I am a moderately fit female (running 6.21 to 9.32 miles). My height is 5' 6" and weight is 149 pounds. I have hypothyroidism, and I am on Levothyroxine. My TSH is around 1.5 mIU/L. My question is about a fluctuating resting heart rate. According to my sports watch, my RHR can be between 80 and 41 bpm. It is the lowest two or three days after a run where I have struggled to keep my heart rate below 150 bpm, even while running slower than a walking pace. Is this something to be concerned about, or is it just a weird glitch? Thank you.