The thyroid-system function in preterm infants of postmenstrual ages of 31 weeks or less: evidence for a "transient lazy thyroid system".

A prospective study was conducted in order to evaluate thyroid function in 20 healthy and 18 sick preterm infants with postmenstrual ages of 31 weeks or less. The clinical condition of both groups was compared using a "Neonatal Special Care Evolution Score". The effect of thyroid hormone treatment, given from D10 on to the sick infants, was also studied. TSH, thyroid hormone levels (TG, T4, T3, rT3, FT4 and FT3) and TBG were measured by radioimmunoassays at D0, D10, D20, D30 and D40. Healthy preterm infants on D0 have a median TSH level of 22 microU/ml and a high TG level of 200 ng/ml; thereafter, median serum levels decrease to 6 microU/ml and 35 ng/ml respectively. During the same period, median serum T4 is maintained at a low level of about 6-8 micrograms/dl, median serum T3 gradually increases from 80 ng/dl on D0 to 150 ng/dl on D40, and median serum rT3 decreases beyond D10 from a plateau of 200 ng/dl to about 100 ng/dl. In the sick preterm infants before treatment, serum TSH is as in the control group but serum T4, T3 and rT3 on D10 are well below the control values (P = 0.005). In all conditions, there is a significant correlation between serum T4 and FT4, and between serum T3 and FT3. Thyroxine, given to the sick preterm infants from D10 on, brings median serum T4 values closely to the ones of the control group whereas serum levels of TSH and TG are unaffected and similar to those of the healthy preterm infants. Furthermore, thyroxine brings serum rT3 within the range of the control group but leaves median serum T3 at a low level of about 50 ng/dl. On T3 treatment, serum T3 normalizes but rT3 and particularly T4 tend to decline further. In the conditions of this study a significant difference in TBG level is not proven. Although an untreated sick group was not enrolled in the study, thyroid hormone treatment brought the "Neonatal Special Care Evolution Score" of the treated sick infants closer to that of the healthy preterm infants. In the sick preterm infant with failure to thrive on D10, there is an impaired thyroid discharge of T4 in spite of serum TSH values not different from those of the control group.(ABSTRACT TRUNCATED AT 400 WORDS)