Internal irradiation (e.g., ¹³¹I or ¹²⁵I) for the treatment of hyper-thyroidism or thyroid cancer can induce a radiation thyroiditis that may abruptly present clinically as pain and tenderness in the thyroid, swelling in the neck, and transient thyrotoxicosis. Indeed, thyroid storm occasionally has occurred after ¹³¹I treatment of thyrotoxicosis.71 These acute changes usually occur within 1 week of the RAI treatment and subside within
3 to 4 weeks.70 As a consequence of ¹³¹I or ¹²⁵I therapy for hyperthyroidism, the most common long-term clinical abnormality relates to the development of hypothyroidism. Therefore, after RAI therapy, patients need to be monitored indefinitely for the occurrence of hypothyroidism. However, there does not appear to be any increased incidence of thyroid cancer in patients receiving RAI therapeutically. This absence of an increased incidence of thyroid cancer after ¹³¹I or ¹²⁵I therapy or high-dose external irradiation for head and neck cancer may relate to the fact that very high levels of intrathyroidal radiation are achieved, thereby destroying thyroid cells and obviating the potential for the later development of thyroid cell carcinoma.