Was taking 150 mcg of synthroid (thyroxine) for over year; recent bw tsh 0.01 had rapid heart rate. on only half tab of 112 mcg for 7d feel better. what happened?

Overcompensated: You went from HYPOthyroid to HYPERthyroid. This could be related to the amount of synthroid (thyroxine) you are taking or improvement of thyroid function by itself (plus thyroid supplements = hyperthyroidism). Discuss with your internist or endocrinologist so either can adjust dose of the medicine

Too much: suggests a little heavy on the thyroid medicine. Speak with your doctor about what dose levothyroxine is right for you.