Leptin is the hormone voice of body fat. Made by adipose tissue and read by the hypothalamus, it reports energy stores and helps the brain coordinate appetite, metabolic rate, fertility, thyroid tone, immune activity, and bone building. It’s a systems signal, not just a “fullness” switch.Reference intervals vary by lab, sex, and body fat. Levels rise with total fat mass and are naturally higher in women than men, increase with puberty, and are elevated in pregnancy due to placental production. For a given body composition, values that fall in the middle of the expected range tend to reflect a clear, effective signal.When leptin is lower than expected for someone’s fat mass, the brain reads scarcity. Hunger rises, resting metabolism and body temperature drift down, and fatigue and cold intolerance are common. The thyroid axis shifts toward lower T3, and the reproductive axis slows—missed periods or infertility in women; low libido and reduced sperm parameters in men; delayed puberty in teens. Bone formation falls, raising stress‑fracture risk, and immune defenses can be muted. Rare genetic deficiency causes profound hyperphagia and early‑onset obesity despite very low levels.When leptin is high—most often with increased fat mass—the signal can be ignored centrally (leptin resistance). Satiety is blunted, sympathetic tone and blood pressure may climb, inflammation increases, and insulin resistance, fatty liver, and cardiovascular risk rise. High levels in pregnancy are physiologic.Big picture: leptin links fat stores to the brain and, through neuroendocrine loops, to metabolism, reproduction, immunity, and bone. Persistently distorted signaling—too little or resistantly high—tracks with risks such as infertility, osteoporosis, type 2 diabetes, fatty liver, and heart disease.
