Based on the chemical nature, hormones are divided into protein hormones, steroids, iodothyronines and amino-acid derivatives. Protein hormones include insulin, glucagon, pituitary hormones and hypothalamic hormones; steroid hormones include cortisol, testosterone, estradiol and progesterone; iodothyronines include thyroid hormones whereas amino-acid derivatives include epinephrine . Hormones act on target organs by binding to hormone receptors located on the target cells.
To enter the target cells, hormones need to bind to the extracellular receptors present on the plasma membrane or intracellular receptors present within the cell. Non-steroid hormones such as protein and amino-acid derivative hormones are not lipid-soluble and so unable to enter the target cells through the plasma membrane. Therefore, such hormones bind to extracellular receptors, forming a hormone-receptor complex. This complex in turn generates second messengers such as cyclic adenosine monophosphate, inositol triphosphate and calcium ions. These messengers bring about biochemical changes in the target tissues and regulate cellular metabolism.
Steroid hormones such as steroids and thyroid hormones are lipid-soluble. Therefore, they can enter the target cells through the plasma membrane by binding to intracellular receptors in the cytoplasm or the nucleus to form a hormone-receptor complex. This hormone-receptor complex binds to a specific part of the DNA and regulates the synthesis of proteins. These biochemical actions result in an overall physiological change in the target cell.