Use of Adjunctive Potassium Iodide after Radioactive Iodine (131I) Treatment of Graves' Hyperthyroidism*

Abstract

One hundred and nineteen patients with Graves' hyperthyroidism who were treated with 131I alone or 131I followed by potassium iodide (131I + KI) were studied retrospectively. Patients in both groups who required only a single dose of 131I for successful treatment of hyperthyroidism had similar age, gland size, 24-h radioactive iodine uptake, pretreatment serum T4 concentrations, and radioactive iodine treatment dose. Seven weeks after 131I, mean serum T4 concentrations were 12.3 +/- 6.1 micrograms/dl (mean +/- SD) in patients who received 131I alone and 8.0 +/- 3.9 micrograms/dl in patients who received 131I + KI (p less than 0.001). Sixty percent of the patients who received 131I + KI and remained euthyroid 1 yr after 131I treatment developed documented transient hypothyroidism while receiving KI (serum T4, 1.4 +/- 0.9 micrograms/dl). Patients with transient hypothyroidism receiving KI had larger estimated thyroid gland weights when hypothyroid than patients whose hypothyroidism was permanent (32 +/- 6 vs. 16 +/- 11 g; P less than 0.001). The overall incidence of hypothyroidism 1 yr after treatment with 131I was 58% in each of the two groups. Sixteen percent of each group were not successfully treated by a single dose of 131I and required further therapy. Adjunctive KI effectively treated thyrotoxicosis more rapidly than 131I alone without adversely affecting outcome at 1 yr; however, patients taking KI more often develop transient hypothyroidism.