Background: \- Thyroid hormone is produced by the thyroid gland, an organ at the base of the neck. Thyroid hormone controls the body's metabolism and the function of many organs. The thyroid gland produces two forms of thyroid hormone: T4 and T3. People who have thyroid cancer are treated with thyroid hormone therapy (synthetic T4, levothyroxine), which at times needs to be stopped to allow for cancer treatments. At these times, a different form of thyroid hormone (synthetic T3, liothyronine) is used to reduce the symptoms caused by low levels of thyroid hormone. Researchers want to know more about how changes in T3 hormone affect the body and organ function. Objectives: \- To study how changes in T3 hormone levels affect the body and organ function. Eligibility: \- Individuals at least 18 years of age who have had most or all of their thyroid removed to treat thyroid cancer who need to stop taking their regular thyroid hormone dose in preparation for the treatment of thyroid cancer. Design: * The study involves a screening visit and a baseline evaluation. It also includes an 11-day inpatient hospital stay. * Participants will be screened with a physical exam and medical history. They will also have blood tests and a neck ultrasound. * Participants will be evaluated with a physical exam, blood tests, and the following procedures: * Glucose tolerance test to measure blood sugar * Tests of body fat, muscle strength, and calorie burning levels * Imaging studies of the heart, liver, and thigh muscles * Quality of life questionnaires * Food preference and diet questionnaires * After 4 weeks of treatment with T3 hormone, participants will have an 11-day inpatient hospital stay to study the effect of thyroid hormone on their metabolism. The stay will involve the same tests done in the baseline evaluation.
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1. To assess the pharmacokinetics of T3 in patients devoid of endogenous or exogenous T4 during acute L-T3 withdrawal at steady- state and after the first L-T3 dose administration
Timeframe: 4 weeks after stopping L-T4 thyroid hormone replacement therapy and starting L-T3 treatment
2. To estimate the minimal duration of L-T3 therapy withdrawal required to achieve a serum TSH level, equal or greater than 30 uIU/mL, to assure effective 131I therapy for treatment of differentiated thyroid cancer
Timeframe: 4 weeks after stopping L-T4 thyroid hormone replacement therapy and starting L-T3 treatment
3. To correlate clinical and biochemical parameters of thyroid hormone action, with circulating levels of T3 during L-T3 therapy withdrawal
Timeframe: 4 weeks after stopping L-T4 thyroid hormone replacement therapy and starting L-T3 treatment