Hormones, chemical messengers that travel in the bloodstream, can be either peptides or steroids.
Organs wait for the arrival of hormones.
Steroids are lipid soluble and pass through the plasma membranes of target cells and combine with receptors in the nucleus. Peptide hormones bind to receptors in the target plasma membrane and then initiate a cascade of biochemical changes within the cell. Each enzyme then begins the next reaction and in the process amplifies the hormone's effect. In negative feedback, an endocrine gland is sensitive to the condition it regulates or to blood levels of the hormone itself. When the product increases, the gland slows the release of the hormone.