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Newborn screening for congenital hypothyroidism (CHT) is generally regarded as a highly successful public health measure. It was officially added to the UK newborn screening programme for phenylketonuria in 19811 though many areas had already started. A two-tier protocol based on assay of thyrotropin (thyroid-stimulating hormone, TSH) was recommended, with a second blood-spot sample taken at 2–6 weeks of age from babies with borderline initial results. As found elsewhere in Europe, the incidence of screening-detected cases in the early years was approximately double that of the clinically diagnosed disorder (table 1).
Presumptive-positive rates (cases referred for clinical evaluation) have increased further over time, partly because many laboratories lowered their primary cut-offs as more sensitive TSH assays became available.6–9 However, other changes have contributed as the Sheffield newborn screening laboratory has experienced increased presumptive-positive rates even though the primary cut-off has remained unchanged.
The Sheffield region laboratory serves the East Midlands and South Yorkshire and has screened between 55 000 and 75 000 babies a year. The screening protocol (figure 1) follows the outline scheme recommended in 1981.1 Between 1980 and 1985 (inclusive), the Pharmacia Phadebas Dry-Spot TSH assay was used, with 10 mU/L primary and second sample cut-offs and 40 mU/L as the secondary cut-off leading to immediate clinical referral. Incidence and sex ratios were broadly similar to those recorded for 1982–1984 by the UK Medical Research Council (MRC) Register5 and from Wales10 and Scotland11 over longer periods.
From August 2001 onwards, Sheffield used the Perkin-Elmer DELFIA assay, with either AutoDELFIA or Perkin-Elmer Genetic Screening Processor instrumentation, and a secondary cut-off of 20 mU/L …