New age-adjusted cutoff levels for thyroid-stimulating hormone (TSH) from birth to term-corrected gestational age in extremely and very preterm infants may facilitate standardization of screening for congenital hypothyroidism diagnosis, according to study results published in Pediatrics.

As there is a wide variation in the recommended TSH cutoff levels to identify cases of congenital hypothyroidism in infants, the goal of this study was to determine TSH reference ranges for preterm infants born <32 weeks’ gestation using a statewide birth cohort of preterm infants.

The researchers included all preterm infants born <32 weeks’ gestation from 2012 to 2016 who underwent newborn screening in the state of Wisconsin. The patients were divided into 2 groups: extremely preterm (22-27 weeks; 1022 infants) and very preterm infants (28-31 weeks; 2115 infants). Serial TSH levels were analyzed (a total of 11,431 TSH values) and TSH percentiles were defined from birth to term-equivalent gestational age.

In very preterm infants, the median TSH value was 3.4 µIU/mL at birth. This value gradually declined, reaching a nadir at approximately 4 to 5 weeks of life, followed by a gradual increase. The 95th percentile TSH level followed a similar trend and was highest at birth, gradually declined until 5 to 6 weeks of life, and then increased gradually toward that of the term-corrected gestational age.

In extremely preterm infants, median TSH levels changed minimally from birth to term-corrected gestational age, but the 95th percentile TSH value was highest at birth and gradually declined until 10 weeks of life, followed by a slight increase.

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At 3 to 4 weeks of life, when repeat newborn screening is often performed, the median TSH level ranged from 2.5 to 2.6 µIU/mL in both infant groups. The 95th percentile TSH level at this point ranged from 11 to 11.8 µIU/mL for extremely premature infants and from 8.2 to 9 µIU/mL in very preterm infants.

The study had several limitations, including inability to generalize the results of the study to all preterm infants, as infants born at gestational ages from 32 to 36 weeks were not included in the study; TSH measurements were based on dried blood-spot and not serum levels; and a lack of data regarding chromosomal or multiple congenital anomalies.

“The current study reveals that [newborn screening of] TSH percentiles vary with increasing gestational age. This information will be valuable for [newborn screening] programs to better define postnatal age-adjusted TSH cutoffs for preterm infants and to standardize diagnosis,” concluded the researchers.

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Reference

Kaluarachchi DC, Allen DB, Eickhoff JC, Dawe SJ, Baker MW. Thyroid-stimulating hormone reference ranges for preterm infants. Pediatrics. 2019;144(2)e20190290.