Leptin is a hormone produced by adipose (fat) cells that signals the hypothalamus to suppress appetite and increase energy expenditure when fat stores are adequate. In this way, leptin acts as a long-term regulator of energy balance and body weight — sometimes called the satiety hormone.

In obesity, leptin levels are often very high — yet appetite is not suppressed. This occurs because of leptin resistance, where the brain no longer responds normally to leptin signalling, similar to how insulin resistance develops. Leptin testing helps quantify fat mass signalling and can reveal leptin deficiency (very rare, causes severe childhood obesity) or leptin resistance (common in metabolic syndrome). It is also used in research into metabolic health, fertility, and immune function.