The major groups of hormones are amine hormones, peptide and protein hormones, and steroid hormones. Amine hormones are relatively small molecules that are structurally similar to amino acids. Epinephrine and norepinephrine, serotonin, dopamine, the thyroid hormones, and melatonin are examples of amine hormones.

Peptide hormones and protein hormones are chains of amino acids. The peptide hormones have 3 to 49 amino acids, while the protein hormones are larger with chains of 50 to 200 or more amino acids. Examples of peptide hormones are antidiuretic hormone and oxytocin. The larger thyroid-stimulating hormone and follicle-stimulating hormone are examples of protein hormones.