Iodine-induced thyroiditis and hypothyroidism in the hemithyroidectomized bb/w rat

Abstract

We have recently reported that iodine administration (0.05% iodine in drinking water) to weanling, diabetes mellitus- and lymphocytic thyroiditis (LT)-prone Biobreeding Worcester (BB/W) rats strikingly increases the incidence of LT without occurrence of iodine-induced hypothyroidism, which frequently results when excess iodine is administered to euthyroid patients with Hashimoto’s thyroiditis. Since hypothyroidism did not occur in the iodine-treated BB/W rats, hemithyroidectomy was carried out in 30-day-old BB/W rats to increase thyroid mass and functional reserve. Iodine administration for 60 days markedly increased antithyroglobulin antibodies (0.40 ± 0.08 vs. 0.15 ± 0.06 OD; P < 0.02), the incidence of LT (68% us. 13%; P < 0.001), and thyroid weight of the residual lobe (10.5 ± 0.7 us. 6.3 ± 0.3 mg/100 g BW; P < 0.001) and induced hypothyroidism (T4, 2.5 ± 0.2 us. 3.0 μg/dL; P < 0.05; T3, 25.1 ± 1.9 us. 37.5 ng/dL; P < 0.001; TSH, 252 ± 49 vs. 61 ± 14 MU/ mL; P < 0.02). Hypothyroidism in the iodine-treated rats occurred primarily in those with LT. Similar studies were carried out in the non-diabetes mellitus-, non-LT-prone, genetically equivalent BB/W rats (W-line), the parent strain Wistar-Furth rats, and Sprague-Dawley rats. Iodine administration did not induce LT or antithyroglobulin antibodies in these three strains and did not affect thyroid function in Wistar-Furth and Sprague- Dawley rats. However, in the W-line rats, iodine excess did induce thyroid enlargement in the residual lobe (8.4 ± 0.2 vs. 6.4 ± 0.2 mg/100 g BW; P < 0.001), a decrease in serum T3 (71.5 ± 2.9 vs. 86.0 ± 2.5 ng/dL; P < 0.001), and an increase in serum TSH (344 ± 65 us. 69 ± 6.0 μU/mL; P < 0.001). It is evident, therefore, that hemithyroidectomy in BB/W rats sufficiently reduces functioning thyroid tissue, resulting in iodine-induced LT and hypothyroidism, similar to iodine-inducod hypothyroidism in euthyroid patients with Hashimoto’s thyroiditis. It is unclear, however, why iodine administration also induced hypothyroidism in hemithyroidectomized, genetically similar, Wline rats in the absence of LT. This observation suggests that iodine-induced hypothyroidism in rats may be genetically determined. © 1987 by The Endocrine Society.