Objective: 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. We therefore studied incidences of subtypes of primary. overt hypothyroidism in a Danish population cohort and compared incidences in two subcohorts with different levels of iodine intake. Design: A prospective population-based study, monitoring a well-defined cohort representative of the Danish population. Methods: The Danish Investigation of Iodine Intake and Thyroid Diseases registry of hyper- and hypothyroidism was established as part of the monitoring of the iodine fortification of salt in Denmark. A computer-based system linked to laboratory databases identified all patients diagnosed with new. biochemically overt hypothyroidism in populations living in Aalborg (moderate iodine deficiency, n = 311 102) and Copenhagen (mild iodine deficiency. n = 227 632). We subsequently evaluated all identified patients to verify incident thyroid disease. and subclassified hypothyroidism into nosological types. Results: During a 4-year period (2 027 208 person-years) 685 new cases of overt hypothyroidism were diagnosed in the cohort; the incidence rate was 32.8 per 100 000 person-years (standardised to the Danish population). Nosological types of hypothyroidism were: spontaneous (presumably autoimmune) 84.4%, post-partum 4.71%, amiodarone-associated 4.0%. subacute thyroiditis 1.8%. previous radiation or surgery 1.8%. congenital 1.6% and lithium-associated 1.6%. Crude incidence rates were 29.0 around Aalborg and 40.6 in an area of Copenhagen. The higher incidence rate of hypothyroidism in the area with higher iodine intake was caused solely by more cases of spontaneous (presumably autoimmune) hypothyroidism, whereas the incidence of non-spontaneous hypothyroidism (all types combined) was significantly lower in the area with higher iodine intake. Conclusion: In a population-based study we observed a higher incidence of hypothyroidism with higher iodine intake. This was due solely to the entity of spontaneous hypothyroidism. The occurrence of overt hypothyroidism was relatively low in Denmark.