Serum Thyroid Hormones in Preterm Infants and Relationships to Indices of Severity of Intercurrent Illness

The purpose of this study was to relate severity of illness at 1, 7, 14, and 28 postnatal days in preterm infants groups, 23–27 (n = 73), 28–30 (n = 160), and 31–34 (n = 208) wk gestation, to the corresponding sera levels of T4, free T4, T4-binding globulin, TSH, T3, rT3, and T4 sulfate. The British Association of Perinatal Medicine and Neonatal Nurses Association 1992 scoring categories (published elsewhere) were used as an index of illness severity: level 1 (maximal intensive care) was compared with level 2 (high-dependency intensive care) combined with level 3 (special care); infants were scored on 1, 7, 14, and 28 postnatal days. In level 1 infants, there were significant reductions in T3 at 7 d (28–30 wk), 14, and 28 d (23–27 and 28–30 wk); T4 at 7, 14, and 28 d (23–27 wk); at 14 and 28 d (28–30 wk); and at 7 d (31–34 wk); and free T4 at 14 d (23–27 wk). TSH was unchanged in all groups at all ages and with reductions in T4 and T3 being the key features of severe illness in extreme preterm infants.