The increased incidence of congenital hypothyroidism: fact or fancy?

Objective: The incidence of congenital hypothyroidism (CH) detected by newborn screening in the US has increased significantly since the early 1990s. We defined the characteristics associated with the increased incidence.

Patients: A cohort of children with CH born during an earlier period of low incidence (1991-94) was compared with a cohort born during a later period when the incidence of CH had doubled (2001-04).

Measurements: Screening was performed with T4 as the primary marker and thyroid stimulating hormone (TSH) on selected specimens. Follow-up on hypothyroid children determined whether they had permanent or transient hypothyroidism. Cases were classified based on laboratory results: initial TSH ≥100 mU/l was 'severe,' initial TSH <100 mU/l but ≥20 mU/l was 'mild' and initial TSH <20 mU/l with subsequent abnormal TSH was 'delayed'.

Results: The overall incidence of CH almost doubled between the two time periods, from 1:3010 to 1:1660. Excess cases were found in the mild and delayed categories, with no increase in severe cases. The proportion of transient cases was <5% in severe cases, 40% in mild cases and 70% among delayed cases. There was no difference in the proportion of transient case between the two time periods. Modifications to the T4/TSH testing protocol between the two time periods resulted in substantially increased numbers of specimens in the younger cohort being selected for TSH testing in both initial and repeat specimens.

Conclusion: The rising incidence of CH in Massachusetts is confined to mild and delayed cases. Our findings suggest that this rise is attributable to enhanced detection rather than an absolute increase in numbers.