Hello... Propranolol is usually prescribed for the adrenergic symptoms of thyroiditis like palpitations, tachycardia, tremors..
It usually decreases heart rate... But in your case heart rate is going to low... Below 60 is bradycardia which could be dangerous...
Immediately consult your Endocrinologist for dose adjustment /stopping of propranolol as per your clinical condition
Next Steps
Consult Endocrinologist as soon as possible