Hypothyroxinemia in the low birth-weight infant in the screening of congenital hypothyroidism

Inside a pilot screening program for Congenital Hypothyroidism, T4 and TSH have been tested in sick and healty preteam and fullterm low birth weight (LBW) newborns during the first two months of life, 36 newborns affected by respiratory distress syndrome and 15 by sepsis have been included in the study. Blood samples were collected by heel puncture on 3rd, 10th, 20th, 40th and, in some cases, up to 60th day of life, and adsorbed on filter paper. Our findings show that hypothyroxinaemia in LBW newborns is strictly related to gestational age. In fact, among preterm infants with GA less than or equal to 33 weeks, 25 subjects (69,44%) showed T4 levels less than or equal to 6 micrograms/dl and 5 infants (13,88%) had T4 concentrations less than or equal to 2 micrograms/dl. The incidence of subjects with T4 values less than or equal to 6 micrograms/dl falls to 42,18% in the group of infants with GA = 34-36 weeks and to 17,27% in the group of fullterm LBW infants. None of these newborns showed thyroxine levels less than or equal to 2 micrograms/dl. All the examined infants showed normal TSH levels. The low T4 values may appear soon after birth or later (3rd-20th day of life) and sometimes persist up to 40th or 60th day, despite of always normal TSH levels. The mean of low T4 values at each sampling time is strictly and directly related to gestational age.