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.

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.

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.

When your body is under long-term stress, trauma, or constant inflammation, it produces more of an enzyme that converts your main thyroid hormone into an inactive form. This leaves your cells starved of active thyroid hormone even though standard blood tests look normal, which can lead to brain fog, low mood, and cognitive problems.