Free thyroxine and thyroid-stimulating hormone reference intervals in very low birth weight infants at 3-6 weeks of life with the Beckman Coulter Unicel DxI 800

Objectives: To establish reference intervals for thyroid stimulating hormone (TSH) and free thyroxine (FT4) at 3-6 weeks of age in very low birth weight (VLBW) infants with the Beckman Coulter Unicel DxI 800 by gender, birth weight (BW) and gestational age (GA) subgroups.

Design and methods: A 4 year retrospective cohort of 308 VLBW infants (GA=27.9 weeks, BW=992.3g) was studied. All blood samples for TSH and FT4 were analyzed using the modified fTSH2 (TSH) and two-step competitive enzyme immunoassay (FT4). Reference intervals were evaluated according to the most recent Clinical and Laboratory Standards Institute (CLSI) guidelines.

Results: The study provides non-parametric 95% reference intervals with associated 90% confidence intervals for FT4 and TSH derived from 308 infants screened at a median of 31 days. The reference intervals for this population are TSH=1.14-11.04 mIU/L and FT4=10.9-21.4 pmol/L. TSH statistically differed according to birth weight (<1000 g vs 1000-1499 g) while FT4 differed according to gender and gestational age at time of testing (<32 weeks vs ≥ 32 weeks); however, these differences were not clinically significant and a single reference interval for each analyte is reported.

Conclusion: The results of this study highlight the importance and complexity of establishing appropriate reference intervals for thyroid function testing for the preterm population.