Giving leptin directly to the brain of mice or rats tells their fat cells to produce more of a specific enzyme that converts thyroid hormones. This shows that signals from the brain can directly control how fat tissue processes hormones.

Giving leptin to rodents boosts the production of a specific protein in their fat tissue that helps generate heat. This happens because the nervous system acts like a messenger, carrying leptin's signals from the brain to the body's tissues to control how they burn energy and convert thyroid hormones.

Giving leptin directly to the brain causes fat tissue to produce more of an enzyme that converts inactive thyroid hormone into the active form, raising active thyroid levels in the body. This shows that fat tissue isn't just passive storage but actively helps control your thyroid hormone levels based on signals from your brain.

Blocking a specific enzyme called D2 stops the hormone leptin from changing thyroid hormone levels in the body. This shows that the D2 enzyme is absolutely necessary for leptin to do its job in regulating thyroid function.

In Chinese adults, the gene that makes the hunger-controlling hormone leptin is less active in fat stored around the organs inside the belly compared to fat stored under the skin, whether someone is lean or obese.