A protein hormone, comprising 167 amino acids in humans, that is secreted by adipose tissue; it suppresses appetite and regulates adipose tissue mass and energy balance. It acts on leptin receptors in the hypothalamus and inhibits expression of neuropeptide Y, thereby countering the appetite-stimulating effects of ghrelin and inhibiting food intake. It also promotes synthesis of the appetite suppressant melanocyte-stimulating hormone. Deficiency of leptin or its receptors leads to severe obesity.