The electrochemical oxidation of sodium levothyroxine (T(4)) has been studied on carbon paste electrode (CPE) with phenyl hydrazine homogenous as mediator, using cyclic voltammetric technique in presence of 0.1 M HCl as supporting electrolyte. The charge transfer coefficient (alpha n(alpha)) for T(4) in the presence and absence of phenyl hydrazine was determined. The oxidation peak currents represented a linear dependence on T(4) concentration from 0.025 mM to 0.1 mM with correlation coefficient 0.997. The effect of concentration and scan rate of sodium levothyroxine in presence of trace phenyl hydrazine concentration was studied. The scan rate effect showed the electrode process is adsorption controlled. The practical application of the phenyl hydrazine mediated CPE in the determination of T(4) in a commercial tablet sample demonstrated that it has good selectivity and high sensitivity. (C) 2010 Elsevier B.V. All rights reserved.