Your daughter’s weight is more than what it should be that could be one reason for her increased heart rate, other possibility could be she does not take enough water and is always dehydrated which also cause a slight increase in heart rate, third cause could be her
thyroid medication, the dose is more than what she requires, as there are no other symptoms heart related problem is less likely.
Next Steps
Kindly meet a good general physician once so that he can examine her pulse, because apart from rate we also need to see the rhythm of the pulse and also we could check her bp, and review her thyroxine dose.
Consult a good dietician and physical trainer to help her reduce weight. According to her height her weight should be 50-55 kg.