  We demonstrate radiofrequency thermometry on a micrometer-sized metallic
island below 100 mK. Our device is based on a normal
metal-insulator-superconductor tunnel junction coupled to a resonator with
transmission readout. In the first generation of the device, we achieve 90
{\mu}K/Hz^1/2 noise-equivalent temperature with 10 MHz bandwidth. We measure
the thermal relaxation time of the electron gas in the island, which we find to
be of the order of 100 {\mu}s. Such a calorimetric detector, upon optimization,
can be seamlessly integrated into superconducting circuits, with immediate
applications in quantum-thermodynamics experiments down to single quanta of
energy.
