Chemical signaling molecules that function , that is, following entrance into the .

Hormones can be differentiated into a number of categories. These include those that received by target cells at the cell surface versus . Another distinction is in terms of how they are found in the blood, which can either be as free (unbound) molecules or instead with other molecules.

Lastly, there exist hormones that are based (), hormones that are derived from which otherwise is an amino acid (), and the which, not surprisingly, are steroid based.

Steroid hormones travel in blood in a bound form and also are received by target cells by internal receptors. Peptide hormones, by contrast, travel in an unbound form and are received by receptor proteins found on cell surfaces. Amine hormones are also received on cell surfaces but, depending on the hormone, may travel in a bound or unbound form.

Hormones also differ in terms of what cells produce them, under what physiological conditions they are produced, what cells receive them, how those cells respond to them, and what are the larger physiological consequences of hormone release and reception. See also as well as pheromone.