Testosterone

Definitions & Key takeaways

Testosterone is the primary male sex hormone and an anabolic steroid. In men, it plays a key role in the development of male reproductive tissue and secondary sexual characteristics. It is produced mainly in the testes and is responsible for several functions, including sexual differentiation during fetal life, and the development of primary sexual characteristics like an enlarged penis and testes, as well as secondary characteristics like a male pattern of hair growth, voice changes, and various anabolic effects.

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Introduction0:00–0:23

When someone mentions testosterone, it might conjure up images of a burly alpha male. That’s because testosterone, the primary male hormone, is an androgen, andro meaning male and gen meaning “to produce”, which means testosterone helps generate the characteristics associated with male sexuality.
The effects of testosterone are first seen in the fetus. During the first six weeks of development, the reproductive tissues of males and females are identical, but in week seven, genes in the sex-determining region of the Y chromosome initiate the development of testicles.

Testosterone in fetal development0:23–1:07

Once they form, the fetal testicles secrete testosterone which guides development of the male urogenital tract and external genitalia, as well as testicular descent through the inguinal canal which happens in the last two months of fetal development.
The fetal ovaries also secrete testosterone but at much lower levels, and this largely explains the differences in fetal development between boys and girls.

Testosterone in puberty1:07–3:05

In puberty, the hypothalamic-pituitary axis takes center stage in regulating testosterone levels and gonadal function - which are the testes in young men.
The hypothalamus secretes gonadotropin-releasing hormone which moves through the bridge between the hypothalamus and the pituitary gland, called the hypothalamo-hypophyseal portal system, and gets to the anterior lobe of the pituitary.
In response, the anterior pituitary secretes luteinizing hormone and follicle-stimulating hormone - two gonadotropic hormones which get secreted into the blood and reach the gonads.
Leydig cells, slowly turn cholesterol into testosterone through a number of steps, and the first step of this process is stimulated by luteinizing hormone.
Two important intermediate molecules in that process are dehydroepiandrosterone, also called DHEA, and the molecule that it gets converted into - androstenedione.
The testes have the enzyme 17β-hydroxysteroid dehydrogenase, which takes androstenedione and turns it into testosterone.
The majority of the testosterone gets bound to plasma proteins like sex hormone–binding globulin and albumin, whereas only a minority remains free and unbound in the blood.
The protein-bound testosterone acts like a reservoir for testosterone, and it’s the unbound testosterone that reaches tissues like the seminal vesicles, prostate gland, muscles, and bone.
In some tissues, testosterone directly affects the target cells, whereas in others, the enzyme 5ɑ-reductase converts testosterone into dihydrotestosterone - and that’s the molecule that ultimately has an effect on the cell.
Testosterone or dihydrotestosterone bind to cell surface receptors and move into the target cell, where they initiate expression of a variety of proteins.

Primary and Secondary Sexual Development3:05–4:27

Testosterone is responsible for primary sexual development, which are the changes necessary for reproduction like enlargement of the penis and testes, as well as increasing libido.
Testosterone also helps with secondary sexual characteristics which aren’t required for reproduction but are associated with masculinity, like a male pattern of hair growth on the face, chest, axillary, and genital areas.
The hair itself changes from thin soft hair of childhood to thick and coarse hair of adulthood. The larynx and vocal folds also change, altering the voice, which can be difficult to control at first and can make it crack - but eventually the voice deepens.
Testosterone also has tissue-building effects, sometimes called anabolic effects. It contributes to the growth spurt in puberty by spurring on growth at the epiphyseal plate to make the bones longer.
It’s also responsible for closure of the plate in late puberty. Other anabolic effects include development of broad shoulders and muscular arms and legs.
Testosterone may be associated with aggressive behavior, something that may be seen in athletes who abuse anabolic steroids in an attempt to increase muscle mass.
Testosterone also stimulates erythropoiesis, giving men a higher red blood cell count than women. After puberty and throughout a person’s life, testosterone levels are regulated through a negative feedback mechanism, where rising levels of testosterone signal the hypothalamus and pituitary to turn off the secretion of luteinizing hormone, which decreases stimulation of the Leydig cells to make testosterone.

Testosterone regulation4:27–5:01

As testosterone levels drop, the hypothalamus and pituitary work together to produce additional luteinizing hormone, which boosts testosterone production.
This feedback cycle keeps testosterone levels normal. Nevertheless, testosterone levels tend to drop slightly as men age.

Aging5:01–5:55

Older men sometimes experience a slight reversal of some testosterone’s effects. For example, testicular size and libido may decrease.
Because of a decrease in the anabolic effects of testosterone, bone mineral density tends to decline, increasing the risk of fractures, muscle mass declines and fat mass increases, contributing to central obesity, and there’s decreased erythropoiesis which can lead to anemia.
Decreased testosterone can also lead to decreased focus and increased irritability. It would seem like testosterone replacement would be an easy solution, but this is an area of research to see if there are benefits and if they outweigh the increased risk of testosterone-dependent diseases like prostate cancer.

Weak Androgens5:55–6:28

Although the majority of testosterone in men is produced by the testes, both men and women produce weak-acting androgens, dehydroepiandrosterone and androstenedione in the zona reticularis layer of the adrenal cortex.
These androgens in women are thought to help maintain normal ovarian function, sexual function, and bone metabolism. These so-called weak androgens aren’t as effective at binding to testosterone receptors, but can be converted into testosterone in the peripheral tissues.
All right, as a quick recap, testosterone is the primary male hormone - it’s made in the testes, but some androgens are also made in the adrenal gland.

Review6:28–6:55

Testosterone causes major effects on the body including sexual differentiation during fetal life, and development of primary sexual characteristics like an enlarged penis and testes, as well as secondary characteristics like a male pattern of hair growth, voice changes, and various anabolic effects.