Radioactive iodine therapy in thyroid disorders.

Radioactive iodine (1311) has a part to play in the therapy of two thyroid disorders, carcinoma and thyrotoxicosis, by virtue of the selective concentration of it which occurs in functioning thyroid cells. The latter represent the only significant site of concentration of the iodide ion in the body, this occurring because of the necessity to incorporate iodine into the molecule of the thyroid hormone thyroxine. "` If, therefore, the gland is" presented with 1311, a source of ionizing radiation is placed within the thyroid gland, and the physical properties -of 131I, the commonly used isotope, are such that there is little tissue irradiation except within the gland. The greater part of the biological effect of the radiation'is due to the beta particles emitted by the source, and as these have only a range of a few millimetres in tissue, there is little appreciable effect-elsewhere, unless, as in the treatment of thyroid carcinoma, the level of thyroid or tumour concentration is very low and appreciable radiation effects occur elsewhere.