Circulating 3,3', 5'-triiodothyronine (reverse T3) in the human newborn.

Serum concentrations of 3,3',5'-triiodothyronine (reverse T3 rT3), 3,3',5-triiodothyronine (T), and thyroxine (T4) were measured in cord blood and invenous blood samples obtained between 2 h and 30 days of postnatal life from healthy full-term newborn infants. The mean serum rT3 concentration of (mean plus or minus SE) 151 plus or minus 12 ng per 100 ml in 18 cord blood samples was significantly higher than the level (41 plus or minus 2 ng per 100 ml) in 27 normal adult sera; the corresponding mean serum T4 of 12.7 plus or minus 0.8 mug per 100 ml in cord blood also was significantly higher than that (8.6 plus or minus 1.9 mug per 100 ml) in 108 normal adults. By contrast, the mean serum T3 concentration in 15 cord blood samples, 24 plus or minus 3 mg per 100 ml, was significantly lower than the value of 126 plus or minus 3.2 ng per 100 ml measured in 108 normal adults. At 4 h of age the mean serum rT3 concentration (165 plus or minus 13 ng per 100 ml) in six newborns was 4ot significantly different from that in paired cord blood samples (194 plus or minus 25 ng per 100 ml); on the other hand, whenever, studied, the mean serum T3 and T4 levels were significantly higher at 4 h than at birth. The failure of serum rT3 concentrations to rise after delivery in response to the early neonatal thyrotropin (TSH) surge and at a time when serum T3 and T4 levels increase significantly prompted a study of the rT3 response to 10 IU of intramuscular TSH in three healthy adult subjects. Just as in the newborns, serum rT3 failed to rise appreciably in these subjects, even though serum T3 and T4 showed the expected increments. Serum rT3 concentrations in 1-4 day-old newborn infants did not differ significantly from values in the cord blood but were significantly lower in older neonates. The mean serum rT3 level in 5-7-day-old infants was higher than that in normal adults, but in 9-11 day and 20-30-day-old infants, mean rT3 values were statistically similar to the adult value. The mean serum T3 concentrations in neonates between 1-30 days old were either higher than or comparable to the values of normal adults. The mean serum T4 concentrations in neonates between birth and 30 days of age were significantly higher than the mean adult level. The mean serum rT3 to T4 ratios (rT3/T4) were elevated in 1-4-day-old neonates; the values in older neonates were similar to those in adults. These results suggest that (a) factors other than TSH are important modulators of serum rT3 in man; (b) high serum rT3 concentration in the newborn becomes comparable to that in the normal adult by 9-11 days of neonatal life.