The objective of this research is to determine the prevalence and risk factors for primary hypothyroidism in a sample of Canadian seniors. The Canadian Study of Health and Aging (CSHA) was a prevalence survey of seniors living at home and in long-term care facilities (n = 10,263). Detailed clinical examinations were conducted on 2,914 seniors, of which 1,415 had serum TSH levels measured.The ten Canadian Provinces were grouped into five geographic regions (Atlantic, Quebec, Ontario, Prairies and British Columbia). Only seniors who underwent detailed clinical examinations (n = 2,914) are included. Hypothyroidism was defined as seniors taking thyroid hormone replacement or seniors having a serum TSH level greater than 10 mU/L. An estimated 9.6 per cent of seniors had hypothyroidism, 5.1 per cent were inadequately treated and 2.1 per cent were untreated. Age, gender, cultural background, geographic region and place of residence were potential risk factors for hypothyroidism. Hypothyroidism was found to be a prevalent problem in this population.