What causes and inability to tolerate thyroid medicine? if i'm not taking too much, what other reasons are there for experiencing increased heart rate

Inability?: What does that mean? Too much replacement or an unusual dose may cause hyperthyroid symptoms such as fast heart rate. Talk to the doctor who prescribed it.

Type of thyroid: Some people cannot tolerate synthroid (thyroxine) due to adverse reactions to the compounds used to make up the pill. Those folks usually do well on tirosint or armour thyroid.