Carry over effects of dietary crude protein and methimazole in broiler chickens

Seven day old male broiler chickens were fed diets containing 12, 18 or 24% crude protein + 0 or 1 g methimazole/kg diet for 21 days to examine the interaction of the birds' thyroid status and crude protein levels on metabolism. Methimazole (1-methyl-2-mercaptimidazole) inhibits thyroidal production of thyroid hormones and results in hypothyroidism. Birds were fed a diet containing 18% crude protein for an additional 21 days to determine the carry over effects of treatments. Birds were killed at 28 and 49 d. In vitro lipogenesis was inversely related (P < 0.05) to dietary protein levels in control birds at 28 d. Dietary methimazole attenuated (P < 0.05) this effect, resulting in a common rate similar to that attained in the birds fed the highest level of protein without methimazole. Birds fed methimazole for an initial 21-day period (7 to 28 d of age) had greater lipogenic rates (P < 0.05) at 49 d than did their control counterparts. In contrast, methimazole increased (P < 0.05) abdominal fat pad (AFP) lipoprotein lipase (LPL) at both age periods, indicating increased ability by the AFP to remove triglycerides from systemic circulation. Observations at 49 d suggest that perturbations in the thyroid of the young bird may substantially change metabolism in later life. Results also show that obesity in hypothyroid birds cannot be explained by increases in de novo lipogenesis, but probably relates to changes in LPL activity.