We have a downstairs bathroom that is a million degrees. Our entire house uses radiant heat in the ceiling and each room has a thermostat except this bathroom. They're all off right now, but the bathroom never shuts off. We thought it might be tied in with the kitchen but it's not. You can't shut the door or you'll literally bake in there. Hot 24/7. Is there any way to shut this off? Why would it be this way? We have no other room like this in our house.

Well that heat system is fed from somewhere(you didn't mention if it was hydronic or electric). From your description it sounds like it is electric? If so, a review of your electrical panel might show that there is a circuit breaker dedicated for that bathroom's heat. There should also be a junction box in the attic that feeds the radiant elements. If there is no breaker for it, it is obviously piggybacked off of another adjacent heating circuit(before where that heater's thermostat is branched off. You will need to find the junction box and install wiring down into a bathroom wall to add a on/off switch or a thermostatic switch(consult with an electrician please)... If it is hydronic, well then there is most likely a manifold port for that zone that you could throttle or secure. Or add a zone valve and thermostat...

Why would it be this way? Someone(builder) was cutting corners or lazy

Good Luck

Thanks! It's electric. We'll have to check that out.

What you will probably find above the bathroom is a junction box with power cable in, power cable out to the heating elements and another cable running down into one of the bathroom walls to a single outlet box.. The wiring and outlet box were probably installed and the electrician spliced the thermostat wires together in the box to ring out the circuit. Then the drywall crew came in and accidently drywalled over the box in the bathroom wall where the thermostat was supposed to be. The electrician then missed his count and final op-check of the heating system...