The most effective treatment for hepatitis C virus (HCV) is interferon-alpha (IFN) therapy in combination with ribavirin. Although symptoms of depression are among the most common side effects of IFN therapy in treating patients with HCV, the mechanisms by which IFN produces these neuropsychiatric side effects remain unclear. In the brain, IFNs are involved in a number of regulatory functions, including but not limited to regulation of the endocrine system via the hypothalamic-pituitary-adrenal and -thyroid axes. The purpose of this study was to assess the effect of IFN therapy on thyroid function and to characterize the relationship between thyroid dysfunction and major depressive disorder during IFN therapy in patients with hepatitis C. Thirty-three patients with HCV were administered the Structured Clinical Interview for Diagnostic and Statistical Manual of Mental Disorders (DSM-IV) Axis I Disorders (SCID) and completed the Beck Depression Inventory (BDI). Patients were on IFN for an average of 6 to 12 months depending on their viral genotype. Serum samples were collected at baseline, during and after IFN therapy, and measured for free thryoxine (FT4) and TSH levels. Patients who developed IFN-induced depression were treated with selective serotonin reuptake inhibitor antidepressants. Only one patient developed transient IFN-induced overt hypothyroidism, but he did not develop depression. Analysis of variance showed that there were no significant differences in either FT4 or TSH serum levels between patients who developed major depressive disorder (MDD) (no.= 10) during IFN therapy and those who did not (no.=23). These results illustrate the frequency and severity of depressive symptoms associated with IFN therapy and the apparent absence of a relationship between IFN-induced MDD and changes in thyroid function.