Sorry to hear that; I guess they would rather sponsor a candidate with a full clean sheet that one with an existing condition.
You can certainly self sponsor to obtain a CPL/MPL and fly with some smaller outfits ( who will be less stringent on medical matters ) to gain sufficient hours to join a major later. The current carrier would still have your records ( if it's anal retentive SQ ) making it difficult for you. As long as your hyperactive thyroid is under control, most airlines have no problems with your condition. Good luck.