Hormones are the body's way of sending signals, or information from one part of the body to another. In a computer, electrons (electricity) act as the signal; in the body (which doesn't have wires!), the signals must be sent with chemicals,
and that is the role of hormones. The term "HPT Axis" refers to the
interaction of the hypothalamus, pituitary, and testes (there are
other axes as well). For the Leydig cells (the cells that produce Testosterone), Luteinizing hormone (LH) is released from the pituitary and it signals the Leydig cells to produce testosterone. Similarly, the pituitary releases FSH, and it tells the Sertoli cells to make sperm (as well as androgen-binding-protein). The pituitary is a gland that produces and stores a number of hormones, under the control of the hypothalamus. The hypothalamus might be considered to be the General (as in military), and the pituitary would be a Colonel under the General's command. The hypothalamus decides how the body's organs should operate, and the
pituitary gives the actual "orders" to the target organs. Some of
the "signalling" hormones made or stored in the pituitary are:
Growth Hormone
IGF-I and IGF-II
Thyroid Stimulating Hormone (TSH)
Vasopressin (or Antidiuretic hormone)
Luteininzing Hormone (LH)
Follicle Stimulating Hormone (FSH)
Adrenocorticotropic Hormone (ACTH)
The hypothalamus and the pituitary are very close together, and are
located at the base of the brain. Just as the pituitary uses hormones
to signal the target organ (testes, thyroid, etc) to do something,
the hypothalamus uses other hormones to signal the pituitary to do
its job. Some of these "Hypothalamic Releasing Factors" are (along
with the pituitary hormones affected):
Hypothalamic Hormone: Regulates:
Gonadotropin Releasing Hormone LH, FSH
Growth Hormone Releasing Hormone GH
Thyrotropin Releasing Hormone TSH
Corticotropin Releasing Hormone ACTH
But how does the hypothalamus know when its commands have been carried out? By what's called a "feedback loop". Just as a General relies on reports from the field, the hypothalamus must monitor the results of its commands. The hypothalamus has sensors (receptors) to determine the levels of the chemicals (hormones) produced by the target organs. For our purposes, we will examine only one feedback loop, the one involving the testes.
The hypothalamus has both androgen (Male hormone) receptors and estrogen (female hormone) receptors. This is important and if you are not already aware estrogen is the female hormone and is required by men albeit but in much smaller amounts.
When the level of either hormone gets too high, the receptors become more highly activated, and the hypothalamus stops sending
Gonadotropin Releasing Hormone to the pituitary. The pituitary, in
turn, stops sending LH and FSH to the testes. Thus, the signal
is, "stop producing testosterone (and sperm)". The important thing to note here is that Estrogen occupies the same receptors that Testosterone does and fools the hypothalamus into thinking there is enough Testosterone. Elevated levels of the female hormone will suppress the axes into lowering Testosterone production. The Testosterone to Estrogen ratio will plummet and all the problems associated with it will begin take hold.
A schematic of the HPTA (and other glands) is shown top right. Note that the other glands are involved in feedback mechanism also.
Confused and want to no more?
See our publications section for recommended reading to learn more about the male endocrine (hormone) system and also our associated links section for online reading.
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