Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: August 15, 2000
Publication Date: N/A
Technical Abstract: There are conflicting results concerning the role of the thyroid hormones in lipid metabolism. For example, rat data indicate that T3 stimulates lipogenesis by increasing the abundance of mRNA for fatty acid synthetase. On the other hand, we have shown that T3 severely restricts lipogenesis in chickens. The purpose of this experiment was to study the effects of T3 on metabolic adaptations in hypothyroid or euthyroid birds. Seven-day-old chickens were fed diets containing 18% crude protein + 0 or 1 g methimazole/ kg to produce either normal or hypothyroid groups of birds at 28 d of age. These two groups were then offered diets containing either 0 or 1 mg T3/kg diets. Birds were sampled at 0, 2, 5 & 8 d following the onset of the T3 treatment. Measurements taken included in vitro lipogenesis (IVL), growth and feed consumption, hepatic enzyme activities and plasma thyroid hormones and metabolites. Iodothyronine 5'-monodeiodinase (5'D; EC 126.96.36.199) 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.