Synthesis and secretion of hormones are the most highly regulated aspect of endocrine control. Such control is mediated by positive and negative feedback circuits, as described below in more detail. Shutting off secretion of a hormone that has a very short halflife causes circulating hormone concentration to plummet, but if a hormone's biological halflife is long, effective concentrations persist for some time after secretion ceases. Feedback Control of Hormone Production Feedback circuits are at the root of most control mechanisms in physiology, and are particularly prominent in the endocrine system. Instances of positive feedback certainly occur, but negative feedback is much more common.
Negative feedback is seen when the output of a pathway inhibits inputs to the pathway.
The heating system in your home is a simple negative feedback circuit. When temperature drops back below the set point, negative feedback is gone, and the furnace comes back on. Feedback loops are used extensively to regulate secretion of hormones in the hypothalamic-pituitary axis . An important example of a negative feedback loop is seen in control of thyroid hormone secretion. TSH binds to receptors on epithelial cells in the thyroid gland, stimulating synthesis and secretion of thyroid hormones, which affect probably all cells in the body.