein known as sex hormone-binding globulin. It is then either transferred to tissues or degraded by the liver. Most of the testosterone is converted into dihydrotestosterone (DHT), which is the more active form, and then utilized by the target tissues. However, this conversion is not always necessary. The testosterone that is not used is then converted into dehydroepiandrosterone (DHEA) or androsterone. It is then conjugated and excreted via the bile into the gut or via the kidneys in the urine.