Plasma iodothyronine levels in lambs during the perinatal period: influence of thyrotropin injection.

Plasma thyroxine (T4) and triiodothyronine (T3) levels were measured in 7 pregnant ewes and their 9 chronically cannulated foetuses during the last 37 days of gestation and the first 5 days after parturition. The highest plasma T4 and T3 levels were measured in lambs during the hours following parturition. In 7 chronically cannulated 139-day old foetal lambs, the intravenous injection of ovine thyrotropin (oTSH; 7 mU per foetus) induced a significantly more intense increase in plasma T4 and T3 levels than that observed after the same dose of oTSH in seven 130-day old foetuses. In newborn lambs, the increase in plasma T4 levels after oTSH injection (3.5 mU per kg body weight) was not different in 8-hour old and 36-hour old animals, but was more intense at 120 h after birth than at 8 or 36 h after birth. The rise in plasma T3 levels after oTSH increased with age at 8, 36 and 120 h following birth. Plasma T4 and T3 measurements made in the same lambs before and after birth confirmed the fragmentary values obtained in foetal and newborn lambs in other previous studies. The rise in plasma iodothyronine levels after TSH injection in lambs increased at the end of gestation and the beginning of neonatal life, except during the first hours after delivery when these levels were high.