Hormones are produced in extremely small amounts and are highly potent. By means of their actions on various target tissues, they affect growth, metabolism, reproductive activity, and behavior.
Chemically, hormones fall into two categories: steroid hormones, made from lipids, and hormones made of amino acids, which include proteins and proteinlike compounds. Steroids are produced by the sex glands (gonads) and the outer region (cortex) of the adrenal glands. All of the remaining endocrine glands produce amino acid hormones.
The production of hormones is controlled mainly by negative feedback. That is, the hormone itself, or some product of hormone activity, acts as a control over further manufacture of the hormone—a self-regulating system. Hormone production also may be controlled by nervous stimulation or by other hormones.
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