Responsiveness of L‐S Ratio of the Amniotic Fluid To Intra‐Amniotic Administran of Thyroxine: Role of fetal age

Abstract

Since 1982 we have accelerated fetal maturation with intra-amniotic thyroxine (T4) in more than 140 patients. The purpose of this analysis was to determine the rate of change of the ratio of lecithin to sphingomyelin (L/S) after administration of T4 at different gestational ages, and to compare the responses to the first and the second administration of T4. Fifty-nine cases in which administration of T4 was continued for 2 weeks or more, and in which at least 3 determinations of L/S had been performed, were identified. Gestational age of the fetus at the inititiation of treatment ranged from 26 to 31 weeks (mean 29.3 weeks). Thyroxine was administered weekly in 200 to 500 mcg doses. Administration of T4 prior to the 27th week did not change the L/S. From the 27th and 31st week of gestation, the slope of the L/S, after the initial dose of T4 increased from 0.33/wk to 1.05/wk. In contrast the slope of the untreated patients changed little reaching a maximum of 0.22 at the 33 week. L/S greater than 2 was observed in 80% of cases after 2 weeks of therapy, when it had been initiated after the 26th week. The response to the second dose was about twice that of the first in fetuses less than or equal to 30 weeks, but was similar to that seen after the initial dose in fetuses greater than 30 weeks. Phosphatidylglycerol (PG) was detected in amniotic fluid in 51% of cases after 2 weeks of treatment, and in 6 instances as early as at the 30th week. Responsiveness of L/S to T4 treatment of the fetus is a function of gestational age and of prior exposure to T4.