Changes in exercise tolerance, metabolic rate, cardiac function and laboratory tests are fairly well understood in patients with severe hypothyroidism and hyperthyroidism, but these changes are less well defined in patients with subclinical disease (abnormal TSH and normal thyroid hormone levels). This study is designed to analyze the above effects on patients before and after treatment for subclinical thyroid disease, using patients with overt disease as controls. Furthermore, a thyroid hormone analog, triac, with tissue specific effects will be tested as replacement therapy in subjects with overt hypothyroidism to better define the tissue-specific effects of this compound.