The high incidence of transient hypothyroxinemia observed in low-birth-weight infants has caused difficulty in the interpretation of total thyroxine screening test for hypothyroidism in premature and low-birth-weight infants.
This problem has resulted in the recall and additional testing of many infants, which is not only time consuming and expensive, but can also delay the determination of the true status of their thyroid funtion.
Present study was carried out to investigated the normal values of T3 and T4 in term infants and change in low-birth-weight infants.
Serum Ts, free T3, T4, free T4 and TSH concentration were determined by radioimmunoassay in 48 cord sera and in 64 newborn sera.
The following sesults were obtained.
1. Cord sera T3 and free T3 levels were all lower than normal adult values and there was significant correlation between birth weight and those levels.
While T4, free T4 and TSH concentration of cord sera showed higher than normal adult values.
2. Newborns were divided into two groups according to birth weight. Group I was infants with birth weight of 2,5009m or more, and group 11 was infants with birth weight less than 2,5009m.
Serum T3 and free T3 levels in group II was significantly lower than group I (p<0.01) and free T3 correlated positively with decreasing T3 levels. Like T3 and free T3, there was. positive correlation between T4 and free T4 and showed significantly lower levels in group 11 than I (p<0.01).
However, ratio of free T3/T3, free T4/T4 and T3/T4 showed no significant difference in those 2 groups.
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