Hepatic 5′ deiodinase activity in the broiler chicken: effects of diet and prior thyroid status

Abstract

Male, Shaver broiler chickens growing from 7 to 49 days of age were used in two experiments to demonstrate the effects of dietary crude protein (CP) and thyroid hormone deficiency-replenishment on 5′D activity. In the first experiment, birds were fed 18% CP ± 1 g/kg methimazole (M) from 7–28 days and then an 18% CP diet ± 1 mg/kg T 3 for 8 days. The second experiment involved feeding three levels of CP (12, 18 & 24%) ± 1 g/kg M from 7 to 28 days and an 18% CP from 28–49 days. Methimazole depressed both plasma T 3 and T 4. Although M depressed and supplemental T 3 increased 5′D per gram of liver in the first experiment, M also increased the relative liver size. Thus, total T 3 productive capacity increased in the face of a decrease in specific activity. Again, in the second experiment, M depressed both T 3 and T 4. In addition, low a low protein diet also depressed T 4. Both CP and M affected 5′D activity in 28-day-old birds. In contrast, only the carry over effect of M was significant in 49-day-old birds. It is suggested from these studies that particular care must be taken in selecting a method to express 5′D activity. Permutations in the thyroid state alter the liver size of the bird, distorting values expressed per unit of liver weight. Values expressed per unit of protein may fail to account for the influence of changes in liver size on total 5′D activity.