Reference intervals for thyroid-stimulating hormone (TSH) and free thyroxine (FT4) in infants' day 14-30 of life and a comparison with other studies

Objectives Established reference intervals of thyroid function in neonates are important; however, studies often consist of a small sample size or lack of clinical information. We aim to define reference intervals for thyroid-stimulating hormone (TSH) and free thyroxine (FT4) for infants aged 14-30 days. We also reviewed follow-up TSH for infants with initial values 10-20 mIU/L. Methods Venous TSH and FT4 of term babies aged 14-30 days with breast milk jaundice that had thyroid function test performed as part of a prolonged jaundice workout from September 2016 to March 2017 were analyzed. Electronic medical records were reviewed to ensure only well babies with no pathological causes of jaundice or conditions that may affect thyroid function were included. TSH and FT4 were analyzed using immunoassay analyzer Dxl 800, Beckman Coulter. Results There were no correlations between FT4 and TSH with gender, birth weight and ethnicity. Correlation coefficient between FT4 and total bilirubin was weak at 0.138 (p=0.001). No association was found between TSH and bilirubin levels. Mean FT4 was higher in the younger age group day 14-21 (p<0.01). There was no significant difference in TSH values between the age groups. Infants with mildly elevated TSH 10-20 mIU/L had normalized values on follow-up (mean, 11.41 vs. 4.42 mIU/L; p<0.01; 95%CI, 5.88-8.09). The following reference intervals (2.5-97.5th percentile) were derived: FT4 day 14-21 (n=513): 11.59-21.00 pmoL/L; FT4 day 22-30 (n=66): 10.14-19.60 pmoL/L; TSH day 14-30 (n=579): 1.90-10.34 mIU/L. Comparison between studies showed variations of reference intervals with different manufacturer assays, age and methodology. Conclusions Our reference intervals would be useful in the clinical setting. Infants with mildly elevated TSH could be monitored first instead of immediate treatment.