peptide hormone

Meaning: A peptide hormone is made of amino acids, from short chains (oxytocin, ADH) to full proteins (insulin). It is made on ribosomes, often as an inactive precursor that is cleaved, stored in vesicles, and released by exocytosis. Being polar, it dissolves in plasma, usually needs no carrier, and has a short half-life. It cannot easily cross the lipid bilayer, so it binds a surface receptor (G protein-coupled or receptor tyrosine kinase). This triggers second messengers such as cAMP or Ca²⁺, or a phosphorylation cascade, that alters existing proteins.

On the MCAT: Passages contrast peptide and steroid hormones: surface versus intracellular receptor, faster versus slower onset, free versus carrier-bound transport. A question may ask what happens if adenylyl cyclase is inhibited or the receptor is absent. In that case, predict that the hormone's response is lost even though the hormone level is normal.

Common confusion: Not every hormone derived from amino acids acts like a peptide hormone. Thyroid hormone is made from tyrosine but is lipophilic and acts through intracellular receptors, much like steroids. Polarity, not amino acid origin, decides where the receptor sits.