Feedback Control Mechanisms in Endocrine Function

As mentioned previously, the endocrine system is concerned with maintaining homeostasis in the body. When a disturbance in physiologic function occurs, hormones are released to rectify the disturbance. As function returns to normal, hormone release is attenuated and homeostasis is resumed. For example, an increase in the blood glucose level initiates the release of insulin from pancreatic beta cells. Insulin increases the incorporation into and storage of glucose in liver, skeletal muscle, and other tissues. Blood glucose levels then return to normal, and insulin release is terminated. 

There are also a few examples of positive feedback mechanisms in the endocrine system.25 43 In a positive feedback loop, rising concentrations of one hormone cause an increase in other hormones, which, in turn, facilitates increased production of the first hormone. The primary example of this type of feedback occurs in the female reproductive system, where low levels of estrogen production increase the release of pituitary hormones (LH, FSH).10 43 Increased LH and FSH then facilitate further estrogen production, which further increases pituitary hormone secretion, and so on (see Chapter 30). Positive feedback mechanisms are relatively rare, however, compared with negative feedback controls in the endocrine system. 

The presence of feedback systems in endocrine function is important from a pharmacologic perspective. Drugs can be administered that act through the intrinsic feedback loops to control endogenous hormone production. A primary example is the use of oral contraceptives, when exogenous estrogen and proges- 

FIGURE 28-1 Negative feedback control in the hypothalamic-pituitary-endocrine pathways. Excitatory and inhibitory effects are indicated by (+] and H, respectively. Negative feedback loops occur owing to inhibition of the endocrine hormone on the pituitary and hypothalamus. 

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