Hard to say, unfortunately. The level of thyroid hormone in your friend's blood would be an important clue, however. A very high level of thyroid hormone in the blood can cause heart rhythm disturbances that could possibly cause the symptoms that you describe. (Very low levels of thyroid hormone could cause a slower heart rate, although this would be less likely to cause the symptoms that you describe unless your friend had other reasons to have a slow heart rate.) Unfortunately, a number of disorders, including non-cardiac disorders, can also cause the symptoms that you describe.
Your friend's cardiologist will, I'm sure, carefully evaluate these symptoms, and will consider all possible reasons for them.
Sincerely/ Robert A. Wascher, MD, FACS
http://doctorwascher.com
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