THYROID-FUNCTION IN VERY PRETERM INFANTS - INFLUENCES OF GESTATIONAL-AGE AND DISEASE

It is not known how immaturity and disease influence postnatal thyroid function in infants <30 wk of gestational age. We performed serial me asurements of plasma thyroxine (T-4), free T-4 (FT4), triiodothyronine (T-3), reverse T-3 (rT(3)), TSH, and T-4-binding globulin (TBG) in 10 0 infants of <30 wk of gestation, during the first 8 postnatal weeks, to investigate the influences of disease and gestational age on the ti me course of thyroid hormones. One hundred infants were divided twice into two groups: 1) in a group of 25-28 and of 28-30 wk of gestation; and 2) in a sick and a healthy group, with similar gestational ages. T he time course of T-4, FT4, T-3, TSH, and TBG, but not rT(3) differed significantly (p < 0.005) between the gestational age groups. T-4 and FT4 decreased to levels below the cord blood value with a deeper FT4 n adir on d 7 in the youngest group. Disease decreased T-4, FT4, T-3, TS H, and TBG concentrations especially during the 1st wk after birth (p < 0.005), However, the FT4 nadir on d 7 was similar in sick and health y infants, After 3 wk, T-4, FT4, T-3, and TBG were higher in the sick group compared with the healthy group, rT(3) levels were not increased in sick infants. We conclude that the extent of the FT4 decrease afte r birth in infants of <30 wk gestation is mainly influenced by gestati onal age and probably reflects a transient depletion of thyroidal horm one reserves. rT(3) cannot be used as a marker of nonthyroidal illness in very preterm infants.