By hypothyroidism, the thyroid gland is producing too less hormones to stimulate the metabolism, or the body is not able to utilize the hormones. The lack of thyroid hormones slows down the metabolism and thus all the activities in the body, giving a combination of many symptoms related to slowness of bodily processes. Hypothyroidism is common, but the frequency of the condition is not well determined. Some authorities estimate that 0.5% of the total American population have the disease to some degree. The frequency is much greater among people over 50 years of age than among young people.

By hypothyroidism the body does not get enough thyroid hormone, or the hormones do not work effectively in the body. This causes the metabolism to slow down. When the metabolism decreases, the processes in the body do not get enough fuel and building materials, and all the body activities will therefore slow down. Energy containing nutrient will also be stored as fat, since they are not broken down. Serious variants of hypothyroidism are called myxedema. This is a rare condition. However, less serious, but painful variants are common.