Hormones

Hormones are molecules that are produced by endocrine glands, including the hypothalamus, pituitary gland, adrenal glands, gonads, (i.e., testes and ovaries), thyroid gland, parathyroid glands, and pancreas. The term “endocrine” implies that in response to specific stimuli, the products of those glands are released into the bloodstream.1 The hormones then are carried via the blood to their target cells. Some hormones have only a few specific target cells, whereas other hormones affect numerous cell types throughout the body. The target cells for each hormone are characterized by the presence of certain docking molecules (i.e., receptors) for the hormone that are located either on the cell surface or inside the cell. The interaction between the hormone and its receptor triggers a cascade of biochemical reactions in the target cell that eventually modify the cell’s function or activity. 

Mechanisms of Action

Several classes of hormones exist, including steroids, amino acid derivatives, and poly-peptides and proteins. Those hormone classes differ in their general cell, where they interact with receptor proteins that are already associated with specific DNA regions. The interaction modifies the activity of the affected genes.

Poly-peptide and protein hormones are chains of amino acids of various lengths (from three to several hundred amino acids). These hormones are found primarily in the hypothalamus, pituitary gland, and pancreas. In some instances, they are derived from inactive precursors, or pro-hormones, which can be cleaved into one or more active hormones. Because of their chemical structure, the poly-peptide and protein hormones cannot enter cells. Instead, they interact with receptors on the

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