tachycardia (or fast heart beat), labile blood pressure, dizziness, flushing, tremor, etc. Since you are also on Synthroid (or levothyroxine), you may experience faster heart rates if the
dose is too high for you.
I do not know if your heart rate readings are due to nortriptyline, combination with Synthroid, or something else. However, I strongly recommend for you to consult your provider as soon as possible.