A passive microwave thermography apparatus uses passive microwave antennas designed for operation, for example, at WARC protected frequencies of 1.400 to 1.427 GHz and 2.690 to 2.70 GHz (for core body gradient temperature measurement) and 10.68 to 10.700 GHz or higher microwave frequency (for surface body gradient temperature measurement) and a related directional antenna or antenna array to measure microwave radiation emanating from an animal, especially, a human body. The antennae may be radially directed toward a point within or on the surface of a human body for comparison with known temperature distribution data for that point and a given ambient temperature. Each frequency band may provide a plurality of adjacent noise measuring channels for measuring microwave noise naturally emitted by the human body. The apparatus measures short-term changes in, for example, core and body surface temperatures to establish a basal metabolic rate. Changes in core body temperature may be stimulated by the administration of food or certain organic and drug-related substances or stress to induce a change in basal metabolic rate over time. These changes correlate directly with a human subject's metabolism rate at rest and under certain dietary constraints and can be used to determine courses of treatment for obesity, metabolic disease, and other disorders. The apparatus can also be used to remotely monitor patients and subjects without physical contact.