I am a very fit, very healthy female long distance runner... however, my last 4 runs, my Average Heart Rate has been through the roof sitting at 183 and my maximum was over 200.. results I would only expect to see when I am sprint training... Causing me to have to cut my runs way short only being able to sustain this heart rate for 8kms.. I have had my Thyroid removed 8 years ago and am now on Thyroxin, however, as per my last blood test my thyroid function is normal and I recently had complete blood checks done and everything was normal.. Any idea why my heart may be doing this ???