Leptin is a cell signaling molecule, or hormone, and in humans the leptin molecule is composed of 167 amino acids. It was discovered in 1994. The word leptin is from the Greek word leptos, which means ‘thin’.  It is often referred to as the ‘satiety’ hormone because appetite, cravings, and metabolic functions are directly influenced by leptin. Leptin also has numerous other signaling influences in the body such as giving a positive influence to thyroid function and another is to reduce anxiety, which we know can also make us want to eat more.