Repeat Thyroid Testing Can Improve Outcomes in Very Low Birthweight Infants

In very low birthweight infants (VLBW) whose initial newborn screening was within normal reference range, retesting for hypothyroidism at 36 weeks of corrected gestational age identified infants needing surveillance for hypothyroidism, according to research findings published in the Journal of Pediatrics.

The study assessed for possible missed hypothyroidism in 398 infants with VLBW (birthweight, 1138 grams; median gestational age, 29 weeks) whose initial newborn screening was within the normal reference range.

In the study, the investigators evaluated serum thyrotropin or thyroid-stimulating hormone (TSH) at 36 weeks of corrected gestational age or at hospital discharge, whichever came first. This retest TSH was assessed to identify the prevalence and factors associated with retest TSH value of ≥5 mU/L, which researchers said was a concentrate that indicated a need for close follow-up and prompt therapy for hypothyroidism. Additionally, the researchers also evaluated the utility of alternative cut-off values for newborn screening TSH.

The retest TSH was obtained at 49.5 days following birth, but this ranged between 12 and 137 days after birth. The median retest TSH was 3.1 mU/L vs an initial newborn screening value of 4.2 mU/L. Approximately 18.3% (n=73) had a retest TSH of ≥5 mU/L. These infants likely “require continued follow-up after discharge from the NICU in order to identify those who require thyroid hormone replacement for true congenital hypothyroidism,” the researchers explained.

Adjustment of the newborn screening cutoff to ≥15 or ≥10 mU/L enabled identification of <50% of infants with TSH ≥5 mU/L, which led to 6% false positives and >70% false negatives. According to multiple regression modeling, approximately 35% of variance in the retest TSH value was explained by birthweight (P <.001), newborn screening TSH concentration (P <.001), and gestational age (P =.005).

Limitations of the study included its retrospective nature as well as the lack of long-term follow-up of the infants discharged from the NICU who had “concerning TSH concentrations,” according to the researchers.

 “Larger studies are needed to ascertain the ideal timing and the optimal method for assessing the risk of hypothyroidism in VLBW infants,” the researchers concluded.

Reference

Rose SR, Blunden CE, Jarrett OO, et al. Utility of repeat testing for congenital hypothyroidism in very low birthweight infants. J Pediatr. Accepted November 1, 2021. doi:10.1016/j.jpeds.2021.11.003