Methimazole and thyroid hormone replacement in broilers.

Abstract

Seven-day-old chickens were fed diets containing 18% crude protein + 0 or 1g methimazole/kg to produce either euthyroid or hypothyroid groups of birds at 28 days of age. These two groups were then offered diets containing either 0 or 1mg triiodothyronine (T(3))/kg diet. Birds were sampled at 0, 2, 5, and 8 days following the onset of the T(3) treatment. Measurements taken at these intervals included in vitro hepatic lipogenesis (IVL), growth and feed consumption, hepatic enzyme activities (malic enzyme, ME; isocitrate dehydrogenase, ICD; and aspartate amino transferase, AAT), plasma hormones (T(3); thyroxine, T(4); insulin like growth factors I, IGF-I; and insulin like growth factors II, IGF-II) and metabolites (glucose; fatty acids, NEFA; triglyerides; uric acid). Hypothyroidism decreased IVL and ME at 28 days of age; however, T(3) supplementation for 2 days restored both IVL and ME. Paradoxically, continuing T(3) replenishment for an additional 3-6 days decreased IVL without affecting ME activity. In contrast, supplemental T(3) decreased IVL in euthyroid birds, regardless of the dosing interval, but had no effect on ME activity. Methimazole decreased plasma T(3), T(4), uric acid, and IGF-I, but did not affect IGF-II at 28 days. Giving T(3) to birds previously on methimazole increased plasma IGF-I as did feeding a control diet. Supplemental T(3) increased NEFA in both euthyroid and hypothyroid birds, but only for a short period following the initiation of supplementation (2 days post-supplementation). 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 T(3) occurred within 2-5 days, suggesting that changes in intermediary metabolism preceded morphological changes. In conclusion, the thyroid state of the animal will determine responses to exogenous T(3).