In this study from a county in southern Sweden 269 of 271 patients (99.3%) treated with radioiodine for hyperthyroidism over a two-year period were followed up five years later. The routine procedures at hospital clinics and primary health care centres were studied. A high number of toxic multinodular goitre (n = 113; 42.0%) and toxic adenoma (n = 76; 28.3%) emerged, 80 patients (29.7%) had toxic diffuse goitre (Graves's disease). During the five-year follow-up the cumulative incidence of hypothyroidism in the various goitre groups was 10.7%, 23.9%, and 57.5% respectively. Altogether 76 patients (28.3%) were diagnosed hypothyroid. Nine patients were lost to regular follow-up; elevated TSH and low or normal T4-concentrations were found in two of them. We propose a register system to enable detection of thyroid dysfunction after radioiodine treatment and other thyroid patients in primary health care.