T3 levels can lead to unpleasant symptoms such as rapid heart beat, insomnia and
anxiety. Too high T-3 has a risk for dangerous cardiac arrhythmias and
heart attacks and
also can harm the heart and the bones.
quote - The Journal of Clinical Endocrinology & Metabolism
Combined Thyroxine/Liothyronine Treatment, T-4 and T-3, Does Not Improve Well-Being, Quality of Life, or Cognitive Function Compared to Thyroxine Alone - jcem.