Hello! Usually there are no interactions seen between metaprolol and synthroid. As you re a nurse I ll explain you what might be happening in your case. In a very few population thyroid drugs cause increase in hepatic metabolism and lead to decreased effectiveness of beta blockers. Hence you might be facing a lack of efficacy of metaprolol leading to increased heart rate which is controlled by metaprolol. Usually it shouldn t cause you much worry if there is no symptoms. Consult your doctor for a change in medication if the symptoms worsen. Dr. Karan Shekar.