The thyroid is a gland in the neck that produces hormones that help to regulate metabolism. Metabolism refers to the body reactions that control how the body uses energy. Hypothyroidism is a condition in which the thyroid makes too little thyroid hormone and body functions slow down. Symptoms of hypothyroidism include tiredness, feeling cold, constipation, hoarse voice, changes in hair and skin, heavy menstrual periods, and weight gain. Thyroid disease is usually easily treated with a drug that contains a synthetic version of the thyroid hormone thyroxine. However, a normal thyroid gland produces both thyroxine and triiodothyronine. Some doctors believe that it is better to treat hypothyroidism using a combination of drugs containing synthetic versions of both hormones than with those containing synthetic thyroxine alone. Previous studies of this issue have had conflicting results, perhaps because they did not always use combinations of synthetic hormones that matched the amounts of hormones produced by a normal thyroid gland.