Testosterone (T)


 

Source: Major source is Leydig cells in adult and fetal life. LH Stimulate T production in males. In females, It is released by theca interna.

Functions of T

In males

  • It is responsible for anabolic growth.
  • It is responsible for the development and maintenance of secondary sex characteristics in males.
  • It is always implicated with libido (sex drive).
  • It has a role in spermatogenesis.
  • It is also involved in sex differentiation in fetal life i.e., to influence Wolffian duct.
  • Growth and secretion of secondary sex glands are under the control of T.

In females

  • It is used as a substrate for E2.

Control of GnRH

P4, T, and E2 affect the secretion of GnRH. In general, These hormones inhibit GnRH secretion.

Mechanism

The hypothalamus releases GnRH that causes the pituitary gland to release LH and FSH. LH and FSH have their effects on gonads and cause them to release P4, T, and E2. All these P4, T, and E2 have negative feedback for the hypothalamus.
Gonads also secrets Inhibin and activin that have negative and positive feedback mechanisms for LH release respectively.
P4, T, and E2 have a negative effect on the hypothalamus also known as long loop inhibition, and Pituitary gland known as short loop inhibition.
  • A high level of T has a negative effect on both the hypothalamus and the pituitary gland.
  • The moderate level of E2 has a negative effect on both the hypothalamus and the pituitary gland. This level mainly has an effect on FSH.
  • Moderate to the high level of P4 has a negative effect on both the hypothalamus and the pituitary gland.
  • A very very high level of E2 has a positive effect on GnRH and LH.

Release of gonadotropin hormone

Gonadotropin can be released in the following ways
  1. Basal or episodic which is normal release.
  2. Pulse, pulsatile secretion, or Surge release (pre-ovulatory LH surge).
Pre-ovulatory LH surge is stimulated by a very very high level of E2. Very very high level of E2 acts on GnRH then GnRH will act on the pituitary gland.
In females, there are two centers for GnRH release.
  1. Tonic center (it is also present in males).
  2. Surge center (it is not present in males).
All the steroid hormones have a negative effect on the tonic center and inhibit its secretion. A very very high level of E2 has a positive effect on surge center for the release of GnRH.
Normally GnRH comes from the tonic center for FSH and LH but for ovulation, it comes from the surge center.

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