Submitted to: Nutrition Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 27, 2001
Publication Date: August 15, 2002
Citation: Nutrition Research 22:375-383, 2002. Interpretive Summary: Excess fat production in the modern broiler accounts for an annual loss to the poultry industry of 800 to 950 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. I is generally acknowledged that the central regulatory hormone system in the chicken involves the thyroid axis. Thus, permutations in the thyroid status of the broiler would allow studies of the importance of the axis in controlling fat production. A strain of male chicks noted for its rapid growth was used to study the effects of prior thyroid status on the endogenous production of the metabolically active form of the thyroid hormones. The background thyroid status of the broiler was found to influence the production of further amounts of thyroid hormones. Regimens described in this study may be of value if the slight growth-depressing effect of triiodothyronine can be overcome.
Technical Abstract: Conversion of thyroxine (T4) to the metabolically active thyroid hormone (T3) is catalyzed by 5''deiodinase (5''D). Male, Shaver broiler chickens growing from 7 to 56 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 ññ 1 g/kg methimazole (M) from 7 - 28 days and then an 18% CP diet ññ 1 mg/kg T3 for 8 days. The second experiment involved feeding three levels of crude protein (CP, 12, 18 & 24%) ññ 1 g/kg M from 7 to 28 days and an 18% CP from 28-56 days. Methimazole depressed both plasma T3 and T4. Although M depressed and supplemental T3 increased 5''D per gram of liver in the first experiment, M also increased the relative liver size. Thus, total T3 productive capacity increased in the face of a decrease in specific activity. Again, M depressed both T3 and T4. In addition, low a low protein diet also depressed T4. Both CP and M affected 5''D activity i 28-day-old birds. In contrast, only the carry over effect of M was significant in 56-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.