Patients with hypothyroidism appear to be sensitive to digoxin while patients with hyperthyroidism appear to be somewhat resistant to the effects of the drug. This study was undertaken to compare blood levels and excretion of digoxin of patients with thyroid disease to patients with normal thyroid function. Eleven patients with hyperthyroidism and 7 patients with hypothyroidism, as determined by clinical symptoms, I131 uptake, and PBI, were given 1.0 mg of tritiated digoxin intravenously to detect any difference in serum concentrations and excretion when compared to a group of 13 euthyroid patients with congestive heart failure. Frequent serum specimens, urine, and