Leptin is a peptide hormone produced by adipose tissue. The more fat mass, the more leptin.
Mechanism: Signals to the hypothalamus that fat stores are sufficient, suppressing hunger and increasing energy expenditure. Most obesity is characterized by leptin resistance, not deficiency. The receptor stops responding to high circulating levels.
Lab range: 5–20 ng/mL men; 10–35 women; varies with body fat percentage.
Caveats: High leptin with high body fat = leptin resistance. Drops dramatically during caloric deficit, driving rebound hunger and the metabolic adaptation that resists weight loss.