Previous studies of hyperthyroid patients suggest that they remain at increased risk of cardiovascular morbidity after restoring euthyroidism.

The study objective is to compare the rate and causes of hospitalization of hyperthyroid patients treated with radioactive iodine (RAI) with those of an age- and gender-matched reference population in a long-term follow-up study.

A population-based cohort study was conducted among 2611 hyperthyroid patients treated with RAI between 1969 and 2002 in Tampere University Hospital, and among 2611 reference subjects. A reference group was randomly selected, with an age- and gender-matched control subject identified for each patient from the Population Register Centre. The control subject had to be alive at the time when the patient received the first RAI treatment.

The causes of hospitalization as well as the diagnosis and date of hospital admission were obtained from the nationwide Hospital Discharge Register (HILMO) maintained by the Research and Development Centre for Welfare and Health (STAKES) using a computerized record linkage, with the personal identification number as the key. The HILMO database covers all dates and causes of hospitalization (hospital admission requiring an overnight stay) of the Finnish citizens since January 1969. New events were analyzed as the main outcome, including only the first hospitalization due to a given indication. The special focus was on the cardiovascular morbidity.

Inclusion Criteria:

• Patients: Radioiodine treatment for hyperthyroidism given in the Tampere University Hospital between January 1969 and June 2002
• Controls: age- and gender-matched control subject identified for each patient from the Population Register Centre. The control subject had to be alive at the time when the patient received the first RAI treatment