Locations of major endocrine glands. Some organs that secrete hormones are not shown, such as the heart and small intestine.

Hormones called amines, including norepinephrine and epinephrine, are derived from amino acids. These hormones are also synthesized in the adrenal medulla (the inner portion of the adrenal gland) from the amino acid tyrosine.

The peptide hormones are short chains of amino acids. This group includes hormones associated with the posterior pituitary gland and some produced in the hypothalamus.

Protein hormones are composed of long chains of amino acids, linked to form intricate molecular structures (see chapter 2, p. 54). They include the hormone secreted by the parathyroid gland and some of those secreted by the anterior pituitary gland (fig. 13.3). Certain other hormones secrete from the anterior pituitary gland are glycoproteins, which consist of proteins joined to carbohydrates.

Another group of compounds, called prostaglandins (pros"tah-glan'dinz), are paracrine substances. They regulate neighboring cells. Prostaglandins are lipids (20-carbon fatty acids that include 5-carbon rings) and are synthesized from a type of fatty acid (arachidonic acid) in cell membranes. Prostaglandins occur in a wide variety of cells, including those of the liver, kidneys, heart, lungs, thymus gland, pancreas, brain, and reproductive organs.

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