Negative feedback system in body regulates
Correct Answer: All the above
Description: Feedback Effect of Thyroid Hormone to Decrease Anterior Pituitary Secretion of TSH Increased thyroid hormone in the body fluids decreases secretion of TSH by the anterior pituitary. When the rate of thyroid hormone secretion rises to about 1.75 times normal, the rate of TSH secretion falls essentially to zero. Almost all this feedback depressant effect occurs even when the anterior pituitary has been separated from the hypothalamus. Therefore, as shown in Figure 76-7, it is probable that increased thyroid hormone inhibits ante- rior pituitary secretion of TSH mainly by a direct effect on the anterior pituitary gland itself. Regardless of the mech- anism of the feedback, its effect is to maintain an almost constant concentration of free thyroid hormones in the circulating body fluids. Antithyroid Substances Suppress Thyroid Secretion The best known antithyroid drugs are thiocyanate, propyl- thiouracil, and high concentrations of inorganic iodides. The mechanism by which each of these drugs blocks thyroid secretion is different from the others, and can be explained as follows. Thiocyanate Ions Decrease Iodide Trapping. The same active pump that transpos iodide ions into the thyroid cells can also pump thiocyanate ions, perchlorate ions, and nitrate ions. Therefore, the administration of thiocyanate (or one of the other ions as well) in high enough concentration can cause competitive inhibition of iodide transpo into the cell--that is, inhibition of the iodide-trapping mechanism. The decreased availability of iodide in the glandular cells does not stop the formation of thyroglobulin; it merely pre- vents the thyroglobulin that is formed from becoming iodi- nated and therefore from forming the thyroid hormones. This deficiency of the thyroid hormones in turn leads to increased secretion of TSH by the anterior pituitary gland, which causes overgrowth of the thyroid gland even though the gland still does not form adequate quantities of thyroid hormones. Therefore, the use of thiocyanates and some other ions to block thyroid secretion can lead to develop- ment of a greatly enlarged thyroid gland, which is called a goiter. Propylthiouracil Decreases Thyroid Hormone Formation. Propylthiouracil (and other, similar compounds, such as methimazole and carbimazole) prevents formation of thyroid hormone from iodides and tyrosine. The mechanism of this is paly to block the peroxidase enzyme that is required for iodination of tyrosine and paly to block the coupling of two iodinated tyrosines to form thyroxine or triiodothyronine. Propylthiouracil, like thiocyanate, does not prevent for- mation of thyroglobulin. The absence of thyroxine and tri- iodothyronine in the thyroglobulin can lead to tremendous feedback enhancement of TSH secretion by the anterior pituitary gland, thus promoting growth of the glandular tis- sue and forming a goiter. Iodides in High Concentrations Decrease Thyroid Activity and Thyroid Gland Size. When iodides are present in the blood in high concentration (100 times the normal plasma level), most activities of the thyroid gland are decreased, but often they remain decreased for only a few weeks. The effect is to reduce the rate of iodide trapping so that the rate of iodination of tyrosine to form thyroid hormones is also decreased. Even more impoant, the normal endocytosis of colloid from the follicles by the thyroid glandular cells is paralyzed by the high iodide concentrations. Because this is the first step in release of the thyroid hormones from the storage colloid, there is almost immediate shutdown of thyroid hormone secretion into the blood. Because iodides in high concentrations decrease all phases of thyroid activity, they slightly decrease the size of the thyroid gland and especially decrease its blood supply, in contradistinction to the opposite effects caused by most of the other antithyroid agents. For this reason, iodides are frequently administered to patients for 2 to 3 weeks before surgical removal of the thyroid gland to decrease the neces- sary amount of surgery, especially to decrease the amount of bleeding. Negative feedback is a regulatorymechanism in which a 'stimulus' causes an opposite 'output' in order to maintain an ideal level of whatever is being regulated. Negative feedback loops occur in a series of steps. You have a stimulus, in which a change occurs. Ref guyton and hall textbook of medical physiology edtion 12 Pg 915
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