Iodine-induced hypothyroidism: a clinical and histological study of 28 patients.

Thirty-three thyroid specimens obtained from 28 patients with clinically and laboratory-proven iodine-induced hypothyroidism were examined clinically, histologically, immunohistochemically, and ultrastructurally. Twenty-eight specimens obtained during the hypothyroid phase showed common histological changes in the thyroid thought to be specific for this disease; hyperplastic change in the follicles with some papillary folding, cuboidal to columnar change of follicular cells with clear and vesicular cytoplasm, scanty or absent colloid material in the large distended follicles, and occasional dilatation of capillary vessels. Lymphocytic infiltration was present in about half of the specimens. No specimens showed either stromal fibrosis or parenchymal atrophy. Immunohistochemical and electron microscopic examination revealed that severe interference with thyroid hormone biosynthesis occurs in the follicular cells. In two patients who had a follow-up biopsy in the recovery (euthyroid) phase after iodine restriction, the histological involvement seen in the hypothyroid phase was no longer present. The histological changes in the thyroid gland seen in patients with iodine-induced hypothyroidism are characteristic. This disease can be diagnosed from laboratory tests, but thyroid biopsy is also a useful tool to differentiate this condition from other diseases causing hypothyroidism. Not only clinicians, but also pathologists, must pay attention to this type of hypothyroidism, because thyroid function may revert to normal by iodine restriction alone.