In this study, we designed an impedimetric immunosensor by modification of disposable ITO-PET (indium tin oxide-polyethylene terephthalate) electrode surface with 3-glycidoxypropyltriethoxysilane (3-GOPE). 3-GOPE silanization agent generates a suitable platform on the ITO-PET working electrode surface for antibody immobilisation and allows a wide determination range for analyte concentrations with the fabricated biosensor. All immobilisation processes, investigation of optimum parameters and characterisation studies of the developed immunosensor were evaluated and followed by Electrochemical Impedance Spectroscopy (EIS) and Cyclic Voltammetry (CV). Furthermore, the interactions between anti-parathyroid hormone (anti-PTH) and Parathyroid hormone (PTH) were simultaneously analysed with the Single Frequency Impedance (SFI) technique. All parameters which affect the biosensor response were optimised. A great wide concentration range (1 fg mL(-1)-2500 fg mL(-1)) and ultra-low detection limit (0.89 fg mL(-1)) were obtained under optimum conditions. Moreover, the impedimetric biosensor allowed high sensitivity in the detection of PTH in human serum. It was proved that the fabricated biosensor system has some advantages such as long-term storage life, very low limit of detection, ability to detect the PTH concentrations at femtogram level (1 fg mL(-1)- 2500 fg mL(-1)), excellent reproducibility and repeatability, reusability and ultra-high sensitivity.