Relevance of Transient Hypothyroxinemia of Prematurity and Congenital Hypothyroidism in Infants Less Than 27 Weeks' Gestation

Premature infants' thyroid stimulating hormone (TSH) progression differs from term infants due to their immature hypothalamic-pituitary-thyroid function (La Franchi, 2014). Limited studies have analyzed the TSH progression in preterm infants, specifically infants less than 27 weeks' gestation. Current screening of thyroid function in most Utah neonatal intensive care units (NICUs) for premature infants includes a state newborn screen (NBS) at 0 to 48 hours of age and a secondary screen at approximately 8 days of age. The NBS primarily screens TSH levels and has cut-off values of 40 milliunits per liter (mU/L) on the first screen and 20 mU/L on the second and third NBS. The objectives of this study were to define a TSH range for infants less than 27 weeks' gestation, to identify those outside the developed TSH range, placing them at risk for transient hypothyroxinemia of prematurity (THOP) and congenital hypothyroidism (CH), and propose a gestational-age-adjusted TSH cut-off value for the Utah Newborn Screen Program. Data were collected from 636 hospital records of infants born between 2013 to 2017, who were less than 27 weeks' gestation. Methods included obtaining de-identified data of participants, collecting demographic characteristics, retrospectively analyzing the data, and developing an interval range for the first, second, and third screen utilizing two standard deviations above and below the mean. The calculated range utilizing two standard deviations for the first, second, and third newborn screen for the entire population was 0 to 21.8, 0 to 12.3, and 0 to 14.1 mU/L, which captured greater than 95 percent of the sample population. Ten percent of the preterm infant population had TSH levels less than one on the initial NBS. The first NBS TSH mean for preterm infants was lower than Utah newborns, suggesting that cases of CH may be missed in preterm infants when using the current 40 mU/L as the TSH cut-off value. Gestational-age-adjusted TSH cut-off of 30 mU/L in the first newborn screen may capture an additional one RELEVANCE OF THOP AND CH 3 percent of preterm infants who are at risk for CH, leading to earlier diagnosis and treatment. Ten percent of preterm infants had TSH values less than 1 mU/L on the first NBS, suggesting that these infants had a blunted or absent TSH surge and should be followed on serial newborn screens. The analysis showed that TSH values of preterm infants remain elevated beyond one month of age, putting them at risk for transient hypothyroidism. Despite being below the TSH value cut-off value in the 2nd and 3rd NBS, those preterm infants who have persistent elevated TSH, classified as above the developed range, should be reported and followed with further serum TSH and T4 testing.