What could be the cause of sudden drop in TSH levels?

Hi doctor,

I am a 53-year-old peri-menopausal woman. My weight is 160 lbs. I had a thyroidectomy ten years before. Since then, I have been taking Levothyroxine 112 mcg daily and was maintaining a TSH level around 3.5 to 5 mU/L. (last good test was four months back with TSH- 4.0 mU/L). However, three months back, I had to change to a different Levothyroxine brand and used it for two months, then the next month woke up with a fast heartbeat in the morning and realized that my TSH level was suddenly down to 0.5 mU/L (also had a sudden drop in blood count of 8 g/dL). So I stopped taking it for three days and continued with the original brand. Every ten days or so, my TSH would go up by 1.0 point. So after ten days, it was 1.5 mU/L, then 20 days later, it was 2.5 mU/L (blood levels also rose to 9 g/dL). However, after another ten days (a full 30 days) from the initial problem, my heart again started to beat fast in the morning while waking up. The TSH levels had dropped again to 1 mU/L (blood levels still getting better- 10.4 g/dL, except for some exceptionally high white blood cells). So it is confusing because we thought the cause could be the switch in medicine, but now we cannot figure out what is causing these drops suddenly. Any thoughts on what could cause these drops in TSH value and cause hyperthyroid and fast heartbeats in the mornings?