How hormones act

A hormone affects only certain cells, its target cells. It works by binding to specific proteins called hormone receptors, found only in those target tissues. Each receptor fits only one hormone, like a lock that only one key opens.

Some receptors sit on the cell membrane (membrane-bound receptors). Others are inside the cell (intracellular receptors), mostly in the nucleus. When a hormone binds its receptor, they form a hormone-receptor complex, which sets off chemical changes in the target tissue.

By their chemistry, hormones fall into four groups:

• Peptides, polypeptides and proteins: insulin, glucagon, the pituitary and hypothalamic hormones.
• Steroids: cortisol, testosterone, estradiol, progesterone.
• Iodothyronines: the thyroid hormones.
• Amino-acid derivatives: epinephrine (adrenaline).

The two ways they work:

• Hormones that bind membrane-bound receptors (mostly the protein hormones) usually do not enter the cell. They knock at the door, and a messenger inside carries the signal on. These inside messengers are called second messengers: cyclic AMP, IP₃ and Ca²⁺. They change the cell's metabolism.
• Hormones that bind intracellular receptors (steroids and iodothyronines) pass through the membrane, walk in, and act with their receptor on the genes (the genome), switching genes on or off. Together these effects bring about physiological and developmental changes.