The thyroid gland's role in the body is very similar to cruise control in the car. Cruise control keeps a car running at a constant steady speed. When we do not want to worry about having to keep a steady foot on the accelerator we turn on cruise control and the car maintains a normal speed without any effort. Thyroid hormone keeps the rest of the body working at the right speed. If thyroid hormone levels decrease, cells throughout the rest of the body decrease in activity. As a result the cells need less energy and thus more energy is available to be stored and the weight increases even though the appetite decreases.