OBJECTIVE To supply normative data for screening thyroxine (T4) and thyrotropin concentrations correlated with birth weight and age at screening 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 infants and have very low total T4 concentrations with normal or elevated free T4 concentrations as a result of deficient protein binding of thyroid hormones. METHODS Both retrospective and prospective studies were used. We conducted retrospective analyses of screening of T4 and thyrotropin concentrations in 9,324 term, 18,946 low birth weight, and 3,450 VLBW infants in Massachusetts, and a prospective study of T4 and thyrotropin concentrations in 48 VLBW infants at 2 weeks of age. 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 T4 concentrations for each weight group (in 250 gm increments) increased progressively and significantly up to 2500 gm. Of the surviving VLBW infants, 1.5% had screening T4 concentrations that were unmeasurably low (<3.9 nmol/L (0.3 microgram/dl)). The mean T4 concentration varied with age at screening, increasing from cord blood concentrations to a peak at 1 to 3 days of age and thereafter decreasing to a nadir 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 concentrations > or = 40 mU/L correlates inversely with weight. The incidence of early, transient hypothyroidism in VLBW infants defined by this thyrotropin concentration was eight times that in term infants. 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 results for primary T4 and primary thyrotropin screening programs. Screening of the concentrations of T4 and thyrotropin in VLBW increases the number of secondary measurements of T4 in a primary thyrotropin screening program and the number of secondary thyrotropin measurements in a primary T4 screening program by 6% and 9%, respectively. We recommend 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 would increase the number of screening analyses by 1.6%.