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

Abstract

Introduction: In adults a decrease in serum T3 and an increase in rT3 are the most common changes in thyroid hormone levels in response to a variety of acute and chronic illnesses. With more severe illnesses serum T4 levels also decrease and this is associated with a worse prognosis. Thyroid hormone levels in infants are crucially determined by the inter-relationship between gestational and postnatal age but it remains unclear whether changes in thyroid hormone status in response to illness are similar across the wide gestational age range of prematurity. Aim: The purpose of this study was to relate severity of illness at 1, 7, 14 and 28 postnatal days in preterm infant groups 23–27 (n=73), 28–30 (n=160) and 31–34 (n=208) weeks gestation, to the corresponding sera levels of T4, FT4, TBG, TSH, T3, rT3 and T4S. Methods: The British Association of Perinatal Medicine and Neonatal Nurses Association 1992 scoring categories (Arch Dis Child; 67: 868–9) 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. Results: In Level 1 infants there were significant reductions in: T3 at 7 days (28–30 weeks), 14 and 28 days (23–27 and 28–30 weeks); T4 at 7, 14 and 28 days (23–27 weeks), at 14 and 28 days (28–30 weeks), and at day 7 (31–34 weeks); FT4 at 14 days (23–27 weeks). TSH was unchanged in all groups at all ages. Conclusion: The clear message from our data is that T4 and T3 are substantially reduced in infants with severe illness, irrespective of gestational age, yet TSH remains unchanged.