In this work T4, also known as thyroxine, a thyroid hormone with many relevant roles in human metabolism, was determined with an antibody-based biosensor, consisting on a multi thiol self-assembled monolayer (SAM) whereas a selective antibody was attached. Electroanalytical measurements were performed by impedance on a ferricyanide redox probe. A limit of detection of 1.5 ng mL(-1) and limit of quantification (LOQ) of 4.0 ngmL(-1) were obtained. This biosensor is a proof-of-concept for the creation of point-of-care electrochemical approach for the quantification of T4. (C) 2017 The Electrochemical Society. All rights reserved.