Thyroid-hormone concentrations after radioiodine therapy for hyperthyroidism.

Fourteen hyperthyroid patients (11 men, three women), ages 28-66, were followed with serial measurements of serum thyroid hormone levels for 1 mo after therapy with I-131. Twelve patients had diffuse toxic goiters (25--70 g in size); two patients had multinodular glands (40--100 g). The patients were taking no antithyroid medications; ten patients were treated with propranolol. All patients received the equivalent of 5000 rad, except the two with multinodular glands, who received larger doses. There was no consistent pattern of serum T4 and T3 levels after the I-131 therapy. For the entire group, there was no significant increase of the mean serum hormone concentration. One group (three patients) had a mean T4 increase of 28% and a T3 increase of 91% above baseline at Days 10--11. Seven patients had minimal increases of hormone levels at Days 2--3, and a third group (four patients) had no increase of thyroid hormones after I-131 therapy. The patients with no rise in hormone concentrations had smaller goiters than the other groups. There was no correlation of the dose of radioactive iodine, or of the initial hormone concentration, with the rises or declines of T4 and T3 levels after I-131 therapy. Radioiodine therapy caused no significant increase of serum T4 and T3 concentrations in the majority of patients.