Comparative effectiveness of 132I and 131I in the rat thyroid gland

The thyroidal effects of short-lived radioiodines such as 132I, 133I, and 135I have not been well defined. Instead, their effects have been assumed to be similar to those obtained by external x-ray exposure, or about 10 times as effective as the longer-lived and better understood radioiodine, 131I. We report here the results of a study done to compare the effects of 132I and 131I and to confirm the suitability of the assumed 10-fold difference. We found the same effect in goitrogen-stimulated rat thyroid glands (in terms of 50% suppression of thyroid gland weight increase) at injected dosages of 94 μCi 132I and 13 μCi 131I. Doses, however, were 1.7 rads/μCi 132I and 175 rads/μCi 131I. The same 50% suppression for thyroid dose was found to occur at about 160 rads from 132I and 2200 rads from 131I, suggesting an approximately 14-fold increase in radiologic effectiveness of 132I over 131I