PYY: A hormone concerned with hunger and the lack of hunger (satiety). Soon after eating, PYY is secreted into the blood by cells lining the lower small intestine (the ileum) and the colon. The release of PYY begins before nutrients arrive in the lower small intestine and the colon. The further release of PYY is stimulated by nutrients within the lower small intestine and the colon. PYY decreases food intake by inhibiting gut motility. It acts as an "ileal brake" to cause a sense of satiety. PYY is a peptide.