Circulating 3 , 3 ' , 5 ' - Triiodothyronine ( Reverse T 3 ) in the Human Newborn INDER

samples was significantly higher than the level (41±2 ng per 100 ml) in 27 normal adult sera; the corresponding mean serum T4 of 12.7±0.8 ug per 100 ml in cord blood also was significantly higher than that (8.6±41.9 big per 100 ml) in 108 normal adults. By contrast, the mean serum T3 concentration in 15 cord blood samples, 24±3 ng per 100 ml, was significantly, lower than the value of 126±43.2 ng per 100 mIl measured in 108 normal adults. At 4 h of age the mean serum rTa concentration (165±413 ng per 100 ml) in six newborns was not significantly different from that in paired cord blood samples (194± 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 T:3 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 rT:3 failed to rise appreciably in these subjects, even though serum T3 and T4 showed the expected increments. Serum rT3 concentrations in 14-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 Xvalues were statisReceivcd for publication 13 Deccminbr 1974 and il rcviscd forin 12 Februarv 197-5. tically 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 xvere 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.