Subclinical hypothyroidism and hyperthyroidism consist of mild disorders of thyroid dysfunction with an abnormal TSH yet normal T4 levels. Previous studies disagree on the impact of these disorders on human physiology (metabolism, cardiac function, exercise tolerance). We are examining the effects of subclinical thyroid disease on these parameters in a rigorous fashion, before and after treatment to normalize the TSH. To date, we have enrolled 5 subjects prior to treatment. We will study 10 subjects in each category (overt hypothyroidism and hyperthyroidism, subclinical hypothyroidism and hyperthyroidism) before and after treatment as well as 10 normal controls. We will also evaluate the tissue-specific effects of the thyroid hormone analog, triac, as replacement therapy for overt hypothyroidism. This analog is believed to strongly influence pituitary and liver function, with less profound effects on cardiac and metabolic function.