Approximately 1 percent of patients who have radioactive iodine therapy for hyperthyroidism develop radiation thyroiditis between five and 10 days after the procedure. The rapid destruction of the thyroid parenchyma results in pain, tenderness, and an exacerbation of hyperthyroidism from the release of stored T4 and T3. A brief course of NSAIDs or, rarely, prednisone in dosages of 40 to 60 mg per day may be used to alleviate pain; a beta blocker often is required to block the peripheral effects of the thyroid hormone. The gland eventually undergoes extensive fibrosis in approximately six to 18 weeks.5
Thyroiditis also may develop from radiation therapy for lymphoma or head and neck cancers. The major risk factors for developing thyroid damage after external irradiation are high-dose irradiation, younger age, female sex, and preexisting hypothyroidism.6,7
Refs HERE