Screening for congenital hypothyroidism: Current controversies

Purpose of review: Several programs in the United States and Canada have switched from primary T4-follow-up TSH screening to a primary TSH test, an approach used in many programs worldwide. This review examines the current controversies regarding the screening approaches for congenital hypothyroidism. Recent findings: In primary T4 screening test programs, selection of the 10th percentile cutoff for TSH testing leads to detection of 98 to 100% of cases, with a false-positive rate of 1.44%. In primary TSH screening test programs, an absolute TSH cutoff greater than 20 mu/L whole blood or greater than 40 mU/L serum leads to detection of 98 to 100% of cases, with a false-positive rate of 0.45%. Delayed TSH rise was detected by one program in 1:324 very low birth weight babies and overall occurs in approximately 1:40,000 infants with permanent congenital hypothyroidism. Most of these babies were born preterm or were sick, requiring admission to an NICU. Summary: Both primary T4 and primary TSH screening test programs do an excellent job of detecting babies with congenital hypothyroidism. Because both ultimately depend on a TSH determination to identify babies for serum testing, the number of missed cases and the false-positive rate are more dependent on individual cutoffs and whether programs choose to undertake second screening tests. A good case can be made for discretionary second testing in very low birth weight babies and sick term babies, and same sex twins. True primary T4 screening programs that choose to follow up babies with tow T4 levels, regardless of the TSH result, will detect babies with delayed TSH rise and central hypothyroidism.