Endocrine reactivity (T3, T4, cortisol) during cold exposure in preterm and full-term lambs

The endocrine reactivity to a short-term cold exposure (+ 4 degrees C during 1 h 30 min) was studied in 8 full-term (FT) and 12 preterm (PT) Limousin x Romanov lambs, obtained by oestrogen injections to the ewes; 4 PT animals died (PTD) during the observation period and 8 remained alive (PTA). The plasma cortisol, T4 and T3 levels were lower in PT lambs (particularly in PTD for T4 and T3) during the first hours of life. Moreover, rectal temperature decreased sharply between birth and 30 min postpartum in the PT group; at this time, plasma T3 levels and rectal temperature were positively related. During cold exposure, the plasma cortisol levels increased in FT lambs, but did not change in the PT group. Moreover, the plasma T4 levels rose slightly in FT animals (+ 12%), increased sharply in PTA lambs (+ 137%), or showed a small decrease in the PTD group (-5%) in which a marked hypothermia occurred; the relative changes in the circulating T4 levels were positively related to the algebraic variations of rectal temperature in PT lambs. However, the changes in the plasma T3 levels did not parallel that of T4 in the three groups of animals, but a positive relationship was observed, in all lambs, between the circulating T3 levels and the rectal temperatures at the end of the cold exposure. These results suggest a reduced sensitivity of the adrenal cortex to stressful conditions in PT lambs. Moreover, they underline the importance of thyroid function for thermoregulation in the newborn lamb, particularly in PT animals.