Studies on doses of methimazole (MMI) and its administration regimen on broiler metabolism

We designed three experiments to determine both the optimal dose of and time on experiment for methimazole (MMI; 1-methyl-2-mercaptimidazole). Our goals were to determine if chicken growth was related to thyroid hormone levels and if intermediary metabolism changed along with changes in thyroid hormone levels. Initiating MMI at one week of age decreased (P < 0.01) plasma thyroid levels and growth in four-week old birds. In contrast, initiating MMI at two and three weeks of age decreased (P < 0.05) hormone levels without affecting growth as severely. Although initiating MMI at two weeks of age depressed (P < 0.05) plasma thyroid hormones at four weeks, there was little change in vitro lipogenesis at four weeks. Again, initiating MMI at one week of age decreased body weight, plasma thyroid hormones and in vitro lipogenesis at four weeks of age. In addition, this treatment also decreased (P < 0.05) malic enzyme activity at this same age period. The second experiment showed that MMI, initiated at 14 days, had no significant effect on 28-day body weight and again decreased both plasma T3 and T4 but T3 replacement increased plasma T3 in both 14–28-day treatment groups. All body weights were similar at 30 days, however. Lastly, diets containing graded levels of MMI decreased thyroid hormones and body weight (0 > 0.25 > 0.5 > 1 g MMI/kg). In contrast, only the two higher levels (0.5 and 1 g MMI/kg) decreased in vitro lipogenesis. Growth depression, caused by MMI feeding, can occur without changes in lipid metabolism. The length of MMI administration may be as important as dose level in obtaining effects (growth, thyroid hormone depression and inhibition of lipogenesis).