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How well does the capillary thyroid-stimulating hormone test for newborn thyroid screening predict the venous free thyroxine level?
  1. Tzveta Pokrovska1,
  2. Jeremy Jones2,
  3. M Guftar Shaikh2,
  4. Sarah Smith3,
  5. Malcolm D C Donaldson1
  1. 1Section of Child Health, University of Glasgow School of Medicine, Royal Hospital for Children, Queen Elizabeth University Hospital, Glasgow, UK
  2. 2NHS Greater Glasgow and Clyde, Royal Hospital for Children, Queen Elizabeth University Hospital, Glasgow, UK
  3. 3Newborn Screening Laboratory, West of Scotland Genetic Services, Queen Elizabeth University Hospital, Glasgow, UK
  1. Correspondence to Dr Malcolm D C Donaldson, Child Health Section of University of Glasgow School of Medicine, Queen Elizabeth University Hospital, Govan Road, Glasgow, G51 4TF, UK; malcolm.donaldson{at}glasgow.ac.uk

Abstract

Objectives To determine, in newborn infants referred with elevated capillary thyroid-stimulating hormone (TSH), a threshold below which a frankly subnormal venous free thyroxine (fT4) level of <10 pmol/L is unlikely, so that treatment with levo-thyroxine (L-T4) might be deferred until venous thyroid function tests (TFTs) become available.

Subjects and methods All infants referred in Scotland since 1979 with capillary TSH elevation were studied, with particular focus on infants screened using the AutoDELFIA assay between 2002 and 2013.

Results Of the 321 infants referred with capillary TSH elevation using AutoDELFIA, 35 were excluded (fT4/TSH unavailable (12), venous sample either preceding or >10 days after capillary sampling (13, 10)), leaving 286 eligible for analysis (208 definite/probable hypothyroidism, 61 transient TSH elevation, 17 of uncertain thyroid status). Capillary TSH and venous T4 were strongly correlated (Spearman's rank correlation coefficient −0.707355). The optimal capillary TSH threshold for predicting a venous fT4 of <10 pmol/L was found to be >40 mU/L (90.3% sensitivity and 65.9% specificity compared with 90.25% and 59.1% for >35 mU/L and 88.3% and 68.2% for >45 mU/L). 93 infants (32.5%) had capillary TSH ≤40 mU/L at referral of whom 15 (9.7%) had venous fT4 <10 pmol/L, comprising seven with true congenital hypothyroidism, five with transient TSH elevation and three with uncertain status, two of whom died.

Conclusion For infants in whom capillary TSH is ≤40 mU/L, it is reasonable to defer L-T4 treatment until venous TFT results are known provided that the latter become available quickly.

  • Endocrinology
  • Screening
  • Neonatology

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