Excess iodine induces the expression of thyroid solid cell nests in lymphocytic thyroiditis-prone BB/W rats

Previous epidemiological studies have suggested that lymphocytic thyroiditis and/or an increased iodine intake may be risk factors for the development of thyroid cancer. We previously reported that excess iodine accelerated the development of thyroid lymphocytic infiltration (LI) in the autoimmune BB/W rat model. We also found that excess iodine increased thyroid cell proliferation in a disordered manner. The present study was designed to further explore these observations and to address the question as to whether excess iodine under certain conditions predisposes the thyroid gland to neoplasia. To test this hypothesis, the lymphocytic thyroiditis-prone BB/W rat was exposed to excess iodine in drinking water. Ten BB/W rats at 4 weeks of age were given iodine water (NaI 0.05%) for 10 weeks, whilst another 10 BB/W rats were given tap water and served as controls. Eighteen normal Wistar rats were also divided into excess iodine and control groups, served as a comparison to the BB/W rats. We found that an excess iodine intake accelerated the development of LI in the BB/W rat. Severe LI was usually accompanied by prominent thyroid cell proliferation, evident as numerous microfollicles and cell masses, not forming normal thyroid follicles. Numerous lymphocytes and plasma cells often encroached on these areas of increased cellular proliferation. The surprising feature, and a possible indicator of activated thyroid cell proliferation, was the high incidence of thyroid solid cell nest-like lesions (SCN) in the iodine treated BB/W rats.(ABSTRACT TRUNCATED AT 250 WORDS)