A 2021 study including collaborators from Yale School of Public Health, Mayo Clinic, and University of Arkansas found that out of the 23 million Americans who are actively taking levothyroxine, approximately 21 million (~90 percent) likely don’t need the prescription. Because levothyroxine can cause side effects including anxiety, heat intolerance, and diarrhea, and in extreme cases, cardiovascular morbidity and death, especially in those over 80, this finding is alarming, says El-Khoury.