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Influence of tri-iodothyronine or lipid administration on the response of the pituitary-thyroid axis to exposure to cold in the newborn lamb.

The influence of the administration of tri-iodothyronine (T3) or a solution of soya oil and egg lecithin on the response of the pituitary-thyroid axis to moderate exposure to cold (4 degrees C for 4 h) was studied in 24-h old lambs. In control lambs, plasma concentrations of TSH. T3 and total and free thyroxine (T4) rose significantly whereas plasma concentrations of reverse T3 remained unchanged during the test. In lambs injected i.v. with a small amount of T3 (1.23 nmol/kg) 30 min before the onset of exposure to cold, plasma concentrations of TSH, reverse T3 and total and free T4 did not change during the test. Administration of lipid 30 min before exposure to cold induced, as expected, a sharp rise in plasma free fatty acid (FFA) concentrations and a transient increase in free T4 concentrations. In these animals, plasma concentrations of TSH increased during the test as observed in control lambs, but plasma concentrations of T3, reverse T3 and total T4 did not show any significant change, whereas free T4 levels decreased during the first 2 h. These results strongly suggest, in contrast to previous results, that T3 exerts a negative feedback upon the hypothalamic-pituitary-thyroid axis in the newborn lamb. Moreover, it appears that a rise in plasma concentrations of FFA could affect neonatal thyroid function.