Your thyroid regulates your metabolism. It does that by making hormones that affect almost every aspect of how your body performs: how fast or slow you burn calories, when or whether you build proteins or store energy as fat, and how your body responds to other hormones.
Not surprisingly, disorders of the thyroid can cause problems for your metabolism. An underactive thyroid ("hypothyroidism") produces fewer-than-normal thyroid hormones, causing a slower metabolism, and, consequently often causes weight gain. An overactive thyroid ("hyperthyroidism"), on the other hand, can cause the metabolism to speed up, often resulting in weight loss.