A microwave handheld radiometer for measuring subsurface tissue temperature of a subject includes a microwave antenna disposed relative to a handheld radiometer housing and adapted to electromagnetically couple to a skin surface of a subject, a circulator having a first circulator input and a second circulator input, wherein the first circulator input is electrically coupled to said antenna via a microwave feedline. A switch is electrically disposed between the antenna and an microwave feedline, said switch having a calibrate state and a measure state. A noise source, electrically coupled to the circulator, is being configured to provide a first source of thermal noise at a first temperature and a second source of thermal noise at a second temperature, wherein the noise source is switched between the first source and second source upon external command wherein a detector provides an output wherein surface temperature effects are corrected for the sub-surface measurement.