Plasma free and total iodothyronine levels in the newborn lamb. Physiological considerations.

Neonatal changes in plasma total and free iodothyronine levels were monitored in 18 Limousin X Romanov suckling lambs (experiment 1) and 24 Limousin X Romanov animals bottle-fed in standardized conditions (experiment 2). In the two experiments, plasma free T3 levels were closely related to total T3 levels, whereas some differences could be observed between total and free plasma T4 levels. The neonatal rise in free T4 in particular was higher than that observed for total T4 levels. Consequently, the enhancement of T4 availability for T3-generating cells could partly explain the neonatal T3 surge. In addition, plasma reverse T3 levels decreased progressively during the first 48 h of life in suckling lambs. These results suggest also that hypotrophy and/or nutritional status could affect neonatal thyroid function: plasma active iodothyronine levels decreased sharply in bottle-fed lambs (birthweight : 2.29 Kg) compared to suckling ones (birthweight : 3.57 Kg), whereas reverse T3 levels increased.