Neonatal screening for congenital hypothyroidism (CH) is based on the measurement of thyroid-stimulating hormone (TSH) in whole dried blood samples on filter paper in all newborns. The objective of screening for CH is to prevent mental retardation, which is irreversible in the event of a late diagnosis, by setting up prompt treatment (before day 15) with levothyroxine.

The threshold value of TSH on filter paper on day 3 is 17 mIU/L in France in the GSP method (GSP, Genetic Screening Processor, Perkin Elmer): It is one of the highest thresholds used in the world. In many countries, the TSH threshold is between 6 and 12 mIU/L. Studies have found that a threshold of > 17 mIU/L may miss as much as 30% of cases of CH, with 30–80% of these being permanent CH.

Recent studies suggest that mild CH (currently missed by the French TSH threshold) is associated with cognitive consequences if left untreated. An inverse relationship between TSH at screening (below the current threshold) and cognitive development at preschool or school age has been shown.

These studies advocate for the evaluation of a lowering of the threshold of TSH on filter paper in France: (a) to determine the number of CH diagnoses with the new threshold and whether these “new cases” would be transitory or permanent; and (b) to analyze the cost-effectiveness of the strategy.