Of 60 known to have died, the post-mortem examination was available on k6. Two thyroid neoplasms were described;. one was a carcinoma, the cther was an adehoma. It as been noted that most of the patients on whom post-mortem examinations were erformed died within 5 years of the radiation therapy--a relatively limited] interval n which to develop tumors. Two patients who have very discrete masses continue to refuse exploratipbn. One ££ these who was recently seen and reluctantly submitted to examination has Heveloped ersistent hoarseness and the mass has increased in size strongly suggesting] a malignant esion. This occurrence of 5 histologicaily identified carcinomas of the thyroi seems ighiy significant in this small selected group of patients exposed to knowrg amounts 0 radiation. A manuscript has been prepared for publication, but it has ndt been submitted for publication because we have thought it preferable to await a ffinal diagnosi n two patients being followed with suspicious lesions but as yet urwilling [to submit to thyroidectomy. The Life of the Normal Thyroid Cell and Its Potential for Replication as Compared to the Radiated Thyroid Cell: The long term follow-up of patients treated with 131] is showing that the of patients if followed long encugh will ultimately suffer from hypothyroidism tyxedema. Our figures show an incidence of only 12% hypothyroidism at the dnd rear, but 4§ - 50% if patients are observed for fifteen years. Unfortunately, woblem of hypothyroidism, if it does not occur promptly, anjses so long after xreatnent that the patient has forgotten that he received I or he became majority or of one the the Fareless about taking prophylactic supplemental hormone earlier and found that he did not need it. Since the patient has forgotten, subsequent physicians are not alerted jto the »ssibility of hypothyroidism, a diagnosis that is difficult to make if theyp is no uistory to suggest its origin. As a result of vigorous follow-up efforts by a few wedical centers,Asurprising number of patients with unrecognized myxedema have turned up. [The symptoms are attributed to, or confused with, senility and considered tq be premature aging. It may be that the life span of the "ideally irradiated" cells in diffuse 1yperplastic thyroid in man is not greatly different from the non-irradiated cells, the mly difference being a failure of replication in the radiated cell. In orcer to learn more about the long, latent thyroid failure after 135, it is recessary first to learn mre about the survival and replacement of normal thyroid celts. Initial studies were made in young, middle-aged and old rats quite some years ago in ar Laboratory, using tritiated thymidine and radiocautography to identify the cq@lls that vere undergoing mitosis. In the young growing rat, the occurrence of labeling was fairly frequent. The incidence declined with age until, in the rats that ware 2% to 3 years of age, only rare muclei were found to contain the labeled thymiding. This pilot 2xperiment illustrated the normal replication rate at different points in tHe life span. Qur first experimental steps to try to determine the life history and wate of ‘placement of the normal thyroid cells were observations on the labeling off thyroid quclei with tritiated thymidine. The objectives were to determine how rapidly the labeling takes place, the duration of the stage of DNA duplication in prepagiation for division and the duration of the total period required for mitosis to becom@ complete.