hormone

A hormone is a chemical messenger that an endocrine gland releases into the blood, from where it reaches cells throughout the body. Only cells with a receptor for that hormone respond, and that is the whole logic of the endocrine system: no receptor, no response. Hormones fall into two classes by how they cross the membrane. A steroid hormone is fat-soluble, so it passes through the cell membrane and binds a receptor inside the cell, usually changing which genes are read. A nonsteroid hormone is a protein or peptide, which cannot cross the membrane, so it binds a receptor on the surface and sets off a second messenger inside the cell. The same hormone can therefore do different things in different tissues, because different tissues carry different receptors. Secretion is usually held steady by negative feedback: the hormone's effect shuts down the signal that produced it, so the level settles at a set point.