THYROID-FUNCTION IN VERY-LOW-BIRTH-WEIGHT INFANTS - EFFECTS ON NEONATAL-HYPOTHYROIDISM SCREENING

Objectives: To supply normative data for screening thyroxine (T-4) and thyrotropin concentrations correlated with birth weight and age at sc reening of infants with birth weights ranging from 400 to 5500 gm, and to document the effects of screening of very low birth weight (VLBW) infants, because VLBW infants comprise 0.86% of surviving newborn infa nts and have very low total T-4 concentrations with normal or elevated free T-4 concentrations as a result of deficient protein binding of t hyroid hormones. Study design. Both retrospective and prospectives stu dies were used. We conducted retrospective analyses of screening of T- 4 and thyrotropin concentrations in 9,324 term, 18,946 low birth weigh t, and 3,450 VLBW infants in Massachusetts, and a prospective study of T-4 and thyrotropin concentrations in 48 VLBW infants at 2 weeks of a ge. Forty of the infants also had hormone measurements at 4 weeks, 29 at 8 weeks of age, and 24 had analysis of cord blood samples. Results: Median T-4 concentrations for each weight group (in 250 gm increments ) increased progressively and significantly up to 2500 gm, Of the surv iving VLBW infants, 1.5% had screening T-4 concentrations that were un measurably low (<3.9 nmol/L (0.3 mu g/dl)). The mean T-4 concentration varied with age at screening, increasing from cord blood concentratio ns to a peak at 1 to 3 days of age and thereafter decreasing to a nadi r at about 2 weeks in both low birth weight and VLBW infants. In VLBW infants the mean concentrations return to the level of 1 to 3 days by 4 to 8 weeks of age. The incidence of screening thyrotropin concentrat ions greater than or equal to 40 mU/L correlates inversely with weight , The incidence of early, transient hypothyroidism in VLBW infants def ined by this thyrotropin concentration was eight times that in term in fants. Two infants had late-onset, transient hypothyroidism at 2 and 7 weeks, respectively. Conclusions: The normative data related to birth weight and age at screening allow proper interpretation of VLBW resul ts for primary T-4 and primary thyrotropin screening programs, Screeni ng of the concentrations of T-4 and thyrotropin in VLBW increases the number of secondary measurements of T-4 in a primary thyrotropin scree ning program and the number of secondary thyrotropin measurements in a primary T-4 screening program by 6% and 9%, respectively, We recommen d screening analyses for VLBW infants in the latter part of the first week of life and again at 2 and 4 to 6 weeks of age. This protocol wou ld increase the number of screening analyses by 1.6%.