They control a variety of biological processes including muscle growth, heart rate, menstrual cycles and hunger. Hormones travel thoughout the body, either in the blood stream or in the fluid around cells, looking for target cells. Once hormones find a target cell, they bind with specific protein receptors inside or on the surface of the cell and specifically change the cell's activities. The protein receptor reads the hormone's message and carries out the instructions by either influencing gene expression or altering cellular protein activity. These actions produce a variety of rapid responses and long-term effects.
Hormones vary in their range of targets.
Some types of hormones can bind with compatible receptors found in many different cells all over the body. Other hormones are more specific, targeting only one or a few tissues. For example, estrogens, the female sex hormones, can regulate function by binding to special estrogen receptor sites in uterine, breast and bone cells. In addition, the same cell can act as a target cell for many different regulatory molecules. For instance, the same uterine, breast and bone cells that accept estrogens, also contain progesterone, androgen, glucocorticoid, vitamin D and vitamin A receptors.