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You should see a cardiologist.
One cause - thyroid
has been excluded.
there are several others, any one of which may be responsible - Anxiety / Alcohol / Tobacco (smoking) / Caffeine (too much coffee, cola) / Medicines like Phenylephrine
used for ‘cold’ / Salbutamol
used for asthma / Illicit ‘drugs’ like Amphetamine
/ anaemia - the list is long.
At times, abnormal nerve paths in the heart (bypass tracts) are responsible!
You must get examined / investigated...
Wishing speedy recovery