THE ACCUMULATION OF RADIOACTIVE IODINE BY HUMAN FETAL THYROIDS*

THE increasing availability of radioactive iodine for treatment of thyrotoxicosis emphasizes a need for establishing limits of safety, especially in regard to the therapy of thyrotoxicosis in pregnant women. Chapman (1) and his group studied the uptake of radioiodine (I131) in human fetal thyroids following its administration to the mothers. From the radioactivity of the macerated fetal thryoids, they concluded that accumulation of radioiodine by the human fetal thyroid was minimal until after the fourth month. Our thyroid clinic group has maintained a cautious attitude toward radioiodine therapy of pregnant patients. Consequently we have not used I131 to treat any patient with thyrotoxicosis during a recognized pregnancy. In order to obtain additional clinical information, we are attempting to determine the age at which the human fetal thyroid begins to accumulate I131. Through cooperation of the departments of Obstetrics, Internal Medicine, and Radiation Research, patients requiring therapeutic abortion were given 500 microcuries of I131 approximately twenty-four hours before surgical intervention. Immediately after the operation the fetuses were photographed and the thyroid glands removed in a block of tissue including the trachea. The radioactivity of the thyroid area was measured in a special counter (Texas Tank). Then the tissue was fixed for histologic study. Routine hematoxylin-eosin sections and radioautograph slides were prepared from each specimen. Maternal thyroidal uptake of the radioiodine was determined in most instances at twenty-four hours. The age of the fetuses was judged in two ways. First, by measuring their crown-rump length on the photographs and then referring to tables and nomographs of size-age relationships (2, 3). Second, by the history, in which the age was computed as the time since the mother's last normal menstrual period minus two weeks—on the assumption that conception was about fourteen day after that date.