Abstract
Abstract. Simultaneous serum concentrations of TSH, total thyroxine (T4) and triiodothyronine (T3) were determined in 93 fullterm (FT), 37 small‐for‐gestational age (SGA) and 38 preterm (PT) babies with a postnatal age from 2 to 144 hours. In addition, TSH, T4 and T3 concentrations were measured in cord sera from 27 FT, 4 SGA and 5 PT babies and in venous blood from 20 mothers at delivery. Cord blood concentrations of TSH were higher and T4 and T3 concentrations were lower than seen in the mothers. Serum concentrations of TSH were high during the first day of life followed by a decline. There was no statistically significant difference between serum TSH concentrations of the three groups of newborns. On the 5th day of life no elevated serum TSH values were found in any of the groups (TSH<5mU/l). Serum concentrations of thyroid hormones increased after birth and reached maximum levels within 24 hours in all groups. The relative increases above cord level were of the same magnitude in the newborns: Two times for serum T4 and six times for serum T3. The thyroid hormone concentrations in blood samples from FT babies decreased from the second day of life, whereas in low birth weight newborns the decreases were more variable. The serum levels of T4 and T3 were significantly different in the three groups of newborns, the highest values were seen in FT and the lowest values in PT babies. In contrast, the ratios between molar serum concentrations of T4 and T3 were found to be highest in PT, lower in SGA and lowest in FT babies, approaching maternal values during the first week of life. The data are discussed with regard to hormone secretion, thyroxine‐binding capacity and peripheral T4 to T3 conversion in the three groups of newborns. It is concluded that from day 5 after birth serum TSH determinations, alone or in combination with serum T4, seem to be the method of choice in screening for congenital hypothyroidism.
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