Purpose: Radiation-induced thyroid dysfunction is considered a late effect. We prospectively assessed acute reactions of the thyroid to external neck irradiation.
Methods and materials: This study included 22 patients in whom the thyroid was incidentally exposed to therapeutic doses of radiation. Thyroid function tests included measurements of serum thyroid stimulating hormone (TSH), free and total triiodothyronine (T3) and thyroxine (T4), thyroglobulin, and antithyroid antibodies. These tests were performed before radiotherapy (baseline values), after approximately 40 Gy had been administrated, 2 weeks after the end of radiotherapy, and 3 and 6 months after the beginning of radiotherapy.
Results: Mean serum levels of TSH were 1.53, 0.55, 0.78, 2.14, and 7.57 microU/ml before radiotherapy, after 40 Gy irradiation, 2 weeks after the end of radiotherapy, and 3 and 6 months after radiotherapy, respectively. Thus, levels of TSH exhibited two phases: a significant decrease during radiotherapy (thyrotoxic phase) and an increase after radiotherapy (hypothyroid phase) (baseline vs. 40 Gy: p < 0.0001, baseline vs. 6 months: p = 0.003). Increases of thyroid hormones were subtle during radiotherapy.
Conclusions: We believe that radiation promotes release of excessive amounts of thyroid hormones during radiotherapy owing to suppression of TSH secretion. In addition to the late damage (hypothyroidism), thyrotoxicosis occurs when the thyroid gland receives a therapeutic doses of external radiation.