The amount of hormone released by an endocrine gland is determined by the body’s need for the particular hormone at any given time. The product of the target tissue exerts an effect on the respective endocrine gland. This effect may be positive (secrete more) or negative (secrete no more or slow down).
Feedback mechanism of thyroid activity
Hypothalamus releases a hormone TSH-RH (TSH-Releasing Hormone) which instructs the anterior pituitary to release TSH (thyroid stimulating hormone). The TSH stimulates thyroid to release thyroxine. If the level of thyroxine in blood increases, the pituitary stops the release of TSH. When the level of thyroxine falls in the blood, the thyroid gets stimulated to secrete more of it. In feedback mechanism the starting point of an activity receives back the information whether to continue or increase, or to slow down or even stop.