Hi im 18 and this is complicated! 2 months ago i started having bursts of fast heart rate, they happend almost everyday for a month. I dont know if its related but i was having fevers and sinus pain at the same time. At first the racing heart rate was txt book svt, somewhere around 140 to 160 bpm. But after a week of them they slowed down to about 110 to 120. After a month of fever and heartracing my thyroid became very enlarged in like 2 days. I had already been to an cardiologist for the tachycardia and had an echo and holter monitor done. The echo came out perfect but the ekg "suggested" i have svt. But the doc did say my heart rate wasnt really fast enough for svt. I also had blood work done for the enlarged thyroid but it camr back normal. On top of all this, once my thyroid became enlarged i became very sick. The doc says i probly have mono, some days are better than others. But ive been very ill for a month. My questions are- can mono cause an enlarged thyroid? If not, could I have thyroiditis? My thryroid has gone down some but not all that much. The doc says whatever im sick with may be causing the svt, is this true?