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Title: PRIOR THYROID STATUS AND THE BROILER'S ADAPTATION TO SUPPLEMENTAL TRIIODOTHYRONINE (T3)

Author
item Rosebrough, Robert
item Kahl, Stanislaw
item Elsasser, Theodore
item McMurtry, John

Submitted to: European Association of Animal Production Proceedings
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/11/2000
Publication Date: N/A
Citation: N/A

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. A strain of male chicks noted for its rapid growth was used to study the effects of prior thyroid status and triiodothyronine supplementation in controlling fat synthesis. Liver tissue when put into a culture system exhibited a high rate of lipid synthetic ability. The thyroid state of the animal will determine responses to exogenous thyroid hormones. These data may help to explain some of the apparent reported dichotomies in lipid metabolism elicited by changes in the thyroid state of animals. In addition, most metabolic changes in response to feeding T3 occurred within 2 to 5 d, suggesting that changes in intermediary metabolism preceded morphological changes. Regimens described in this study may be of value if the slight growth-depressing effect of triiodothyronine can be overcome.

Technical Abstract: Seven-day-old chickens were fed diets containing 18% crude protein + 0 or 1 g methimazole/kg to produce either euthyroid or hypothyroid groups of birds at 28 d of age. These two groups were then offered diets containing either 0 or 1 mg triiodothyronine (T3)/kg diet. Birds were sampled at 0, 2, 5 & 8 d (d 28, 30, 33 & 36 d of age) following the onset of the T3 treatment. Measurements taken at these intervals included in vitro lipogenesis (IVL), growth and feed consumption, hepatic enzyme activities and plasma thyroid hormones and metabolites. Iodothyronine 5'-monodeiodinase activity was measured to determine if the native thyroid state would affect this regulatory enzyme's response to thyroid hormone repletion. Hypothyroidism decreased IVL at 28 d of age; however, T3 supplementation for 2 d restored IVL. Paradoxically, continuing T3 replenishment for an additional 3 to 6 d decreased IVL. In contrast, supplemental T3 decreased IVL in euthyroid birds, regardless of the dosing interval. Hypothyroidism increased 5'D activity. Although T3 replenishment for 8 d decreased 5'D activity, this activity was still greater than that of the controls. In conclusion, the thyroid state of the animal will determine responses to exogenous thyroid hormones. These data may help to explain some of the apparent reported dichotomies in lipid metabolism elicited by changes in the thyroid state of animals. In addition, most metabolic changes in response to feeding T3 occurred within 2 to 5 d, suggesting that changes in intermediary metabolism preceded morphological changes.