Hello jennyvee.....first of all, your question as related to your tachycardia should be addressed to your Physician immediately rather than on a Healthboard forum. Since you state that you are Hypothyroid, I would imagine that your Endcrinologist has put you on Thyroid replacement medication such as Synthroid. It is possible that your level of Thyroid replacement medication might be too high causing a rapid heartbeat to take place. Again these questions must be addressed to your Physician. The only way he knows what is going on and how to correct the current problem, is for you to communicate with him/her.