Why do I have palpitations and breathlessness while walking

Some medical conditions that could cause palpitations are, among others, anemia, anxiety, thyroid problems, and hypoglicemia (low blood sugar). According to the data you've given, and the fact that you've been diagnosed with hypothyreosis , it seems probable that your difficulties could be the result of the hypofunction of the thyroid gland, usually followed by anemia. However, you may perform a laboratory examination and check up on the status of the thyroid. You can consult an internal medicine specialist in order to request such an exam and to follow up on the condition.