||OBJECTIVE: In a mortality follow-up of the HUNT Study, serum TSH within the reference range was positively associated with the risk of coronary death in women. We now aimed to confirm the association of high serum TSH with the risk of coronary heart disease, using hospital-based diagnoses of myocardial infarction. DESIGN: Prospective population-based study with linkage to hospital information on myocardial infarction and to the national Cause of Death Registry. PARTICIPANTS: A total of 26, 707 people without previously known thyroid or cardiovascular disease or diabetes at baseline. MEASUREMENTS: Hazard ratios (HR) of coronary death and HRs of hospitalization with a first-time acute myocardial infarction, by baseline thyroid function. RESULTS: During 12, years of follow-up, 960 (3.6%) participants had been hospitalized with first-time myocardial infarction and 558 (2.1%) had died from coronary heart disease. High TSH within the reference range was associated with increased risk of coronary death in women (P(trend) 0.005), but not in men. The risk of coronary death was also increased among women with subclinical hypothyroidism or subclinical hyperthyroidism, compared to women with TSH of 0.50-1.4 mU/l. However, thyroid function was not associated with the risk of being hospitalized with myocardial infarction. CONCLUSIONS: High serum TSH was associated with increased mortality from coronary heart disease in women, but we found no association of thyroid function with the risk of being hospitalized with myocardial infarction. Thus, the morbidity finding does not confirm the suggestion that low thyroid function within the clinically normal range is associated with increased risk of coronary heart disease.