[en] Purpose: Exposure of the thyroid to therapeutic doses of external irradiation has been demonstrated to induce thyroid dysfunction. This study was designed to assess the relationship between irradiation and early thyroid dysfunction, prospectively. Methods and Materials: Twenty patients in whom the thyroid was incidentally exposed to therapeutic doses of irradiation were studied. The dose given to the thyroid was 40-54 Gy over 4-7 weeks. Thyroid function tests, including serum thyroid stimulating hormone (TSH), free thyroxine (free T4), free triiodothyronine (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 μU/ml before irradiation, 1.74 μU/ml at 3 months, p = 0.11). However, a significant elevation was noted at 6 months (mean TSH: 3.50 μU/ml at 6 months, p = 0.0001, vs. preirradiation), when TSH levels were higher than preirradiation 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 hormone-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. Thyroid autoantibodies did not turn positive in any patient during follow-up. Conclusions: Damage of the thyroid develops in most patients when the organ is exposed to radiation. This radiation-induced damage is initially manifested within 6 months after irradiation