Among different kinds of cancer theranostic mediators, gadolinium (Gd) doped iron oxide nanoparticles are one of the most promising candidates in combining diagnostics (imaging) and therapeutics (molecular therapy) functions in a single, multimodal platform. Due to its larger size, the doping of Gd into the Fe₃O₄ is difficult. We have overcome this difficulty by modifying a polyol based reflux method that has been previously used for, for example, cobalt–zinc (Co–Zn) doping of ferrites but not for doping with Gd. This modified approach allowed a facile synthesis of Gd-doped superparamagnetic iron oxide (Fe₃O₄) nanoparticles (GdSPIONPs) with a lower Curie temperature (T_c) for hyperthermia superparamagnetism with low coercivity, both T₁ and T₂ based MRI contrast enhancements, low cytotoxicity and optimal hemocompatibility. Such a combination of theranostics properties in a single nanosystem is unprecedented and highly desirable for heat controlled magnetic hyperthermia in minimizing treatment resistance, and maximizing treatment efficacy.