Lack of effect of soy isoflavone on thyroid hyperplasia in rats receiving an iodine-deficient diet

We have reported a dramatic synergism between soy intake and iodine deficie ncy regarding induction of thyroid hyperplasia in rats. Because isoflavones are active constituents of soybeans, in the present study, their possible contribution was examined, Female F344 rats were divided into 8 groups, exp osed to diet containing a 0.2% soy isoflavone mixture (SI), 0.2% SI+iodine deficiency (ID), 0.04% SI, 0.04% SI+ID, 20% defatted soybean (DS) alone, 20 % DS+ID, ID alone or basal diet alone for 5 weeks, Thyroid weight was not i nfluenced by SI, but was increased by the ID and DS diets with a further si gnificant increment in the DS+ID group (P<0.01). Compared to the control va lue, serum T-4 was significantly (P<0.01) increased by 20% DS alone and dec reased in all groups given the ID treatment (P<0.001). Serum thyroid stimul ating hormone (TSH) level was increased by ID, and further enhanced by DS ( P<0.01) but not SI, Histopathologically; diffuse hypertrophy and/or hyperpl asia of thyroid follicles were observed in the ID-treated groups, the sever ity being enhanced by DS but not SI. Proliferating cell nuclear antigen lab eling indices (%) mere elevated in the ID diet groups and again enhanced by DS, but not SI. These results thus suggest that isoflavones may not be inv olved in the mechanisms underlying the synergistic goitrogenic effect of so ybean with iodine deficiency.