Age-dependent cutoff values in screening newborns for hypothyroidism.

Objectives: We wanted to develop age-related reference (cutoff) values and an algorithm to identify babies at low, moderate, and high risk for hypothyroidism of any cause. We used thyroid-stimulating hormone (TSH) as the primary tool, and thyroxine (T4) as part of a confirmatory test. Our data permitted us to estimate cutoff values for newborns at <24 h, 24 to 47 h, 48 to 71 h, 72 to 95 h, and > or =96 h after birth.

Methods: We used a time-resolved fluoroimmunoassay method for TSH and T4 with the AutoDELPHIA instrument (Perkin-Elmer Life Sciences, Turku, Finland). TESTING ALGORITHM: We developed a conservative algorithm for TSH and T4 testing. In the initial screening, we used a > or =20 microIU/ml cutoff for TSH to identify those babies of any age who required confirmatory testing on a repunched filter paper blood specimen.

Results: In 161,244 newborns tested during 2002, we found 8,035 babies with TSH values > or =20 microIU/ml. Graphs of the values for TSH vs. age in hours revealed the possibility of using more than one cutoff value. The general finding was that the cutoff values decreased with increasing age of the newborn.

Conclusions: Based on our findings, we conclude that testing babies who are <24 h old is not recommended and should only be performed if no other specimen is available. A high TSH in babies <24 h old is unreliable for screening newborns for hypothyroidism. We routinely stipulate that the infant be at least 48 h old for TSH and T4 testing. If not, the cutoff value must be set to a higher value to prevent an excessive number of false-positive results; however, this increases the chance of missing a truly hypothyroid baby. We designated newborns as being at "low" (LR), "moderate" (MR), or at "high" risk (HR) for hypothyroidism. The TSH test continues to be a screening test; and follow-up quantitative testing and clinical evaluation are needed for all babies identified as being at MR or HR for hypothyroidism.

Setting: Newborn Screening Laboratory of the Ohio Department of Health, Columbus, Ohio.