Thyroid Hormone to Induce Non-Insulin Mediated Glucose Disposal in People With Insulin Receptor Mutations

Background: \- Insulin receptor mutation causes high blood sugars and sometimes diabetes complications. Researchers want to see if thyroid hormone helps. Objectives: \- To see if thyroid hormone treatment changes how the body handles sugar in people with insulin receptor mutation and improves blood sugar in people with diabetes. Eligibility: \- People ages 12 65 with an insulin receptor mutation. Design: * Study part 1:19-day clinic stay. Participants will be monitored for 4 days. Then for 15 days they will take a thyroid hormone pill 3 times a day. Participants will have: * Blood tests. * Heart rate and skin temperature monitored. * All their food provided. * Two 5-hour sessions in a special room. They will wear special clothes and sometimes sit still. * Two small tubes inserted in veins. One will deliver tiny amounts of sugar and fat with a non-radioactive tracer. Participants will also drink water with a tracer. The other tube will collect blood. * A sweet drink. Participants may have finger stick blood sugar tests. * Glucose-monitoring device inserted into body fat for two 24-hour periods. * Adults may have samples of fat and muscle taken. * Heart ultrasound. * PET-CT scan in a machine. An intravenous catheter will be placed in an arm vein. A small amount of radioactive substance will be injected. * DEXA scan of body fat and bone density. * Participants with poorly controlled diabetes will then take thyroid hormone at home for 6 months. They will have blood drawn and sent to the study team monthly. * After about 3 months, they will have an overnight visit. After 6 months, they will have a 4-day visit.