OBJECTIVE Thyroid disorders are highly prevalent in patients with schizophrenia. Changes in the levels of thyroid hormones cause the occurrence of psychiatric disorders and affect the response to treatment. The aim of this study was to investigate the changes in thyroid hormone levels in patients with chronic schizophrenia, and to demonstrate any association between psychiatric symptoms and hormones. METHODS Sixty-three patients with schizophrenia and 53 controls were included in this study. The serum levels of free triiodothyronine (FT3), free thyroxine (FT4) and thyroid-stimulating hormone (TSH) in patients and controls were tested by using the chemiluminescence immunoassay. Symptoms were assessed via the Positive and Negative Syndrome Scale (PANSS). RESULTS The level of FT4 in the schizophrenia group was statistically significantly lower than the control group (p = 0.045). There was no significant difference between the clinical subtypes of schizophrenia and thyroid hormone levels. There was a mild negative correlation between PANSS negative subscale scores and levels of TSH (p = 0.023). CONCLUSIONS These findings suggest that the subclinical hypothyroidism observed in the chronic schizophrenic patients may be associated with the treatment of neuroleptics. The possible alterations of thyroid hormones should be considered in chronic schizophrenic patients for treatment with neuroleptics.