Classically, a hormone is defined as a chemical substance secreted into the bloodstream that acts on distant tissues, usually in a regulatory way. Hormones act as messengers and regulators, and they can signal in three main ways:
Most endocrine hormones are peptides, with the following exceptions:
| Type | Hormone |
| Amines | T3 and T4, dopamine, NE and epinephrine |
| Steroids | Cortisol, aldosterone, DHEA, androstenedione, testosterone, estradiol, progesterone, Vit D |