Low incidence rate of overt hypothyroidism compared with hyperthyroidism in an area with moderately low iodine intake

In areas with relatively high iodine intake, the incidence rate of hypothyroidism is several-fold higher than that of hyperthyroidism. Recently, we found a similarly high prevalence rate of subclinical hypothyroidism compared with hyperthyroidism in a high iodine intake area, while a relatively low prevalence of subclinical hypothyroidism was observed in a low iodine intake area. In the present study we compared the incidence rate (newly diagnosed in primary care and at hospital) of overt hypothyroidism with that of hyperthyroidism in a well-defined geographical area in Jutland, Denmark, with an iodine intake around 60 microg/day. The number of personsxyears studied was 569,108. Data on hyperthyroidism have been published previously. The overall incidence of hypothyroidism was 13.5/100,000 per year (F/M 22.9/3.6), hyperthyroidism 38.7/100.000 per year (F/M 63.0/13.0). The incidence of hypothyroidism was steadily increasing with age up to 80/100,000 per year in subjects older than 70 years of age, but apart from congenital hypothyroidism it was lower than that of hyperthyroidism at all ages. The majority of patients (79%) was diagnosed to have spontaneous autoimmune hypothyroidism (16% with goiter, 84% with no thyroid visible or palpable). In conclusion, in an area with moderately low iodine intake, hypothyroidism was considerably less common than hyperthyroidism. This is in contrast to findings in high iodine intake areas. The iodine intake of an area seems to be of major importance for the pattern of thyroid disorders observed.