Many hormones are peptides. The anterior and posterior pituitary glands produce many peptide hormones. The anterior pituitary gland secretes luteinizing hormone, follicle stimulating hormone, prolactin, adrenocorticotrophic hormone (ACTH), and growth hormone. The posterior pituitary gland secretes antidiuretic hormone and oxytocin. Additional peptide hormones include atrial-natriuretic peptide (ANP), produced by the heart; insulin and somatostatin, secreted by the pancrease; and cholecystokinin and gastrin secreted by the the gastrointestinal tract.

A crucial distinction exists between peptide hormones and steroid hormones. Peptide hormones, being polar, cannot cross the cell membrane, but steroid hormones can. This is a crucially important distinction to keep in mind for the MCAT. A peptide hormone must bind to receptors on the surface of the cell triggering a second messenger to be released in the cytoplasm.