Radiation and thyroid disease.

External irradiation of the thyroid causes the subsequent development of thyroid malignancies at an approximate rate of 5 per rad per million people per year. There is no certain lower threshold, but possibly doses above 3000 rads are less effective. The tumours produced are primarily differentiated malignancies typical of non-irradiated patients, and the clinical course is similar. Most radiation-associated malignancies followed treatment given for enlarged thymic glands, enlargement of the tonsils or adenoids, and acne. Currently patients are seen with radiation-associated thyroid disease following treatment of malignant tumours in the neck. Diagnostic manoeuvres are similar to those used in non-irradiated individuals, but multinodularity is probably an indication for operation. Because of the high incidence of multicentricity, we prefer near-total thyroidectomy if cancer is found. Prophylactic thyroid hormone administration is of uncertain value in preventing the development of tumours. X-ray to the thyroid also can induce other histologic abnormalities, including a high incidence of adenomas and hypothyroidism, and is associated with an increased incidence of autoimmune thyroid disease and possibly the development of exophthalmos and Graves' disease. In contrast to the obvious effects of external irradiation, 131I diagnostic tests and treatments have not been associated with a clearly increased incidence of thyroid malignancy.