Nonobese diabetic (NOD) mice and a derived strain, NOD.H.2h4, have been used as a model for experimental spontaneous thyroiditis and thyroiditis induced by iodide excess after a goiter-inducing period. Some authors have proposed that iodide, given after methimazole or propylthiouracil, is capable of inducing apoptosis in thyroid cells and that anti-thyroid drugs can modulate the expression of apoptosis components such as Fas and its ligand (Fas-L). Here we evaluated the effect of potassium iodide (20 microg/animal for 4 days, i.p.) given to NOD mice at the 10th week of life after exposure to methimazole (1 mg/ml) in drinking water from the 4th to the 10th week of life. Fas, Fas-L and Bcl-w expression were analyzed semiquantitatively by RT-PCR immediately after potassium iodide administration (group MI44D) or at week 32 (MI32S). Control groups were added at 10 (C10) and 32 weeks (C32), as well as a group that received only methimazole (CM10). An increase in the expression of Fas-L and Bcl-w (P<0.01, ANOVA) was observed in animals of group MI44D, while Fas was expressed at higher levels (P = 0.02) in group C32 (72.89 +/- 47.09 arbitrary units) when compared to group C10 (10.8 +/- 8.55 arbitrary units). Thus, the analysis of Fas-L and Bcl-w expression in the MI44D group and Fas in group C32 allowed us to detect two different patterns of expression of these apoptosis components in thyroid tissue of NOD mice.35289-95