Abstract
The present study was designed to examine the effects of a low iodine diet (LID) on tumorigenesis in F344 rats treated with N-methyl-N-nitrosourea (MNU). Four-week-old male F344 rats were exposed to 100 ppm MNU in their drinking water for 15 weeks. Thereafter Group 2 animals received a low iodine diet LID while Group 1 received no further treatment after MNU. Non-carcinogen control animals received the LID without prior initiation (Group 3). At sacrifice, 36 weeks after starting MNU administration, nerve tumors were present in 16 of 20 (80%) animalls, and 19 of 32 (59%) in Groups 1 and 2, respectively, mostbeing malignant schwannomas. Thymic lymphomas appeared in 7 of 20 (35%) rats in Group I and 15 of 32 (47%) in Group 2, and also gastric tumors were present in 6 of 20 (30%) in Group I and 10 of 32 (31) in Group 2. There were no significant intergroups differences regarding these lesions. However, the incidence of thyroid carcinomas in Group 2 was significantly higher than that in Group I (P<0.01). Also cumulative incidence curves indicated a significantly earlier appearancein Group 2 and the number of rats bearing 3 different tumors was significantly increased in this group (P<0.01). Moreovar, administration of the LID was itself associated with a small incidence of thyroid lesions.
