**1. Introduction**

The term Hormones refer to substances or molecules that are produced by the endocrine system. This system sends messages to various parts of the body and hormones are the messengers used to do so. Hormones help to regulate body processes, like hunger, blood pressure and sexual desire, among others. Whereas hormones are key molecules in reproduction in human, they are fundamental to *all* the body systems [1]. Chemically, hormones may be classified as either proteins or steroids. All of the hormones in the human body, except the sex hormones and those from the adrenal cortex, are proteins or protein derivatives [2].

Hormones flow through the whole body by blood but only affect certain cells designed to receive their messages \_ target cells. These specific cells that respond to a given hormone have receptor sites for the said hormone [1, 2]. This is a sort of lock-and-key mechanism, in that if the hormone fits the receptor site then there will be an effect. It is synonymous with lock and key, where if the key fits, then the lock opens. If a hormone and a receptor do not match, there will be no reaction. In some cases, target cells are localized in a single gland or organ whereas in other instances, they are diffuse and scattered throughout the whole body in which case many areas will be affected. Through modifying activities of the cells, hormones are able to bring about their associated effects on those target cells.

When certain types of hormones called protein hormones react with receptors on the surface of the target cell, the sequence of events that results in hormone

action is relatively fast. On the other hand, steroid hormones basically react with receptor sites inside a target cell. Since this method of action involves synthesis of proteins, the actions of these hormones are relatively slow [2].
