Submitted to: Domestic Animal Endocrinology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 5, 2002
Publication Date: April 1, 2003
Citation: Domestic Animal Endocrinology 24:231-242, 2003.
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. A strain of male chicks noted for its rapid growth was used to study the effects of thyroid hormone oblation and restoration on growth and metabolism. Liver tissue when put into a culture system exhibited a high rate of lipid synthetic ability. Tissue was taken from chickens fed triiodothyronine after a period of hypothyroidism. This tissue exhibited a decrease in the ability to produce fat at the onset of thyroid replenishment. Triiodothyronine first increased lipogenesis and then decreased this very same process. It is evident that normal thyroid function is required to elicit our previously noted decrease in lipogenesis accompanying supplemental thyroid hormones.
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 and 8 d 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. 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. 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.