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Tropic Hormones - Hormones produced and released by the anterior pituitary:
1. adrenocorticotropic hormone (ACTH): stimulates the production and release of hormones (including androgens) from the adrenal cortex.
2. thyroid-stimulating hormone (TSH): stimulates the production and release of hormones from the thyroid (Thyroid controls metabolic rate and skeletal growth.)
3. growth hormone: acts at liver and stimulates the growth of the musculature and skeletal system.
4. melanocyte-stimulating hormone (MSH): stimulates pigment-producing cells of the skin.
5. beta-lipotropic hormone (BLPH): precursor of the endorphins or the bodies natural opiate system.
***The next 3 tropic hormones are in a subclass called the gonadotropic hormones. These are the ones most involved in the monthly cycle of women.
6. follicle-stimulating hormone (FSH): 1) stimulates the growth of the ovarian follicles and 2) works with LH to stimulate ovulation
7. luteinizing hormone (LH): 1) stimulates estrogen (estradiol) production by the ovarian follicle, 2) works with FSH to cause ovulation, 3) stimulates the formation of the corpus luteum and 4) stimulates estrogen (estradiol) production by the corpus luteum.
8. prolactin (PRL): 1) stimulates progesterone production and release by the corpus luteum and 2) stimulates milk production by the mammary glands during lactation.
Releasing Hormones: Hormones produced by the hypothalamus which control the production and release of the tropic hormones by the anterior pituitary. Releasing factors are sent from the hypothalamus to the anterior pituitary through a specialized local blood-transport system which runs between the two structures through the pituitary infindibulum. This blood-transport system is called the hypothalamo-hypophysial portal system.
Releasing Hormones:
1. corticotropin releasing hormone (CRH): stimulates production and release of ACTH
2. thyrotropic hormone releasing hormone (TRH): stimulates the production and release of TSH
3. somatostatin: inhibits the production and release of growth hormone
4. growth hormone releasing hormone: stimulates the production and release of growth hormone
5. gonadotropin releasing hormone (GnRH): stimulates the production and release of FSH and LH
(Note: Given the GnRH stimulates the production and release of both FSH and LH, you would expect the pattern of release for both hormones to be identical. It is not. The pattern of production and release of FSH and LH differ because inhibin (from the ovarian follicle and the corpus luteum) acts directly at the anterior pituitary to inhibit the production and release of FSH.
6. dopamine: inhibits the production and release of prolactin
7. prolactin stimulating factor: stimulates the production and release of prolactin
Hormone relationships you should know:
------> = stimulates production and release
---/---> = inhibits production and release
GnRH------>LH
GnRH------>FSH
inhibin---/--->FSH (directly at the anterior pituitary, not through GnRH)
LH---/--->GnRH
FSH---/--->GnRH
LH------>estrogen (ovarian follicle and corpus luteum)
low estrogen------>GnRH
moderate estrogen---/--->GnRH
high estrogen---->GnRH
dopamine---/--->prolactin
prolactin stimulating factor------>prolactin
prolactin------>progesterone
progesterone---/--->GnRH
progesterone---/--->prolactin stimulating factor
Feedback Systems: The levels of hormones in the body at any given time are regulated by feedback systems. When you hook two or more of the above relationships together you get a feedback system. Most feedback systems are negative feedback systems. In this case, when the level of a given hormone gets too high, the systems responsible for its production are shut down.
Negative Feedback System example:
GnRH------>LH------>moderate estrogen---/--->GnRH
GnRH stimulates the production of LH which stimulates the production of estrogen. When estrogen levels get high enough (moderate levels, not very low and not very high), the estrogen acts at the cells in the hypothalamus that make GnRH and inhibit GnRH production.
Some rare feedback systems are positive feedback systems. These systems basically run out of control until some other event, such as another hormone or loss of one of the hormone-producing sites involved, comes into play to stop the process.
Example:
GnRH------>LH------>very high estrogen------>GnRH
GnRH stimulates the production of LH which in turn stimulates the production of estrogen. When estrogen levels are very high, they stimulate further GnRH production. This process occurs at mid-cycle just before ovulation takes place and ends with the rupture of the ovarian follicle. Once the follicle is ruptured, there's no estrogen-producing site for a short time, so the positive feedback loop is broken.
Changes of Hormone Levels Across the Monthly Cycle:
Estrogen:
1. very low on Day 1 (no follicle and no corpus luteum to produce estrogen
2. increase gradually throughout most of the preovulatory phase as the ovarian follicles (and later only the Graafian follicle) grows
3. surges 24 hours prior to ovulation (When the Graafian follicle is mature, it signals its readiness to the hypothalamus by releasing all of its store of estrogen.)
4. decreases sharply after surge and very low through ovulation and first couple of days after ovulation (After releasing stores, estrogen levels depleted. The follicle ruptures and there is no source of estrogen until corpus luteum develops.)
5. rises in postovulatory phase and then plateau at moderate level (Increase corresponds with development of corpus luteum. Plateau at moderate level due to negative feedback with GnRH and, thus, LH.)
6. drops off just before Day 1 (With the death of the corpus luteum, no source of estrogen exists.)
Progesterone:
1. low level of Day 1 (Corpus luteum has died and there is no source of progesterone.)
2. stays low through preovulatory phase (No corpus luteum, therefore, no source of progesterone production.)
3. increase during postovulatory phase with plateau at mid-postovulatory period (Increase as corpus luteum develops and then plateau due to inhibition of prolactin stimulating factor.)
4. drops of at end of cycle just before Day 1 (When corpus luteum dies, there is no source of progesterone.)
FSH:
1. starts to increase on Day 1 (No estrogen or progesterone inhibition of GnRH. No inhibin. Stimulates follicle development.)
2. after gradually increasing the early preovulatory phase, levels gradually decrease for rest of preovulatory phase (Combination of the effect of moderate estrogen inhibiting GnRH, which would be enough to hold levels steady, and the effect of inhibin directly inhibiting FSH production at the anterior pituitary, which causes the levels to actually decline. Prevents multiple follicles from maturing at one time.)
3. surge at ovulation. (Increase not as great as that for LH. Caused by the massive release of estrogen just before ovulation which stimulated a surge of GnRH. The surge of FSH and LH at mid-cycle is what causes the follicle to rupture or ovulation to occur.)
4. gradual decline to very low levels after ovulation and the very low levels are maintained for the rest of the postovulatory phase (Both estrogen and progesterone from the corpus luteum are inhibiting GnRH. Also, inhibin from corpus luteum directly inhibiting FSH production at anterior pituitary. Prevents more follicles from maturing when a pregnancy may have already been initiated.)
LH:
1. gradual increase beginning on Day 1 (No estrogen or progesterone to inhibit GnRH.)
2. rises to moderate level and stays there through the rest of the preovulatory phase (Inhibition of GnRH by moderate estrogen levels holds LH levels steady.)
3. surges at ovulation (Increase greater than for FSH. Caused by the massive release of estrogen just before ovulation which stimulated a surge of GnRH. The surge of FSH and LH at mid-cycle is what causes the follicle to rupture or ovulation to occur.)
4. gradual decline through postovulatory phase (Due to inhibition of GnRH by both estrogen and progesterone.)