Effect of laboratory practices on the incidence rate of congenital hypothyroidism

Newborn screening (NBS) laboratories in the United States expanded their programs to include primary congenital hypothyroidism (CH) in the 1970s. An increase in the national CH-incidence rate since 1987 has been reported. Our goal was to analyze national data reported by state NBS programs and laboratories from 1991 to 2000 to determine the extent to which changing laboratory methods might have contributed to the reported rise in CH-incidence rate.

We used generalized estimating equations to analyze the association between the rate of confirmed cases of CH per 100000 live births and the initial screening method (thyroxine [T4] or thyrotropin [TSH] assay), the T4- and TSH-assay methods, the screening-test cutoff value used to report abnormal T4- or thyrotropin-assay results, and the performance of a second screen on >or=80% of newborns in the state. We then evaluated the association of CH rate with year after adjusting for any screening methodology or parameter that was significant in the univariate analysis.

During 1991-2000, laboratories that used a TSH assay for initial screening reported a 24% higher incidence rate of CH than those that used a T4 assay. The assay type also affected the incidence rate. Screening for T4 by enzyme immunometric assay (EIA) or fluoroimmunoassay (FIA) methods resulted in 38% and 24% higher incidence rates of CH, respectively, compared with the radioimmunoassay (RIA) method, whereas screening for TSH by the FIA method resulted in a 20% higher incidence rate of CH than did screening with radiochemical methods. During the decade studied, many laboratories changed their T4-assay method from RIA to either FIA or EIA; this particular change seemed to have the greatest impact on the CH-incidence rate.

Although the use of different laboratory methods and screening practices by NBS laboratories affected the incidence rate of CH, after adjusting for screening methodologies and parameters, an increasing incidence rate still persisted during the decade studied. Thus, there seem to be additional unknown factors that contributed to the reported increase in incidence rate.