Hello and thank you for using HCM. I carefully read your question and I understand your concern. I will try to explain you something and give you my opinion. As you describe i think we are dealing with a rhythm issue. person might experience spiking beats or palpitations for different reasons. They might be what we call exstra-systolic beats. This are electrical impulses generating in parts of the heart different from that of normal sinus rhythm. hey cause this escaped, skipping beat.The genetation of this extra-systolic beats might be from different reasons to simple emotional stress to other pathology like anemia, hyperthyroidism, ishemic heart desease ect. Another reason of this skipped beats might be the atrial fibrillation or atrial falter. This is an arrhythmia, when the heart beats is a chaotic way. You feel skippered beats, like your heart stops and begins again. It might happens for the same reasons as extra-systoles. You might experience chest heaviness because of this rhythm disorders. You mention a previous history of atrial fibrillation and a thyroid dysfunction. This to reasons make me suspect that maybe you make episodes of paroxysmal atrial fibrillation. This means that your heart beats normaly and during some short moments passes in atrial fibrillation and that beats normal again. So if I was your treating doctor I will recommend you an holter monitoring to see what are this episodes and haw much are they because this determines your treatment. All sow you have to bring the thyroid function in normal values because it exacerbate the atrial fibrillation. You don t kneed to be very concern but I advise you to make all the examinations because if it is atrial fibrillation again and if it is not treated it rises the possibility of having an insult. Hope I was helpful.Wish you good health, Best regards.