Relationship between circulating tri-iodothyronine and cortisol in the perinatal period in the foal.

Pre- and post partum changes in plasma T3 have been examined in relation to plasma cortisol in 23 newborn foals (12 full term, 6 premature and 5 'twilight'), and in 5 fetuses catheterized in late gestation. Blood samples were collected daily from the fetuses and from the neonates at 30-min intervals for 2 h after birth; the plasma was assayed for T3 and cortisol by standard radioimmunoassay methods. In the full-term foals, plasma cortisol and T3 concentrations were high at birth (67.4 +/- 6.1 and 4.3 +/- 0.3 ng/ml respectively) and rose to a maximum during the following 2-h period to 141.0 +/- 8.8 ng/ml and 8.4 +/- 0.5 ng/ml. The corresponding changes in the premature foals over the same period were significantly lower (P less than 0.01; 17.2 +/- 2.2 to 27.0 +/- 4.3 ng cortisol/ml and 2.9 +/- 0.5 to 5.4 +/- 0.6 ng T3/ml). In the 'twilight' group, intermediate increases in both cortisol and T3 were observed. In the 5 chronically catheterized fetuses, both plasma cortisol and T3 were low in the last few weeks of gestation (7.0 +/- 1.1 ng/ml and 1.2 +/- 0.2 ng/ml respectively). In 2 fetuses there was little or no increase in plasma cortisol or T3 before induction of labour at about 320 days; one foal was premature and the other 'twilight'. Infusion of adrenocorticotrophic hormone for 4.5 days into the third fetus from 309 days led to an increase in both cortisol and T3 and the birth of a viable, full term foal. The 2 remaining fetuses were delivered spontaneously, one at term and one at 300 days (well before term). The latter was viable but dysmature; it showed a slight prenatal rise in plasma cortisol with little change in T3. When all the data were combined from both fetuses and neonates a significant positive correlation between plasma T3 and cortisol was found (P less than 0.001). These results demonstrate a relationship between circulating cortisol and T3 in the perinatal period and show that prematurity is associated with low concentrations of both hormones.