The thyroid status of 249 patients with chronic schizophrenia (males = 136, females = 113) with a median age of 36 years (range: 16 to 58 years) and a median duration of hospitalisation of 10 years (range: 1 to 30 years) was assessed. Thyroid antibodies (TAb) were found in 51 patients (20%). In female patients, 32 (28%) were TAb positive compared to 13% (n = 152, p = 0.01) in healthy female blood donors. In male patients, the prevalence of TAb was 14% compared to 7% (n = 449, p = 0.01) in healthy male blood donors. Of the 183 patients who had thyroid hormone measurements, 60% had normal test, 5% had elevated TSH and 17% had low TSH. The T4, FT41 and FT31 were significantly lower in those with low or high TSH (p < 0.001) compared to those with normal TSH. Of the 143 patients with normal TSH, 33 (23%) had low T3. In conclusion, there is a spectrum of thyroid function test abnormalities in chronic schizophrenia; this may be related to an abnormality in the central regulation of the hypothalamo-pituitary thyroid axis as well as at the peripheral level. However the association between chronic schizophrenia and the presence of thyroid antibodies, and the clinical relevance of these biochemical abnormalities, are still not clear.