Hormones

Hormones are intercellular messengers made by endocrine glands and released straight into the blood, which carries them to where they act.

Chemically they fall into three kinds: steroids (estrogens, androgens), polypeptides (insulin, endorphins) and amino acid derivatives (epinephrine, norepinephrine).

They keep body activities in balance. Insulin is released when blood glucose rises fast; glucagon raises blood glucose. Together they hold it in a narrow range.

Responses to outside stimuli run through epinephrine and norepinephrine; growth and sex hormones steer growth and development.

Thyroxine, from the thyroid, is an iodinated derivative of the amino acid tyrosine. Too little causes hypothyroidism (lethargy, obesity); too much, hyperthyroidism. Low dietary iodine can cause hypothyroidism and an enlarged thyroid, now largely prevented by adding sodium iodide to table salt.

Steroid hormones come from the adrenal cortex and the gonads. Glucocorticoids control carbohydrate metabolism, modulate inflammation and act in stress; mineralocorticoids control how much water and salt the kidney excretes.

A failing adrenal cortex can cause Addison's disease (hypoglycemia, weakness, poor tolerance of stress), fatal unless treated with glucocorticoids and mineralocorticoids.

Gonad hormones drive secondary sex characters. Testosterone is the main male hormone (deep voice, facial hair, build); estradiol the main female one, also helping control the menstrual cycle; progesterone prepares the uterus to receive a fertilised egg.