Thyroxine screening values in premature infants.

OBJECTIVES: Premature infants often have low thyroxine levels when compared to fullterm infants. We sought to determine gestational age specific normal ranges for thyroxine screening results for premature infants in neonatal intensive care units. METHODS: Thyroid screening results for infants less than 38 weeks gestation admitted to two NICUs were examined. For each sample the thyroxine Z-score was computed using parameters from fullterm infants. The mean thyroxine Z-score was calculated for each gestational age for days of life 1, 2, 3-7, 8-14, 15-21, 22-28, and 29-60. RESULTS: There were 1144 specimens obtained from 543 premature infants. The mean thyroxine Z-score was below 0 for almost every gestational age and days-of-life category. The mean thyroxine Z-score increased with gestational age, but did not rise with increasing postpartum age. CONCLUSION: The data show that normal thyroxine Z-scores for premature infants are lower than for fullterm infants and remain low at least as long as the infants remain ill.

Thyroid screening for early discharged infants.

OBJECTIVE: As neonatal discharge before 24 hours of life becomes commonplace, the rejection of congenital hypothyroidism (CH) screening specimens obtained too early has created the need for numerous additional tests. We sought to determine whether the specimens obtained before 24 hours could be used safely. METHODS: During a 31-day period we measured thyrotropin in all thyroid-screening specimens that had been obtained before 24 hours. We also examined the early specimens from every infant diagnosed in New Jersey with CH during 1993 or 1994. RESULTS: Among the 663 specimens, those obtained at or before 12 hours and those from infants with birth weights less than 2500 g had too many low thyroxine results to be useful. Among the 515 specimens obtained at more than 12 to 24 hours from newborns weighing 2500 g or more, 37 (7%) had low thyroxine levels and 12 (2.3%) had thyrotropin levels of 20 microIU/mL (mU/L) or higher. Four hundred seventy-one of the 515 infants had subsequent specimens obtained at more than 24 hours, and none of the results were abnormal. There was no child weighing more than or equal to 2500 g who was diagnosed with CH in 1993 and 1994 whose specimen obtained at 24 hours or less was normal. CONCLUSIONS: Accepting specimens obtained at more than 12 to 24 hours from infants weighing 2500 g or more would have resulted in more than the usual number of false-positive results but no false-negative results. This would have decreased the requests for additional specimens by more than 90%.