AimTo determine whether thyroid hormone levels are associated with a specific clinical phenotype in patients with first-episode psychosis (FEP).MethodsNinety-eight inpatients experiencing FEP and with less than 6 weeks of antipsychotic treatment were included in the study and were followed up for one year. Baseline psychiatric evaluation included assessment of prodromal symptoms, positive and negative symptoms, depressive symptoms, stressful life events and cycloid psychosis criteria. Thyroid function (thyroid-stimulating hormone (TSH) and free thyroxin (FT4)) was determined at admission. Partial correlation analysis was conducted to analyse the correlation between levels of TSH/FT4 and symptoms. Logistic regression was performed to explore the association between psychopathological symptoms, 12-month diagnoses and thyroid hormones while adjusting for covariates.ResultsPatients with prodromal symptomatology showed lower baseline FT4 levels (OR = 0.06;p= 0.018). The duration of untreated psychosis (DUP) was inversely associated with FT4 concentrations (r=  − 0.243;p= 0.039). FEP patients with sudden onset of psychotic symptoms (criteria B, cycloid psychosis) showed higher FT4 levels at admission (OR = 10.49;p= 0.040). Patients diagnosed with affective psychotic disorders (BD or MDD) at the 12-month follow-up showed higher FT4 levels at admission than patients diagnosed with nonaffective psychosis (schizophrenia, schizoaffective) (OR = 8.57;p= 0.042).ConclusionsOur study suggests that higher free-thyroxine levels are associated with a specific clinical phenotype of FEP patients (fewer prodromal symptoms, shorter DUP duration and sudden onset of psychosis) and with affective psychosis diagnoses at the 12-month follow-up.