TMJ is a possible symptom of hypothyroidism.
I find quite a bit information that says flexeril should not be taken when one has hyperthyroidism due to the possibility of causing heart attack, but I can't find no evidence that it triggers hypothyroidism, so I would say it did not trigger your hypothyroidism.
The best thing to do is talk to your doctor or pharmacist to find out about interaction between the flexeril and hypothyroidism.