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What is feedback control ? Illustrate feedback mechanism of homeostasis by an example of hormones.  

When the secretion of hormones is under the control of factors or other hormones, it is called feedback control. The regulation of secretion of thyroxine from the thyroid gland is an example of such feedback control mechanism.

Homeostasis means maintenance of static or constant conditions in the internal environment of the body. Hormones help to regulate cellular functions.

(1) Positive feedback control : If the level of thyroxine is less than the normal limits in the blood, thyroxine level stimulates the hypothalamus to secrete more of TRH which results in the increased secretion of thyroxine. Such regulatory effect is called positive feedback control.

(2) Negative feedback control : The thyroxine releasing hormone (TRH) from the hypothalamus stimulates the anterior lobe of the pituitary gland to secrete the thyroid stimulating hormone (TSH). The TSH in turn, stimulates the thyroid gland to secrete thyroxine. A high level of thyroxine in the blood exerts an inhibitory effect on hypothalamus in such a way that less of TRH and TSH is produced resulting in decrease of thyroxine. This is called negative feedback control.