My question is Is it possible to have an Underactive Thyroid, But also have Inappropriate Sinus Tachycardia. The reason Im asking is because over the past couple of months I have had extremely fast heart rates and finally the dr Diagnosed it as Inappropriate Sinus Tachycardia, But also had the Thyroid tested....well thinking that it would be overactive as it would suggest due to the fast heart rate, it was not I had an underactive thyroid???? Strange thing is I had the thyroid tested before and Nothing showed up, and it was just a few months ago when it was tested. I am just curious to what would cause the fast heart rate???? I did not think It was common with the thyroid being underactive