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 hypothyr oidism is several-fold higher than that of hyperthyroidism. Recently, we fo und 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 intak e area. In the present study we compared the incidence rate (newly diagnose d in primary care and at hospital) of overt hypothyroidism with that of hyp erthyroidism in a well-defined geographical area in Jutland, Denmark, with an iodine intake around 60 mu g/day. The number of personsxyears studied wa s 569,108. Data on hyperthyroidism have been published previously. The over all incidence of hypothyroidism was 13.5/100,000 per year (F/M 22.9/3.6), h yperthyroidism 38.7/100.000 per year (F/M 63.0/13.0). The incidence of hypo thyroidism was steadily increasing with age up to 80/100,000 per year in su bjects older than 70 years of age, but apart from congenital hypothyroidism it was lower than that of hyperthyroidism at all ages. The majority of pat ients (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 i odine intake areas. The iodine intake of an area seems to be of major impor tance for the pattern of thyroid disorders observed.