Hypothyroidism is a condition in which the body lacks sufficient thyroid hormone. Since the main purpose of thyroid hormone is to "run the body's metabolism," it is understandable that people with this condition will have symptoms associated with a slow metabolism. The estimates vary, but approximately 10 million Americans have this common medical condition. In fact, as many as 10% of women may have some degree of thyroid hormone deficiency. Hypothyroidism is more common than you would believe, and millions of people are currently hypothyroid and don't know it.