If it's underactive, you may have a condition called hypothyroidism. It can lead to weight gain from a buildup of salt and water in your body. An overactive thyroid is called hyperthyroidism. Many people with it lose weight, but others pick up extra pounds because it can make you feel hungrier. How your thyroid affects your metabolism, energy, and weight is complicated. Other hormones, proteins, and chemicals may also play a role. Check with your doctor if you think it's a problem.