Influence of Lymphocytic Thyroiditis on Iodine Metabolism in the Beagle 1

Summary Tracer studies of thyroidal 131I metabolism were conducted in beagles some of which had lymphocytic thyroiditis. The percentage of each thyroid lobe represented as epithelium, colloid, and stroma, as well as the severity of the lesions, was determined by microscopic examination. The high level of iodide in the commercial diet fed resulted in low thyroidal 131I uptake values in all animals. Dogs with severe thyroiditis had biological half-times (BT 1/2) of 131I significantly shorter than dogs with mild or no lesions. The maximum thyroidal uptake of 131I was significantly lower and also occurred earlier in dogs with severe thyroiditis. Fourteen months after one thyroid lobe was removed the BT 1/2 of 131I was shortened compared to values obtained in the intact dog. There is little colloid in thyroid glands affected with severe lymphocytic thyroiditis; hemithyroidectomy also results in a reduction of the relative amount of colloid with a corresponding increase in the percentage of epithelium in the opposite lobe. Thus, the BT 1/2 of 131I in the thyroid gland appears to be related to the amount of follicular colloid. Lymphocytic thyroiditis in the dog can be diagnosed by tracer studies with 131I even when the dietary level of iodide is high.