• Yes I can. Oh, you mean you want me to do that now? OK, here goes... Negative feedback is a simple enough concept. In a simple model, you have hormone A, that has an effect on a tissue. As the effect increases, it exerts a negative feedback upon the hormone that inhibits its secretion/release, causing the levels of the hormone to fall. This means that the effect on the tissue decreases, so the negative feedback is lost. This means the inhibtion on the hormone is no longer there, so it begins to rise again, etc etc. In reality, negative feedback cycles are much more complex, because they may involve multiple organs, hormones that influence other hormones, and so on, but the basic principle remains the same. No, you wanted to know about thyroxine (also called T4, so I'll just call it that from now on). T4 makes up 90% of the hormone output from the thyroid gland, but in actual fact T3 has most of the effect on tissue because although it is secreted in much smaller amounts, it is much more potent than T4. OK, so a *basic* overview of the negative feedback of thyroxine release. The hypothalamus gland in the brain secretes a hormone called thyrotrophin releasing hormone (TRH). This stimulates the anterior pituitary gland to secrete a hormone called thyroid stimulating hormone (TSH). This in turn stimulates the thyroid gland to secrete T4 (thyroxine) and T3. Being hormones, these go through the whole 60,000 miles of blood vessels in your body and do their actions to help regulate metabolism and so on. In doing so, some of it goes through the blood vessels that supply the hypothalamus and the anterior pituitary. Because the levels of T3 and T4 are now quite high, they exert negative feedback at both these locations (i.e. they inhibit TRH secretion from the hypothalamus and also TSH secretion from the anterior pituitary). This causes the thyroid gland to be no longer stimulated as much, and so it releases less and less T3 and T4. Eventually, the levels of T3 and T4 in the plasma go down so much that they no longer exert a strong negative feedback on the hypothalamus and anterior pituitary, so these begin to secrete their hormones in greater levels again and the process starts again. In reality, as I said before, it's much more complicated. Furthermore, the glands never really stop producing the hormones,and then start again, but rather the cycle is kept in a constant balance. If still not sure, see the picture I've done.
  • the gland secretion stimulate other gland to produce hormone,the increase level of that hormone in the blood cause a drop productin of the first gland secrete,when it is on a right level,or else it shutdown the release of that secrection

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