Etiology of Increasing Incidence of Congenital Hypothyroidism in New Zealand from 1993–2010

BACKGROUND: Recent reports suggest that the incidence of congenital hypothyroidism (CHT) is increasing in some countries. The etiology of this change is unclear, and it may relate to changes in screening thresholds. We aimed to determine whether the incidence of CHT in New Zealand has changed and whether ethnic-specific rates and the rates of CHT subtypes have also changed. METHODS: The New Zealand neonatal TSH-based screening program has prospectively identified cases of CHT using the same assay and screening thresholds since 1993. Thyroid scintiscans are routinely recommended. We retrospectively identified all cases of CHT requiring levothyroxine treatment from 1993-2010 recorded by the national newborn screening program (>99.5% coverage). Among other parameters, ethnic and CHT subtype-specific incidence rates were calculated. RESULTS: There were 330 new cases of CHT and 1,053,457 live births registered in New Zealand in the 18-yr period, and 86% of cases had a scintiscan, 67% of which had thyroid dysgenesis (female to male ratio 5.0:1.0) and 33% dyshormonogenesis (0.9:1.0). The overall incidence of CHT rose from 2.6 to 3.6 per 10,000 live births (P < 0.01). The incidence of dyshormonogenesis (P = 0.01) increased but not of dysgenesis (P = 0.13). This was mediated by a 2-fold increase in Asian births and 40% increase in Pacific Island births. Both ethnic groups displayed higher rates of dyshormonogenesis compared with New Zealand Europeans (odds ratio 3.3 and 2.6, respectively). There was no change in the ethnic-specific incidences of CHT. CONCLUSION: Although the incidence of congenital hypothyroidism in New Zealand has increased, this is due to changes in the country's ethnic composition.