-5forms were present, but not abundant in the extranodular tissge thyroid. A copy of the manuscript describing these findings of this gnd entitled "DNA Content Associated with Nuclear Changes in 1511 Radiated|Human Thyroids" was submitted to the Journal of Clinical Endocrinoldgy and Metabolism and is attached to this report. Our experimental results in snimals suggest that there is dose range of 13ly which for a time after the radiation is given, neitheg completely destroys the function of the thyroid cell, nor interferes witl| of those cells to multiply and make a larger gland. After a the capacity Jonger lapse of time and long after the dose of 1311 is dissipated, a defedt develops in the ability of the radiated cell tc divide, although DNA may build up. Clinical observations in the husen show that although the subfle damage may be caused to the thyroid cell, it continues to survive and maMe thyroid hormone maintaining the individual in ea euthyroid state. Suparficially, it may appear that an ideal euthyroid state is achieved in such clinical subject. In fact, the euthyroid state psreists for a good many years. However, we now are beginning to observe at 12, 15 and more years after 131I therapy thet these husan glands, which appeared to have alec capacity to manufacture hormone, ultimately begin to fail and ithe begins to suffer from hypothyroidiem. This has become apparent indfvidual from our long term study of these patients. It is thus a reasonable assumption from the animal experiments that the expected nonnal replacement off thyroid cells is not taking place and explains the ultimate failure of the thyroid. A Study of Nucles Lnanges & I in Rat Thyroids » Time of Neoplasm Formation Following Since we know that neoplasms sometimes develop in rat thyrods following small doses of 1317, and since the frequency of the occurrence] of these tumors is enhanced by giving thiouracil, it has seemed epproprgsate to use the tritiated thymidine technique to observe the behavior of thyroid nuclei as tumors are begiming to develop. We have in progress two rather extensive experiments on thel frequency of individual cells forming DNA at various intervels of time during which neoplasme may be expected to be developing. The purpose is tojwatch the development of changes in DNA formation in a population of radjated thyroid cells. A rate of mitotic activity should be manifested by upteke of tritiated thymidine. After a brief period of iodine deficient diet to insure a high uptake of 1311, a large series of approximately 100 rats were injecteq 5, 10, or 50 pe of 1351z, Others received ncee. with either Following the]1311 and a brief respite, chronic administration of thiouracil in the d#inking water was begun in some rats at each radiation dose level. ‘This serjes of rats were pubescent and weighed 120 to 140 grams when 1311 was gives. Another large series of rets which were somewhat younger and weighed f#om 80 to 100 g grems were prepared with similar doses some months following tile above series. Representatives of the various groups were killed soon after 1317 was given to determine the actual uptake of 1511 in the averaga gland. Rats representing the various experimental groups were subdivided following 13ly so that in addition to thosewhich received thiouracil dnically, others received it acutely shortly before sacrifice, and still bothers received none. Each animal was given tritiated thymidine four fhours before sacrifice, so that contact radioautographs might be made to determine which