I am sorry to hear about the problems you are having. You have provided information regarding 'abnormal ECG' which was requested by your GP after you presented to them with a history of intermittent palpitations ('lasting a second or two') with a background of thyroid diesease as described. I assume that your Thyroid profile was recently checked (to ensure that Thyroxine 100microgram is still the appropriate dose for you and does not need adjusting).

You describe 'short pr interval' on the ECG - hopefully the 'printed' autoated ECG reading (which most 12 lead machines now produce) was confirmed by the Cardiologist who would have double-checked that in fact the P-R interval was 'short'. I understand that the Cardiologist communicated with your GP saying that they did not see 'an av nodal rhythm' and the p-wave (morphology / shape) was normal - if this is a fact, it is indeed REASSURING. Just to explain - 'short pr interval' implies that the impulse or 'spark' starting each heart beat (which normally originates in the 'sino-atrial node' located at the top of the Right Upper Chamber of our heart) is actually starting much Lower Down and closer to the middle of the heart where the next node called 'A-V, or atrio-ventricular' node is located' hence the time it takes the spark from the abnormal starting point, possibly lower down, is much shorter and if, another spark starts from a higher or more 'normal' Sino-Atrial (SA) node position, the risk is that both may cause the two lower, 'pumping' chambers (One Pumps fresh blood to Brain and rest of our body-Left Ventricle, and the other pumps de-oxygenated blood to the lung circulation for oxygenation as we breath); may contract very rapidly resulting in a much faster and irregular rate which may cause the chambers not to pump out fully, but due to the fast rate, pump out only partially, sometimes resulting in symptoms such as fainting (syncope), or chest pain etc- which you have NOT described. Here is a link for further informationhttp://medical-dictionary.thefreedictionary.com/Ventricular+pre-excitation+with+arrhythmia andhttp://my.clevelandclinic.org/heart/disorders/electric/wpw.aspx  

As you can gather from the information, these conditions come under a group of cardiac conduction abnormalities also known as 'Ventricular Pre-Excitation Syndroms' - the p-wave (which shows 'electrically' the two upper chambers or, Atria contracting and / or the QRS complex (which is due to the two lower chambers or Ventricles contracting) are usually abnormal in shape or morphology - not suggested by the Cardiologist to your GP; again, a Reassuring fact. 

With regard to exercise and activity, if I had been able to see the ECG myself, I could comment further, but at this time, I am unable to do so hence would suggest you please check back with your GP. However, I can share the fact with you, that Coffee, Chocolate, Tea (any any caffeinated beverages etc.) can increase heart rate and sometimes ' bring out' palpitations. Also in some people who have been on Thyroxine, for various reasons, over time, the dosage may need adjusting downward as even a slight excess of Thyroxine may predispose to faster heart rates, and sometimes, a condition called Atrial Fibrillation or, Paroxysmal Atrial Fibriallation; hence the Cardiologists recommendation to your GP (re. 24 hour cardiac 'tape' etc. as above) is a very good one. There are other conditions that affect heart rates (including anemia, leading to hyperdynamic circulation etc.) but the approach being suggested is the most reasonable and appropriate one.

I do hope the information provided is helpful to you and suggest you proceed with the investigations being suggested by your GP (and the Cardiologist). I wish you good health.