Why does excessive thyroxine lead to tachycardia?and specifically what kind of drug would neutralize this?

Increases metabolism: Thyroid hormone is esstential for cellular functions, but when it excess, it works as a catabolic (breakdown) hormone and speeds up virtually all cellular processes including the pace-maker in the heart, thus rapid heart rate (tachycardia). Meds such as metoprolol or Cardizem can slow down the heart rate, but there is no med to neutralize it--it must be stopped/lowered. Consult doc. Good luck.

Hyperadrenergy: It leads to too much adrenaline and adrenaline-like chemicals. Drugs called beta blockers are very effective at controlling the cardiac effects.