The endocrine system is a very important and very involved system within our bodies. It allows our body to communicate over long distances and is a major player in regulating a stable environment or in other words preserving homeostasis. This system employs the use of hormones which are chemicals secreted from glands and enter the bloodstream where they circulate until exerting an effect on a downstream target cell. We're going to be specifically talking about the two types of mechanisms in which hormones exert their effect on target cells. But to start off here's a brief overview what hormones are and how they work. There are three basic group of hormones. 1. Amino acid derivatives. 2. Peptide hormones and 3. Lipid derivatives or steroid hormones. For amino acid derivatives, these are small molecules and they come from amino acids like tyrosine and tryptophan. One hormone derived from tryptophan is melatonin. A hormone involved in our sleep-wake cycles. You can get tryptophan from foods like turkey, chocolate or milk. Peptide hormones are made up of a bunch of amino acids. Some examples would be thyroid-stimulating hormone, oxytocin or prolactin. Lipid derivatives or steroid hormones are considered lipophilic meaning they have an affinity for lipid structures which we'll take a look at shortly. They circulate in the blood bound to carrier proteins so they usually last in the circulation longer than other types of hormones. There are two main mechanisms for which a hormone can affect a target cell. The first is by and non steroid action. This mechanism is employed by an amino acid hormone or a peptide hormone. These hormones cannot freely cross a membrane on a target cell so it first acts by binding a cell surface receptor. There is then an intracellular signaling cascade that occurs in order for the desired effect to take place. There are proteins on the intracellular side of the target cell that are associated with the receptor. Most often this involves what's called a G-protein. This G protein is found next to an enzyme that converts ATP to a molecule called cyclic AMP or cAMP. This is something that makes this mechanism unique. Cyclic AMP is called a second messenger because it signals a cascade of events that eventually change the enzymatic activity in a cell to cause the target cell response. Examples of hormones that use the cyclic AMP second messenger system include ACTH, calcitonin, epinephrine, glucagon, parathyroid hormone and ADH. The second mechanism is the steroid action. Because lipid derivative or steroid hormones are lipophilic they can freely move through the membrane and into the cell. From there they can either bind to a receptor within the cell or they can move freely into the nucleus where the hormone or hormone complex causes a change in gene activity. This then increases transcription and mRNA production which then leads to an increase in protein production within the cell. Examples of hormones that exert effects via steroid hormone mechanisms are testosterone, estrogen, progesterone, aldosterone, and calcitriol.