What it means
Fat cells release leptin roughly in proportion to their size, so leptin levels rise as fat stores grow. Leptin acts on the hypothalamus, a brain area that controls hunger, to signal fullness and allow normal energy use.
People born without working leptin, a very rare genetic condition, have extreme hunger and severe early obesity. That shows how important the signal is.
Most people with obesity have high leptin levels, not low ones, but the brain responds less strongly to the signal. Researchers call this leptin resistance, and they are still working out how it happens.
When people lose weight, leptin falls, sometimes faster than fat mass. The brain can read that drop as a sign of scarcity, raising hunger and lowering calorie burn. This is one reason weight regain is common after diets end. Leptin tests are not part of routine weight-loss care.
Why it matters
Falling leptin after weight loss is part of the biology that makes maintenance hard after stopping a GLP-1.