The epidemiology of thyrotoxicosis in New Zealand: incidence and geographical distribution in north Canterbury, 1983-1985.

In a 3-year (1983-1985) epidemiological study of thyrotoxicosis in North Canterbury, New Zealand, the annual incidence was 25.8 per 100,000 (female 40.7, male 10.5). Thyroid scintiscanning showed that 64% had diffuse hyperplasia (DH), 27% toxic multinodular goitre (TMG), 7% toxic uninodular goitre (TUG), and 2% zero uptake. The calculated annual incidence of toxic diffuse goitre (DH) was 15 per 100,000, and for toxic nodular goitre (TMG and TUG combined) was 8 per 100,000. The age-related incidence for toxic diffuse goitre peaked in middle life whereas toxic nodular goitre showed an increasing incidence with age. There was no significant seasonal variation or rural/urban difference in incidence. Analysis of geocoded addresses did not identify areas of high incidence. The variable duration of symptoms prior to diagnosis limits the search for possible environmental trigger factors. North Canterbury was an endemic goitre area prior to the introduction of iodized salt 50 years ago, and the incidence of toxic nodular goitre is likely to fall in future.