Thyroid disorders are known to manifest occasionally as isolated psychiatric disorders. In order to determine whether thyroid dysfunctions could play a significant role in the pathogenesis of psychiatric disorders in the elderly, the prevalence of thyroid disorders was compared in a group of psychogeriatric patients and in a group of nonpsychiatric elderly patients. Thyroid function screening was performed in 157 patients consecutively admitted to a psychogeriatric unit, and the prevalence of hypothyroidism was determined in the different groups of psychiatric disorders (senile and multi-infarct dementia, organic brain syndrome of other etiologies, psychotic depression, neurotic depression, chronic delusional state, acute confusional state, and personality disorder). Thyroid function screening was performed similarly in 104 unselected elderly patients admitted to the medico-surgical admission unit of the University hospital to assess the prevalence of hypothyroidism in a general, nonpsychiatric, elderly population. Eight patients were diagnosed as hypothyroid based on an elevated basal thyrotropin (TSH) and on thyrotropin-releasing hormone (TRH) test in the total number of patients, two in the nonpsychiatric and six in the psychiatric group. Three had clinical hypothyroidism, with decreased total and free T4 and T3 plasma levels in addition to increased basal TSH, five had biochemical hypothyroidism, with normal T4 and T3 levels and an excess TSH response to oral TRH. The prevalence of hypothyroidism in the two groups did not differ significantly (1.9% in the nonpsychiatric versus 3.8% in the psychiatric group). The prevalence of hypothyroidism in a subgroup of 88 patients with senile and multi-infarct dementia was 2.3%.(ABSTRACT TRUNCATED AT 250 WORDS)