Female control values are significantly lower (confirmed by others). Experinentral ~~ al Liked o ? animals whose thyroids had been meticulously removed or deliberately destroyed with 131; ranged as high as 3,500 ng/mg. A series or about 300 surviving radiated male rats of one exper nment were sacrificed 14 to 16 months after a range of small doses of i Sly. As per routine, all animals were individually identified and all data was collected including th e rad dose of 1 31, originally received by the thyroid. direct heart puncture. laboratory. The animals were anesthetized é nd exsanguinated by The serum was frozen until the IRA was full) operational’ in our Information was subsequently assembled regarding the pi resence of micro- scopic neoplasms in each thyroid. The results of TSH assays among hose animals that had developed neoplasms and those that had no neoplasms are the bas ls for a report in manuscript form. neoplasms. The levels of TSH in general were higher among 1s which developed The incidence of neoplasms in this series was 79.7%. elevation in serum TSH was often found in rats whose thyroids had been irradiated wit 131, and also had become the site of one or more growing neoplasms. er, numerous in- There were, how stances of neoplasms having developed without accompanying elevati of TSH. From this Study it may be concluded that both elevation in TSH and neoplasm mation results from irradiation but may not have a cause and effect relationship. TSH elevation was not often found until the rad dose received by the thyroid was 3,0§0 rads or more. Thyroid neoplasms were, however, readily produced by doses between 1,500 and 2,000 rads. As a preliminary conclusion, it may be said that 131 I radia’ lion in the thyroid May result in a rise in TSH and may also initiate changes that res t in neoplas™ formation but it cannot be:assumed that the rise in TSH resulting fran irradiation initiates neoplasms. On the other hand, once a neoplasm is initia red, TSH may be a growth promoting factor. Another group of 450 female rats were given small doses of Ly The details of preparation and observations on these animals followed the plan pr eViously used.