Testosterone

Fetal Life

In the fetus, androgens (male sex hormones, including testosterone) play a key role in sex differentiation and physical development. The male gonads form between 6 and 12 weeks of gestation within the abdomen, then move toward the inguinal canal, which is the route for the testicles to pass down between the outer skin and the abdomen [Page 2233]and then, between months 7 and 9 gestation, to descend into the scrotal sac.

The level of testosterone in the amniotic fluid surrounding the fetus influences fetal development in both males and females. Two examples of this influence involve bones and the brain. Higher testosterone exposure in males is associated with fingers in the hand with lower ratios of second to fourth digit length. This value can be used as a guide to androgen exposure in the uterus. Studies also have shown that greater levels of amniotic fluid testosterone increases male-related play behavior in early preschool life. Testosterone also has a positive effect on the ability to coordinate body movements with vision.

Childhood Through Puberty

After birth, the hypothalamus releases GnRH in a pulsatile fashion into the bloodstream. This hormone then stimulates the secretion of FSH, LH, and testosterone during the first 6 months of life. The HPT axis is then switched off until being reactivated for puberty to commence.

Testosterone plays a key role in male pubertal development. First, there is an increase in production of androgens from the cortex of each adrenal gland over 3–4 years prior to the onset of puberty. Second, usually between 10 and 14 years of age in males, activation of pulsatile release of hypothalamic GnRH commences, which leads to the increase in size of the testes and to testosterone synthesis and secretion.

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