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The Hypothalamic-Pituitary-Thyroid Axis in Preterm Infants; Changes in the First 24 Hours of Postnatal Life

Department of Paediatrics (N.M., T.G.M.), The Rotunda Hospital, Dublin 1, Ireland; Tayside Institute of Child Health (R.H.) and Department of Epidemiology and Public Health (S.A.O., F.L.R.W.), University of Dundee, Ninewells Hospital and Medical School, Dundee DD1 9SY, Scotland; Department of Internal Medicine (H.v.T., T.J.V.), Erasmus University Medical School, 3000 DR Rotterdam, The Netherlands; and Nuclear Medicine Service (S.-Y.W.), Veterans Affairs Medical Center, University of California–Irvine Medical Center, Long Beach, California 90822-5201

Address all correspondence and requests for reprints to: Dr. F. L. R. Williams, Department of Epidemiology and Public Health, University of Dundee, Ninewells Hospital and Medical School, Dundee DD1 9SY, Scotland, United Kingdom. E-mail: f.l.r.williams{at}dundee.ac.uk.

The purpose of this study was to measure serum T₄, free T₄, TSH, T₃, rT₃, T₄ sulfate, and thyroxine binding globulin at four time points within the first 24 h of life (cord and 1, 7, and 24 h) in infants between 24 and 34 wk gestation. The infants were subdivided into gestational age groups: 24–27 wk (n = 22); 28–30 wk (n = 26); and 31–34 wk (n = 24). The TSH surge in the first hour of postnatal life was markedly attenuated in infants of 24–27 wk gestation [8 compared with 20 (28–30 wk) and 23 mU/liter (31–34 wk)]. T₄ levels in the most immature group declined over the first 24 h, whereas levels increased in the more mature groups [mean cord and 24-h levels: 65 and 59 (NS) vs. 70 and 84 (P < 0.002) vs. 98 and 125 (NS) nmol/liter]. Free T₄ and T₃ showed only small, transient increases in the most immature group and progressively larger and sustained increases in the other gestational groups. rT₃ and T₄ sulfate levels in cord serum were higher in the most immature infants, and in all groups levels decreased initially and then variably increased. The features of a severely attenuated or failed hypothalamic-pituitary-thyroid response to delivery critically define this 24- to 27-wk group as distinct from more mature preterm infants.

This work was supported by Commission of European Community (QLG3-2000-00930), Chief Scientist’s Office Scottish Executive (K/MRS/50/C741), Paediatric Metabolic Fund, Wellcome Trust, and Tenovus (Scotland).

Abbreviations: D3, Type III iodothyronine deiodinase; FT4, free T₄; TBG, thyroxine binding globulin; T4S, T₄ sulfate.

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