You should get some new tests done. Specifically you need to test for both Free T4 and Free T3. When taking thyroid med, TSH does not reveal much. If possible it would also be good to test for Reverse T3 as well. You also should test for Vitamin D, B12 and ferritin, since those need to be optimal as well. Can you get these done?
Also please check and make sure of the selenium dosage. I hope it is 200 mcg, not mg.
Does this pain show up during exertion, severe emotional stress or after a heavy meal?
Since you have obviously been diagnosed as hypothyroid and are taking thyroid med I think we should start by reviewing your thyroid related test results and reference ranges shown on the lab report. Also, have you been tested recently for Vitamin D? Are there any other symptoms you have?