Effects of Thyroid Hormone Enzyme Blocking on Hypothyroidism

Background: \- Hypothyroidism is a condition caused by the loss of function of the thyroid gland. The thyroid gland produces two hormones, T4 and T3. These hormones control the metabolism and function of many organs. Lack of energy, depression, and constipation are common symptoms of hypothyroidism. T4 is converted into T3, the active form of thyroid hormone, by two enzymes called deiodinases. People with hypothyroidism are treated with a synthetic T4 hormone, which the enzymes convert to T3. This treatment is usually effective, but some people continue to have symptoms even after treatment. Some researchers think that this may be caused by a problem with the enzymes that convert T4 into T3. They want to look at how the enzymes regulate the levels of T4 and T3 in the blood. They will do so by using a drug that blocks the action of one of the two enzymes. Objectives: \- To look at how thyroid hormone enzyme blocking affects hypothyroidism treatment medication. Eligibility: \- Individuals at least 18 years of age who have hypothyroidism and are on thyroid hormone replacement therapy. Design: * The study consists of one screening visit, 9 days of inpatient hospital admission, and a follow-up visit 2 weeks after discharge. * Participants will be screened with a physical exam and medical history. They will provide blood samples. * Participants will receive balanced-diet meals to take home for the 2 days before they enter the hospital. They will continue this diet while in the hospital. * During the inpatient stay, participants will be monitored with regular blood tests. They will have the following procedures: * Continued thyroid hormone replacement for all 9 days. * Drug to block thyroid enzyme for 7 days. * Metabolism test, with room temperature changes, on days 1, 2, 5, 8, and 9. * Measurements of body fat on days 2, 5, and 8. * Blood glucose tests on days 1 and 9. * Muscle contraction tests on days 1, 2, 4, 5, 8, and 9. * Heart imaging studies on days 2, 5, an