Results of a population-based investigation have shown that the areas of Denmark with the highest incidence of hypothyroidism are also those that have the highest iodine intakes.
Furthermore, results revealed that hypothyroidism in these regions was due solely to spontaneous hypothyroidism that was most likely of autoimmune origin, as 95% of these individuals tested positive for thyroid peroxidase auto-antibody.
The authors of the study write in the European Journal of Endocrinology: "Studies of hypothyroidism are often based on referred patients, and limited information is available on the incidence rates of subtypes of hypothyroidism in the general population."
The team therefore identified the levels of incident thyroid disease among populations living in Copenhagen and Aalborg from medical records contained in the Danish Investigation of Iodine Intake and Thyroid Diseases registry of hyper- and hypothyroidism.
There were 311,102 people living in Aalborg, an area of moderate iodine deficiency, and 227,632 people living in Copenhagen, an area of only mild iodine deficiency, due to differences in iodine supplementation in drinking water.
In these two areas, 685 individuals developed overt hypothyroidism, equating to an incidence rate of 32.8 per 100 000 person-years, during the 4-year study period.
Importantly, the crude incidence rates of hypothyroidism calculated for the area of Aalborg and Copenhagen were 29.0 and 40.6 per 100, 000 person-years, respectively.
In addition, there were more cases of spontaneous hypothyroidism in Copenhagen, where there was also a higher iodine intake, than in Aalborg, with rates at 35.0 versus 23.1 per 100, 000 person-years, respectively.
Causes attributed to hypothyroidism among these individuals were spontaneous or autoimmune in 84.4%, post-partum in 4.7%, amiodarone-associated in 4.0%, subacute thyroiditis in 1.8%, previous radiation or surgery in 1.8%, congenital in 1.6%, and lithium-associated in 1.6%.
"We observed a higher incidence of hypothyroidism with higher iodine intake," Allen Carlé, from Aarhus University Hospital in Aalborg, and colleagues state.
"This was due solely to the entity of spontaneous hypothyroidism. The occurrence of overt hypothyroidism was relatively low in Denmark," they add.