Effects of hyper- or hypothyroid status on growth, adiposity and levels of growth hormone, somatomedin C and thyroid metabolism in broiler chickens.

Relatively few methods involving dietary manipulation of thyroid status have been used to study the effect of that status on growth and body composition as well as on changes in hormone levels and metabolism. The present work examines the influence of either a hyperthyroid status (induced by feeding triiodothyronine: T3 or thyroxine: T4) or a hypothyroid status (induced by feeding methimazole: MMI) on growth, feed consumption and body composition as well as on changes in thyroid hormone levels and metabolism and in hormone levels of the somatotroph axis of broiler chickens. MMI depressed growth but increased fatness. Long-term administration of thyroid hormones decreased both growth and fat deposition, T3 being more effective than T4. The reduced growth of MMI-treated birds was not only related to the hypothyroid state but also to decreased somatomedin C (Sm-C) production, although growth hormone (GH) levels remained high. Thyroid hormone-treated birds showed slightly depressed GH levels but unchanged Sm-C levels. MMI induced an increase in hepatic 5'-monodeiodination activity, while T3 and T4 reduced that activity. The T4 administered was largely eliminated as inactive reverse T3. Both of the statuses (hypothyroid and hyperthyroid) profoundly changed peripheral thyroid hormone metabolism and influenced GH/Sm-C relationships, but in the opposite way. Hormonal changes could be related to the observed changes in growth and adiposity of broiler chickens and illustrate the negative correlation between T3 and body fat.