Why can a hormone affect some cells but not others?

A hormone can affect some cells but not others because hormonal action is dependent on the presence of specific receptors within or on the surface of a target cell.1

Each hormone has a strong, highly specific chemical affinity for its corresponding receptor. When a hormone circulates through the body, it can only exert an effect on tissues that express the specific receptor gene required to recognize and bind that hormone.12

The location of these receptors further dictates how a cell responds:
* Cell Surface Receptors: Protein and peptide hormones typically bind to receptors located on the cell membrane, as these hormones are often water-soluble and cannot easily penetrate the cell.13
* Intracellular Receptors: Steroid and thyroid hormones, which can cross the cell membrane, bind to receptors located within the cytoplasm or the nucleus of the target cell.13

Once the hormone-receptor complex is formed, it triggers a chain of biochemical responses—such as enzyme activation or the regulation of gene expression—that results in the hormone's specific physiological action. Cells that lack the appropriate receptor remain unaffected by the hormone.13