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OBJECTIVE: To test the association between thyroid dysfunction and psychiatric diagnostic group in a large sample of consecutive patients, while controlling for the effects of age, sex, medication and concomitant medical conditions. METHODS: We compared the distribution of psychiatric diagnostic groups according to the International Statistical Classification of Diseases and Related Health Problems, 10th revision (ICD-10), and of selected psychopathological symptoms in 100 newly admitted psychiatric patients who had genuine thyroid disease and 92 psychiatric patients who had nonspecific alterations of thyroid function with the corresponding items for the whole group of admitted patients (n = 1134) during the observation period. This analysis was then repeated using an age-matched and sex-matched comparison group selected from all admitted patients in a random order. RESULTS: When all admitted patients were considered, the presence of a genuine thyroid disorder was associated with the diagnosis of a mood disorder (ICD-10 category F3). This effect was no longer detectable when the age-matched and sex-matched comparison group was used, indicating a simple effect of these 2 variables. Nonspecific alterations of thyroid-stimulating hormone (TSH) were associated with the ICD-10 diagnostic group F4 (neurotic, stress-related and somatoform disorders), with demographic variables being similar in this subgroup to those of all admitted patients. These patients also tended to display more symptoms of a mild depressive syndrome. When only patients with nonspecifically decreased TSH concentrations were tested, these findings could not be reproduced. Nonspecifically decreased concentrations of thyroxine and free thyroxine index were found significantly more often in the diagnosis group F1 (mental disorder due to substance use), reflecting results for alcohol-dependent patients. This result could not be replicated using an age-matched and sex-matched control group. CONCLUSION: In newly admitted psychiatric patients with genuine thyroid disease, there was no notable association of thyroid disease and major psychiatric diagnostic groups according to ICD-10, especially depression. This argues against the hypothesis of thyroid disorders being a major risk factor for psychiatric illnesses. Nonspecific alterations of TSH were more frequently seen in patients of diagnostic group F4 and with mild depressive syndromes, possibly representing an altered influence of stress-regulating systems on thyroid function. [ABSTRACT FROM AUTHOR]