In a double-blind study, 114 patients with clinically benign nontoxic goiter were treated either with liothyronine (T3), 50 μg/day, or thyroxine (T4), 200 μg/day. After 12 weeks of therapy, patients whose goiters decreased in size were continued on the same therapy for an additional 16 weeks. Those who did not respond were randomly divided into two groups: in one group the same dose of the same medication was continued, and in the other twice the dose of their original medication was given. By the end of 28 weeks, 40 of 54 patients treated with T3 and 29 of 59 patients treated with T4 showed a significant decrease in goiter size. The difference in effectiveness of the two agents was statistically significant (p T3 produced a uniform and consistent depression of radioiodine uptake and circulating hormonal iodine levels, and was effective shrinking both small and large thyroid nodules, whether or not radioiodine uptake was reduced to hypothyroid levels. T4 uniformly increased circulating hormonal iodine levels. However, in T4-treated patients who responded with regression of thyroid nodules, suppression of radioiodine uptake was substantially greater than in those who did not respond.