Hormones

Hormones as Intercellular Messengers

Hormones are molecules that carry signals between cells, allowing one part of the body to instruct another. They are manufactured by the endocrine glands — ductless glands that release their secretions straight into the bloodstream rather than through a duct. Once in circulation, the hormone travels through the blood and acts at its target site, which may lie far away from the gland that produced it.

Chemical Diversity of Hormones

Hormones do not share a single chemical identity; they fall into a few distinct classes.

• Steroidal hormones — built on a steroid skeleton, such as the estrogens and androgens.
• Polypeptide hormones — short chains of amino acids, such as insulin and the endorphins.
• Amino-acid derivatives — small molecules derived by modifying a single amino acid, such as epinephrine and norepinephrine.

Regulatory Roles in the Body

Hormones work together to keep the body's internal activities in balance.

Insulin is the clearest example of this balancing act: it is released when blood glucose rises sharply and pulls the glucose level back down, while the hormone glucagon pushes the level back up when needed. Acting in opposite directions, insulin and glucagon jointly keep blood glucose within a narrow range.

Epinephrine and norepinephrine mediate the body's response to external stimuli. Growth hormones and the sex hormones govern growth and development.

Thyroxine, secreted by the thyroid gland, is an iodine-containing derivative of the amino acid tyrosine. Too little thyroxine produces hypothyroidism, marked by lethargy and obesity, while an excess produces hyperthyroidism. Because thyroxine synthesis depends on dietary iodine, low iodine intake can itself cause hypothyroidism along with enlargement of the thyroid gland — a condition now largely prevented by fortifying common table salt with sodium iodide ("iodised" salt).

Steroid Hormones from the Adrenal Cortex and Gonads

Steroid hormones originate from two sources: the adrenal cortex, and the gonads (the testes in males, the ovaries in females).

The adrenal cortex secretes two functionally distinct groups of steroid hormones:

• Glucocorticoids regulate carbohydrate metabolism, moderate inflammatory responses, and are involved in the body's reaction to stress.
• Mineralocorticoids control how much water and salt the kidney excretes. …