Hormonal status in the newborn lamb (cortisol, T3, T4). Relationships to the birth weight and the length of gestation: effect of the litter size.

Abstract

The experiments reported involve 18 Romanov lambs born spontaneously between 135 and 145 days of gestation. Their birth weights ranged between 1.22 and 3.55 kg. The neonatal changes in plasma cortisol, triiodothyronine and thyroxine levels were observed from birth to 30 days postpartum, and related to the length of gestation and the birth weight. Positive correlations were found between: (1) plasma T3 (2, 4, 8 h and 11 days after birth), T4 levels (4 h after birth) and the birth weight; (2) plasma T3 levels (2, 4 and 8 h after birth) and the weight gain during the 1st month of life, and (3) plasma cortisol levels at birth, 8 and 48h postpartum and the litter size. Negative correlations were found between: (1) plasma cortisol levels (at birth, 2, 4, 8 and 48 h postpartum) and the birth weight; (2) plasma cortisol levels (2, 8 and 48 h after birth) and the length of gestation, and (3) values of the T3/T4 ratio and the litter size. A tentative scheme of the interactions between litter size, hormonal plasma levels, length of gestation and birth weight is proposed. Moreover, the effects of placental insufficiency and insufficient plasma levels of thyroid hormones could explain partly the high frequency of disease which occurs in low birth weight lambs.