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Methimazole is given for Thyrotoxicosis
There is no proved herbal medicine.
will not help
The only alternative is operation.
Your heart problem is most likely from over-active thyroid.
Check with the cardiologist - to make sure there is no underlying heart disease
Failure to treat can lead to serious consequences.
You have to see Endocrinologist
/ you need regular follow up - medication and testing
Wishing speedy recovery