Demented individuals are traditionally investigated for potentially reversible causes. Hypothyroidism is generally accepted as being a condition that, if identified and treated, may be associated with improvement in mental state. A literature search was carried out to determine if data exist to show that this is so. Analysis of 2781 cases from studies of etiology in dementia revealed only one case of reversible dementia due to hypothyroidism. Problems associated with interpretation of previous reports include lack of acceptable criteria for diagnosis of dementia and inadequate follow-up. A randomized controlled trial will be needed to resolve this issue.