CONTEXT: Active surveillance of primary congenital hypothyroidism (CH) in a multi-ethnic population with established newborn bloodspot screening. OBJECTIVE: To estimate performance of newborn screening for CH at different test thresholds; to calculate incidence of primary CH. DESIGN: Prospective surveillance undertaken from June 2011 to June 2012 with three-year follow-up of outcomes. Relative likelihood ratios (rLRs) estimated to compare bloodspot thyroid-stimulating hormone (TSH) test thresholds of 6mU/L and 8mU/L, with the nationally recommended standard of 10mU/L for a presumptive positive result. SETTING: UK National Health Service. PATIENTS: Clinician notification of children aged under five years investigated following clinical presentation or presumptive positive screening result. MAIN OUTCOME MEASURE(S): Permanent primary CH status determined by clinician report of continuing thyroxine requirement at three-year follow-up. RESULTS: 629 newborns (58.3% girls; 58.7% white ethnicity) were investigated following presumptive positive screening result and 21 children (52.4% girls; 52.4% white) after clinical presentation; 432 remained on treatment at three-year follow-up. Permanent CH incidence was 5.3 (95%CI 4.8, 5.8) per 10,000 infants. Using locally-applied thresholds, sensitivity, specificity and positive predictive value were 96.76%, 99.97% and 66.88% respectively. Compared with TSH threshold of 10mU/L, positive rLRs for 8mU/L and 6mU/L were 1.20 (95%CI 0.82, 1.75) and 0.52 (95%CI 0.38, 0.72), and negative rLRs 0.11 (95%CI 0.03, 0.36) and 0.11 (95%CI 0.06, 0.20) respectively. CONCLUSIONS: Screening programme performance is good, however a TSH threshold of 8mU/L appears superior to the current national standard (10mU/L) and requires further evaluation. Further research should explore the implications of transient CH for screening policy
