1996 Volume 9 Issue 4 Pages 385-390
A study was conducted to determine the mode of action for promoting effect of large amounts of vitamin A (VA) on cell proliferation of thyroid proliferative lesions induced by thiourea (TU) (Mitsumori et at, Cancer Letter 103: 19-31, 1996). Groups of male F344 rats (16/group) received 0.1% TU in water (TU group), 0.1% VA in diet (VA group), both 0.1% TU in water and 0.1% VA in diet (TU+VA group) or distilled water and basal diet (control group) for 4 weeks. Increased weights of thyroids (absolute and relative), liver (absolute and relative) and pituitary (relative), decreased serum T4 levels, and decreased thyroidal iodine uptake, and organification were observed in TU and TU+VA groups, as compared to the control group. Thyroidal iodine organification was also decreased in VA group. However, there were no differences in these parameters between TU and TU+VA groups. No treatment related effects were noted in serum T3 and TSH. No remarkable differences were observed in the histopathological findings of thyroids, liver, and pituitary between TU and TU+VA group, although diffuse follicular cell hyperplasia was induced in these groups. The results in the present study suggest that the enhancement effect of VA on thyroid proliferative lesions may not be attributable to the modification of thyroidal iodine uptake.
