On the basis of their chemical nature, hormones can be divided into groups:
(i) Peptide, polypeptide, protein hormones (e.g., insulin, glucagon, pituitary hormones, hypothalamic hormones, etc.)
(ii) Steroids (e.g., cortisol, testosterone, estradiol and progesterone)
(iii) Iodothyronines (thyroid hormones)
(iv) Amino-acid derivatives (e.g., epinephrine).

Hormones which interact with membrane-bound receptors normally do not enter the target cell, but generate secondary messengers (e.g., cyclic AMP, IP₃, Ca⁺², etc.) which in turn regulate cellular metabolism.
Hormones which interact with intracellular receptors (e.g., steroid hormones, iodothyronines, etc.) mostly regulate gene expression or chromosome function by the interaction of hormone-receptor complex with the genome. Cumulative biochemical actions result in physiological and developmental effects.