Content Reviewers:Rishi Desai, MD, MPH
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.
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.
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.
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.
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