THE EFFECT OF IODINE ON LYMPHOCYTIC THYROIDITIS IN THE THYMECTOMIZED BUFFALO RAT

Excess iodine has been associated with an increased incidence of lymphocytic thyroiditis (LT) in the BB/W rat, obese strain chicken, and hamster. The spontaneous incidence of LT in the Buffalo (Buf) rat is increased by neonatal thymectomy. In the present study, the effect of combined thymectomy and excess iodine ingestion on the incidence of LT in Buf rats has been examined. Buf rats were thymectomized at 1 day of age and randomized at 4 weeks of age to receive either standard rat chow with tap water (controls), or standard rat chow with 0.05% iodine in the drinking water (iodine group) for 12 weeks. The serum was assayed for TSH, antithyroglobulin antibodies, and iodine. The thyroids were fixed in Bouin’s solution and stained with hematoxylin and eosin, and the presence of thyroiditis was determined. Iodine increased the incidence of LT from 31% in the control group to 73% in the iodine-treated group (P < 0.05). Serum TSH concentrations and levels of thyroglobulin antibodies were significantly higher in the iodinetreated rats, primarily due to the increased incidence of LT and subsequent iodine-induced hypothyroidism. These data suggest that iodine enhances the effect of neonatal thymectomy on LT in Buf rats and support the concept that iodine may play an important role in the expression of LT in predisposed animals. © 1990 by The Endocrine Society.