It seems that your aunt has had hyperthyroidism (increased thyroid) that lead to heart related complications and a very weak heart, along with atrial fibrillation- a heart rhythm abnormality. After treatmetn with radio-active iodine, she seem now to be suffering from a common complication of this treatment, hypothyroidism (LOW thyroid levels).
She will need life long replacement thryoxine treatment, and followup for her heart problem, on which she should be alright...
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Dr. Gupta