Effect of iodine intake and methimazole on lymphocytic thyroiditis in the BB/W rat

Spontaneous LT and elevated serum anti-Tg occur in the diabetes prone BB/W rat, but thyroid function is essentially normal in the rats with LT. Prolonged low dose MMI decreases the incidence of LT in BB/W rats. The administration of excess iodine beginning at 30 days of age markedly accelerates the occurrence of LT and anti-Tg at 90 days of age. Low iodine intake decreases the incidence of LT. Excess iodine intake did not induce LT in W-line, Wistar-Furth, and Sprague-Dawley rats. This suggests that iodine induced LT occurs only in genetically susceptible rats. Despite the increased incidence of LT during iodine administration, thyroid function remains essentially normal. This is in contrast to the frequent induction of hypothyroidism following iodine administration to euthyroid patients with Hashimoto's thyroiditis. In order to decrease thyroid reserve, rats were hemi-TX at 30 days of age. The administration of iodine markedly increased the incidence of LT and serum anti-Tg, increased the weight of the remaining lobe, and induced hypothyroidism as determined by significantly lower serum T4 and T3 concentrations and elevated serum TSH concentrations. Excess iodine administration to hemi-TX W-line rats (genetically equivalent, non-diabetes, non-LT prone BB/W rats) did not induce LT but did induce hypothyroidism, suggesting that BB/W and W-line rats are susceptible to iodine induced hypothyroidism, perhaps unrelated to the induction of LT. Excess iodine did not induce LT or affect thyroid function in hemi-TX Wistar-Furth and Sprague-Dawley rats.