Is untreated thyroid disease fatal?

When thyroid meds were reduced, I went into deep, deep sleep, woke in great pain in legs and unable to move, or open eyes. Took some time to slowly regain movement. I was in living room and my mother and adult son were there. I didn't move at all during the 2 1/2 hours I was 'alseep' nor did I snore or make any sound. The feeling was like a bad 'B' movie where I'd turned to stone. This was on a lower dose of 100mcg. Can this disease cause my heart to slow to point of stopping?