I have the Costochondritis at times.
Back in 2006, after exercising too hard and being under medicated, I got a condition which sometimes goes with hypothyroidism, pericarditis. It took one and half years to get a diagnosis and numerous visits to the emergency department at various hospitals. The pain appeared to be Costochondritis so it was ignored. By the time I got an official diagnosis it had moved into the heart muscle. I was diagnosed with perimyocarditis with a pericardial efusion.
One diagnosis was an enlarged heart which is common with hypoT. Another was atypical chest pain. And one ED doctor told me to never come back there with these symptoms; take an aspirin and go lie down. Essentially the message there was "you're a hypochondriac. Don't come back here." By the time I got diagnosed I was close to dying from severe heart palpitations, a racing heart, an irregular heart beat, and soaring blood pressure.
Be persistent and keep going to find out what it is.