Submitted to: Comparative Biochemistry and Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/12/2005
Publication Date: 2/10/2006
Citation: Rosebrough, R., Russell, B.A., McMurtry, J.P. 2006. Studies on doses of methimazole (MMI) and its administration regimen on broiler metabolism. Comparative Biochemistry and Physiology. 143:35-41. Interpretive Summary: Excess fat production in the modern broiler accounts for an annual loss to the poultry industry of 1000 to 1500 million dollars annually. The original source of this problem relates to selection genetic practices that emphasized rapid growth at the expense of other carcass characteristics. The literature is of limited value in determining methods to depress fat synthesis and allow lean tissue synthesis to remain at an elevated rate. The thyroid axis is more important in regulating intermediary metabolism in birds than in mammals because of a questionable role for insulin in birds. The purpose of this set of experiments was to 1) chemically inhibit thyroid hormone production and 2) examine protein nutritional status as a moderator of this inhibition. Although hypothyroidism decreased lipid synthesis in 28-day old chickens, restoration of thyroid function in these birds increased synthetic ability in market-age birds (49 days of age). Assigning a role to the thyroid hormone axis may be difficult because an interplay between protein nutrition and background thyroid status.
Technical Abstract: 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 to 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 (Experiment). 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 gene expression and malic enzyme activity at this same age period. Lastly, diets containing graded levels of MMI decreased thyroid hormones and body weight (Experiment 3; 0> 0.25> 0.5 >1 g MMI/kg). In contrast, only the two higher levels (0.5 & 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).