The endocrine system relies on feedback mechanisms to control the hormone levels in the circulatory system. 

The physiological activity of a hormone depends mainly on its concentration within the circulatory system. The effects of too high or too low a concentration of hormones can be damaging—this level must be tightly controlled. 

Most endocrine glands are under negative feedback control that acts to maintain homeostasis, i.e., prevent deviation from an ideal value. A key example of a negative feedback system is the regulation of the thyroid hormone thyroxine, which regulates numerous key metabolic processes.

Positive feedback mechanisms control self-perpetuating events; that is, they encourage deviation from the mean. Positive feedback systems are much less common, although they do exist. A key example occurs during childbirth.

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Key Points

• The endocrine system relies on feedback mechanisms to control the hormone levels in the circulatory system.

• Negative feedback systems prevent deviation from an ideal mean to maintain homeostasis.

• Positive feedback systems facilitate deviation from the mean.

Key Terms

negative feedback: a system that prevents deviation from a mean value

positive feedback: a system that promotes deviation from a mean value

hormone: a chemical that is made by specialist cells, usually within an endocrine gland

homeostasis: the state of steady internal, physical, and chemical conditions maintained by living systems