EN
Decuypere E. and Kühn, E.R.: Alterations in thyroid hormone physiology induced by temperature and feeding in newly hatched chickens. Acta physiol. pol., 1988, 39(5-6): 330-394. In.the chicken the transition of a poikilotherm to a homeotherm reaction upon cold exposure takes place in the perinatal period between pipping and hatching However, newly hatched chicks cannot maintain their body temperature within narrow limits after cold exposure. The fact that relatively little attention was payed on the role of thyroid hormones in the thermoregulatory reaction to cold of young chicks was probably due to the hypothetically long latention time that was thought to be necessary to bring about changes in secretory activity by cold stimulation. However, more recently, rapid changes (within hours) of thyroid hormone concentrations upon cold exposure were described in the chickens and the quail. In this study, changes in circulating T₃ and T₄ concentrations upon cold exposure of young chicks during the first two weeks were followed, that means during the period wherein NST (non-shivering thermogenesis), if it exists at all, should be progressively replaced by ST (shivering thermo- genesis). Because of the importance of feeding condition on thyroid hormone levels, the experiments were carried out with and without a preceeding fasting period. In all experiments a short-term cold exposure of young chickens (1-11 days) fed ad lib decreased T₃ but increased T₄ levels while a reversed picture was found after short cold exposure of the fasted animals. However, after prolonged cold stimulus (15°C) of young chickens fed ad lib, plasma T₃ was also significantly elevated over that of controls whereas T₄ levels returned to normal values. A prolonged warm treatment (37°C) of young chickens fed ad lib resulted in significantly lower T₃ and higher T₄ concentrations. After a prolonged cold treatment no differences in T₄ or T₃ response upon TRH were found whereas the warm treatment abolished these responses upon TRH. However, a cold treatment at the jstage of incubation during which the hypothalamo-hypophyseal control of thyroid function is established (dag 10-l4) enhanced the T₄ response to TRH with a long lasting effect extending to the posthatch period. Since T₃ is thought to be the active form of thyroid hormones with regard to thermopoiesis we have studied more specifically the effect of. blocking peripheral Conversion of T₄ on thermoregulatory abilities in young chicks and the influence of temperature treatment on monodeiodination capacity. The lower rectal temperatures following the interference with the peripheral monodeiodination of T₄, the effect being more pronounced at the lower ambient temperature, are indicative for a preponderant role of T₃ on thermogenesis. Thyroid hormone levels are the integrated results of production and release on one hand and elimination, tissue utilization or peripheral interconversion on the other. Moreover, all these processes can be and are probably influenced by the relative amounts of T₄ and T₃ as well as by the ratio of free/bound hormones. Some exogenous factors, acting on thyroid hormone activity, could preferentially increase T₄ conversion while others could primarely act on enhanced gland secretion. Therefore further study on the underlying mechanisms of thyroid hormone activity control and regulation are needed.