Thyroid indices in arterial and venous cord blood: significantly greater levels of reverse triidothyronine in venous blood than in arterial blood.

Paired arterial and venous cord blood samples were obtained from 42 normal newborns (24 males and 18 females). T4 was determined in all paired samples. In addition, other indices were determined: T3 in 40, RT3 in 29, TBG in 29, thyroglobulin (Tg) in 14, and TSH in 11. Gender difference in any of the thyroid indices was not found. Arterial and venous cord serum thyroid indices correlated positively (T4, r = 0.743; T3, r = .907, rT3, r = .920; TBG, r = .752; Tg, r = .934, and TSH, r = .989; P less than 0.005). The difference between the means +/- SD of arterial and venous levels was significant (P less than 0.01) only for rT3 (191 +/- 43.2 v 224 +/- 55.8 ng/dL). Arterial (T4 v T3, r = .453, P less than .005; T4 v RT3, r = .660, P less than 0.005) and venous (T4 v T3, r = .620, P less than 0.005; T4 v rT3, r = .612, P less than 0.005); T3 and rT3 levels correlated positively with T4 levels. In contrast, T3 and rT3 levels for arterial (r = .216, P greater than 0.1) and venous (r = .216, P greater than 0.1) samples did not show a significant correlation. These data are in keeping with earlier reports for animal placental models studied in vitro, suggesting that placental inner ring deiodination of maternal thyroxine is a source of fetal RT3.