Amniotic and allantoic fluid concentrations of thyroxine, 3,3′,5′-tri-iodothyronine, 3,3′-di-iodothyronine and 3′,5′-di-iodothyronine in the pig during the period of gestation

Abstract

Thyroxine (T4), 3,3',5'-tri-iodothyronine (reverse T3; rT3) and di-iodothyronines (3,3'-T2 and 3',5'-T2) were measured in pig amniotic fluid (AF) and allantoic fluid (Al) between 32 and 113 days of normal pregnancy. Low but measurable quantities of T4 in AF and Al (2.1 +/- 0.3 and 3.2 +/- 0.5 nmol/l respectively) were found before the onset of fetal thyroid gland function, which indicates the maternal source of T4. The presence of rT3 (55.8 +/- 4.1 pmol/l in AF and 49.8 +/- 5.3 pmol/l in Al), 3,3'-T2 (45.5 +/- 0.6 pmol/l in AF and 49.2 +/- 9.2 pmol/l in Al) and 3',5'-T2 (20.8 +/- 2.6 pmol/l in AF and 24.0 +/- 2.2 pmol/l in Al) may be attributed to the monodeiodinase system already active in fetal pig tissues in early pregnancy, as demonstrated previously. T3 concentration was undetectable in both AF and Al. An approximately twofold increase in the levels of T4, rT3 and T2s in AF and Al at mid-gestation was observed. T4 and rT3 in AF showed a positive correlation with protein concentrations. AF rT3 concentration (but not T4) correlated with rT3 in the cord and maternal serum. The 3,3'-T2 and 3',5'-T2 in AF and Al showed parallel changes to rT3, while the rT3/3,3'-T2 and rT3/3',5'-T2 molar ratios remained constant. T4 concentrations in AF and Al were markedly lower than in corresponding maternal and fetal serum; the rT3 concentration in Al was equal to that in AF and two to four times lower than in fetal serum. In spite of differences between serum hormone patterns in the pig and human near term, iodothyronine concentrations in AF showed some similarities, mainly the following: undetectable T3, a strong correlation between rT3, T4 and AF total protein and the presence of 3,3'-T2 and 3',5'-T2 in measurable levels. Comparative data for Al, except the ones in the present study in the pig, are not available.