I have felt unwell for a few weeks . I've been on thyroxine since 2002 . I nearly collapsed at the wheel at work , which felt like a panic attack (never had one before) . Heart was racing , flushed , faint . This went on for an hour . Doctor has upped my thyroxine to 125mcg . I've gone low before but never had these symptoms . I've lost weight , eat like a horse , felt full of energy . I found this strange . I'm now waiting for an urgent cardiology appointment as doctor says my heart palpitations are not related . Has any one else had this problem ?