A PROSPECTIVE ANALYSIS OF SUBACUTE THYROID-DYSFUNCTION AFTER NECK IRRADIATION

Purpose: Exposure of the thyroid to therapeutic doses of external irra diation has been demonstrated to induce thyroid dysfunction. This stud y was designed to assess the relationship between irradiation and earl y thyroid dysfunction, prospectively. Methods and Materials: Twenty pa tients in whom the thyroid was incidentally exposed to therapeutic dos es of irradiation were studied. The dose given to the thyroid was 40-5 4 Gy over 4-7 weeks. Thyroid function tests, including serum thyroid s timulating hormone (TSH), free thyroxine (free T4), free triiodothyron ine (free T3), antithyroglobulin antibody, and antimicrosomal antibody , were performed prior to irradiation and at 3, 6, and 12 months after radiotherapy. Results: Serum TSH levels did not change significantly at 3 months after irradiation (mean TSH level: 1.33 mu U/ml before irr adiation, 1.74 mu U/ml at 3 months, p = 0.11). However, a significant elevation was noted at 6 months (mean TSH: 3.50 mu U/ml at 6 months, p = 0.0001, vs. preirradiation), when TSH levels were higher than preir radiation levels in 19 of 20 patients. After irradiation, 13 patients remained in a euthyroid state (euthyroid group), while in the other 7 patients hypothyroidism occurred (hypothyroid group) and thyroid hormo ne-replacement therapy was performed. After 6 months, elevation of TSH was less significant in the euthyroid group, whereas elevation of TSH persisted continuously and exponentially in the hypothyroid group. Th yroid autoantibodies did not turn positive in any patient during follo w-up. Conclusions: Damage of the thyroid develops in most patients whe n the organ is exposed to radiation. This radiation-induced damage is initially manifested within 6 months after irradiation.