Peptide hormones are a class of peptides that are secreted into the blood stream and have endocrine functions in living animals. Like other proteins, peptide hormones are synthesized from amino acids according to an mRNA template, which is itself synthesized from a DNA template inside the cell's nucleus. Peptide hormone precursors (pre-prohormones) are then processed in several stages, typically in the endoplasmic reticulum, including removal of the N-terminal signal sequence and sometimes glycosylation, resulting in prohormones.

These prohormones often contain superfluous amino acid residues that were needed to direct folding of the hormone molecule into its active configuration but have no function once the hormone folds. Specific endopeptidases in the cell cleave the prohormone just before it is released into the blood stream, generating the mature hormone form of the molecule. Mature peptide hormones then diffuse through the blood to all of the cells of the body, where they interact with specific receptors on the surface of their target cells.

Notable peptide hormones

• atrial-natriuretic peptide (ANP) or atrial natriuretic factor (ANF)
• antidiuretic hormone, also called vasopressin
• growth hormone
• insulin
• oxytocin
• somatostatin

Many neurotransmitters are secreted and released in a similar fashion to peptide hormones, and some peptides may be used as neurotransmitters in the nervous system in addition to acting as hormones when released into the blood.