I have Hashi's thyroiditis too, and last year, when my levels were really hypo, (T4 was even below the range) I came down with that costochondritis curse too. It was thee worst pain ever...my entire chest felt like it was beaten up...like broken ribs, or something horrible like that. My sternum, especially, felt like it was in a vise! SO VERY PAINFUL!! There was a small spot on my sternum, if barely touched, I practically jumped and screamed! I remember that I couldn't wear a bra, as it hurt even more to wear one...so I had to go braless, which I hate.
I also noticed that being in a cold place made it seem to hurt worse...like when we went into cold restuarants. (It was summer when it was at it's worst)
At the same time, I was suffering from acid reflux, really extreme. My whole chest was killing me, and it seemed like it would never go away!! I cried a lot!
All the Dr. told me to do was take 800 mg Motrin and ice it. (I tried the motrin for about a month, it never helped, even a little bit). I bought a microwave heating pad, (Bed Buddy), and kept that thing on my chest all day long there, for a while. It truly sucked!
I'm not sure how long it lasted, but it seemed at least 6 to 8 months, it was really bad. Then it was moderate pain for a few months. Today, it's gone, thank God!
My levels were optimized about 2 months ago...after being on meds for 27 months. I will never forget that pain! Altho, it's not the only place I've had extreme pain.
Hang in there girl...it will go away, I promise you...