[ hormone ]
A substance that acts as a signal between organs and tissue to regulate physiology and behaviour.
Hormones are secreted directly into the circulatory system by specialised cells in glands and other organs. They regulate metabolic activities, growth and tissue repair, sexual function, sleep and emotions, to name a few.
Hormones are either Amino acid–based (amines and peptide or proteins) or steroid-based. Amino acid–based hormones are water-soluble acting on the surface of target cells. Steroid-based hormones are lipid-soluble, penetrating membranes of target cells and acting directly within their nuclei.