Magnetic and superparamagnetic colloids represent a versatile platform for the design of functional nanostructures which may act as effective tools for biomedicine, being active in cancer therapy, tissue imaging and magnetic separation. The structural, morphological and hence magnetic features of the magnetic nanoparticles must be tuned for optimal perfomance in a given application. In this work, iron oxide nanocrystals have been prepared as prospective heat mediators in magnetic fluid hyperthermia therapy. A procedure based on the partial oxidation of iron (II) precursors in water based media has been adopted and the synthesis outcome has been investigated by X-Ray diffraction and Transmission electron microscopy. It was found that by adjusting the synthetic parameters (mainly the oxidation rate) magnetic iron oxide nanocrystals with cubic and cuboctahedral shape and average size 50 nm were obtained. The nanocrystals were tested as hyperthermic mediators through Specific Absorption Rate (SAR) measurements. The samples act as heat mediators, being able to increase the temperature from physiological temperature to the temperatures used for magnetic hyperthermia by short exposure to an alternative magnetic field and exhibit a reproducible temperature kinetic behavior.