Which hormone is NOT classified as a polypeptide hormone?

Epinephrine is the hormone that is not classified as a polypeptide hormone because it is a catecholamine, which is derived from the amino acid tyrosine. Unlike polypeptide hormones, which are made up of chains of amino acids and typically have a more complex structure, epinephrine is classified as an amine hormone due to its structure and synthetic pathway.

Polypeptide hormones, such as insulin, vasopressin, and prolactin, are all proteins or peptides that exert their effects through receptor binding and subsequent signaling pathways. Insulin, for instance, consists of two polypeptide chains linked by disulfide bonds and plays a crucial role in glucose metabolism. Vasopressin, also known as anti-diuretic hormone (ADH), is a nine-amino-acid peptide involved in regulating water balance. Prolactin, a peptide composed of 198 amino acids, is important for lactation.

These distinctions highlight why epinephrine stands out as not being a polypeptide hormone, while the others are defined by their polypeptide structure and functions.