Based on their chemical nature, hormones can be divided into the following groups: (i) Peptide, polypeptide, protein hormones: E.g., insulin, glucagon, pituitary hormones, hypothalamic hormones, etc. (ii) Steroids: E.g., cortisol, testosterone, estradiol, aldosterone and progesterone. (iii) Iodothyronines (thyroid hormones) (iv) Amino-acid derivatives: E.g., epinephrine (adrenaline), nor-epinephrine (nor-adrenaline).
Steroid hormones and iodothyronine are hydrophobic hormones that interact with intracellular receptors and mostly regulate gene expression or chromosome function by the interaction of the hormone-receptor complex (HRC) with the genome.