There is accumulating evidence that individuals with schizophrenia show altered levels of thyroid hormones. However, a qualitative and quantitative synthesis of findings in this field has not been performed so far. Therefore, we aimed to perform a systematic review and meta-analysis of studies investigating the levels of thyroid-stimulating hormone (TSH), free thyroxine (fT4), total thyroxine (tT4), free triiodothyronine (fT3) and total triiodothyronine (tT3) in multiple-episode schizophrenia (MES) and first-episode psychosis (FEP). Electronic databases were searched from their inception until 30th May 2020 by two independent reviewers. Random-effects meta-analyses and meta-regression analyses were performed. Altogether, 19 studies were included. Persons with FEP had significantly lower TSH levels (5 studies, g = −0.26, 95%CI: −0.47 to −0.06, p = 0.013, I2 = 21.3%), higher fT4 levels (3 studies, g = 0.58, 95%CI: 0.15–1.01, p = 0.008, I2 = 64.6%) and lower tT3 levels (2 studies, g = −0.60, 95%CI: −0.82 to −0.37, p < 0.001, I2 = 0%) compared to controls. Elevated TSH levels were found in persons with MES (13 studies, g = 0.20, 95%CI: 0.02–0.39, p = 0.031, I2 = 50.0%). Our findings imply that the levels of TSH might be decreased in persons with FEP and increased in those with MES. Other alterations need to be confirmed by additional studies. These findings imply the need to monitor the levels of TSH and thyroid hormones from the onset of psychosis.
