Why did my doctor lower my thyroid medication dosage?

Think the reverse. When your thyroid hormones are low your thyroid is over acting “hyper” and thinks you need less hormones. When thyroid stimulating hormone is high it is actually too low and under active.

It's a balancing act and when the tsh is low they decrease the dosage so your thyroid can put out more tsh. Meaning the medication suppresses the thyroid from producing tsh. The lower dosage should cause more output from your thyroid. I have been hypothyroid for 25 years. .75 keeps mine right at 1.0