Neonatal changes in plasma cortisol, free and total iodothyronine levels in control and hypotrophic lambs.

Neonatal changes in plasma free and total iodothyronines, cortisol, glucose and urea levels have been studied in 8 control (birthweight greater than or equal to 2.5 kg) and 16 hypotrophic lambs (birthweight less than 2.5 kg) receiving limited amounts of colostrum during the first 36 h of life and then fed ad libitum. During the period of colostrum feeding, plasma glucose levels were low in both groups and increased after the onset of ad libitum feeding; they were significantly lower in hypotrophic animals from birth to 36 h. Plasma urea levels increased during the period of colostrum feeding and decreased thereafter in all animals. At birth, they were significantly higher in hypotrophic lambs. Over the entire period studied (20 d), plasma levels of total T4, free T4, total T3 and free T3 were markedly lowered in hypotrophic lambs without alterations in the values of the T3/free T4 ratio. No differences could be observed in plasma reverse T3 and cortisol levels. For all blood parameters recorded, the neonatal changes were parallel in the two groups of lambs. In agreement with hypoglycemia and hyperuremia observed at birth in hypotrophic lambs, with the litter size recorded for each experimental group and with previous results, placental insufficiency linked to a large litter size gestation could be at the origin of low thyroid hormone levels.