The injectable formulation for treatment by hyperthermia comprises a liquid carrier and heat-generating superparamagnetic iron oxide nanoparticles having a mean diameter not greater than 20 nm. Said injectable formulation is able to form in-situ a hyperthermic solid or semi-solid implant upon contact with a body fluid or tissue. Said hyperthermic solid or semi-solid implant may be useful for treating a tumor or a degenerative disc disease by hyperthermia.