The Effect of Methimazole on the Development of Spontaneous Lymphocytic Thyroiditis in the Diabetes-Prone BB/W Rat

Methimazole (MMI) administration decreases the incidence and intensity of experimentally induced lymphocytic thyroiditis (LT) in the female August rat and male A/J mouse. Spontaneous LT frequently occurs in the insulin-dependent type-I diabetic (DM) BB/ W rat. Experiments were carried out to determine whether MMI administration to BB/W rats from 30 to 120 days of age would affect the incidence and intensity of spontaneous LT and the incidence of spontaneous DM. A total of 870 ng MMI/gm BW or saline (C) were administered ip daily to 77 BB/W rats beginning at age 30 days. Rats were killed at 120 days, blood obtained for measurement of serum T4, TSH, and anti-Tg Ab (ELISA), and thyroids removed for histology. MMI administration was associated with a decrease in the incidence of LT (31% vs. 55%; p < 0.05) but no difference in the severity of the LT. Serum T4 was similar in the MMI and C groups, but serum TSH was slightly but significantly higher in MMI treated rats [43 ± 6 (mean ± SE) μU/ml vs. 30 ± 2.5; p < 0.05]. Serum anti-Tg Ab levels increased with age but MMI administration did not affect this rise. There was no significant difference in the incidence of insulin-dependent DM between the MMI and C rats (MMI, 56%; C, 74%). Conclusion: MMI ad-ministration to the genetically predisposed insulin-dependent diabetes and LT prone BB/W rat during the age when LT spontaneously occurs reduced the incidence of LT.