What is a hormone and what do they do?

A hormone is a chemical messenger which travels in the blood.  Hormones cause other tissues and organs to either produce other hormones or carry out chemical reactions. Water-soluble hormones cause this by binding to receptors on the surface of cells and causing another chemical to be produced the cell which triggers reactions. Receptors are proteins on the surface of the cell which have the exact same shape as the hormone. Lipid-soluble proteins can diffuse into the cell because it can pass through the plasma membrane because they are both fats and they will bind to receptors on the inside of the cell to cause an effect. For example, the pituitary gland in the brain produces LH which causes eggs to be released from the ovaries.