CORD BLOOD REVERSE T3 IN NORMAL, PREMATURE, EUTHYROID LOW T4, AND HYPOTHYROID NEWBORNS

To assess whether 3,3’,5’-triiodothyronine (reverse T3, rT3) can predict neonatal thyroid status we have determined rT3,thyroxine (T4), thyrotropin (TSH), and rT3 /T4 (X1000) ratio in 57 full-term normal (group N), 32 premature (group P) with birth weights 1048 to 2430 g and gestation ages 28 to 37 weeks, 95 hypothyroxinemic (T4<7/ig/100 ml) full-term euthyroid (confirmed after 2 years) infants (group L), and 6 hypothyroid infants (group H). Alt values were expressed as mean ± SD. N had T4 10.2±2.2 /yg/100 ml, TSH 11.6±14.4 μU/ml, rT3 224.6±70.2 ng/100 ml, and rT3/T4 22.5 ±6.4. P had T4 8.1 ±2.2 /yg/100 ml, lower than N (p<0.01) but similar TSH 13.5 ±6.9 μU/ml. P had higher rT3 264.4 ±72.4 ng/100 ml (p< 0.05) and rT3/T4 34.3 ±13.1 (p<0.01) than N. L had T4 6.2±1.0 μg/100 ml, lower than N or P (p<0.01) yet similar TSH 10.3±5.9 /AJ/ml. rT3 was lower (160.9±72.0 ng/100 ml) than N or P (p<0.01). rT3/T4 ratio (27.6±15.6) was lower than P (p<0.05) but no different from N. H had Tt ranging from 2.9–7.2 //g/100 ml with TSH > 70 /AJ/ml in all. In group H rT3 was low in 50% (of 6) and infants with normal rT3 (false negative) values had iodide-induced myxedema, sepsis, or lingual thyroid. Low rT3 (false positives) were noted in 3.5% N, 3.1% P, and 12.6% L. These observations suggested that (a) an increased rT3/T4 ratio indicates prematurity, (b) a normal rT3 in the hypothyroid infant may indicate either a less severe degree of hypothyroidism or co-existing prematurity and systemic illness, (c) because rT3 varies with fetal maturity and available T4 precursor, rT3 is of limited value in screening for hypothyroidism. © 1978, Italian Society of Endocrinology (SIE). All rights reserved.