THYROID ABNORMALITIES AFTER THERAPEUTIC EXTERNAL RADIATION

The thyroid gland is the largest pure endocrine gland in the body and one of the organs most likely to produce clinically significant abnorm alities after therapeutic external radiation, Radiation doses to the t hyroid that exceed approximately 26 Gy frequently produce hypothyroidi sm, which may be clinically overt or subclinical, as manifested by inc reased serum thyrotropin and normal serum-free thyroxine concentration s. Pituitary or hypothalamic hypothyroidism may arise when the pituita ry region receives doses exceeding 50 Gy with conventional, 1.8-2 Gy f ractionation. Direct irradiation of the thyroid may increase the risk of Graves' disease or euthyroid Graves' ophthalmopathy, Silent thyroid itis, cystic degeneration, benign adenoma, and thyroid cancer have bee n observed after therapeutically relevant doses of external radiation. Direct or incidental thyroid irradiation increases the risk for well- differentiated, papillary, and follicular thyroid cancer from 15- to 5 3-fold. Thyroid cancer risk is highest following radiation at a young age, decreases with increasing age at treatment, and increases with fo llow-up duration. The potentially prolonged latent period between radi ation exposure and the development of thyroid dysfunction, thyroid nod ularity, and thyroid cancer means that individuals who have received n eck or pituitary irradiation require careful, periodic clinical and la boratory evaluation to avoid excess morbidity.