Patients with thyroid function abnormalities (hyperthyroidism, hypothyroidism) are studied and treated in this protocol. A limited number of patients with a personal history of signs, symptoms or laboratory studies consistent with thyroid dysfunction or a strong family history of thyroid abnormalities are also enrolled. Patients undergo routine history and physical examination, standard endocrine blood and urine tests, a standard TRH test, thyroid nuclear medicine scans, thyroidal radioiodine (RAI) or technetium (99mTc) uptake measurements, as well as X-ray, computed tomography (CT) or magnetic resonance imaging (MRI) studies or other standard diagnostic procedures, as clinically indicated. Patients will be screened for entry into other research studies.