Welcome to Just Answer and thanks for your question. What is considered normal for thyroid hormones varies from lab to lab. Both TSH (thyroid stimulating hormone) and free T4 are looked at together to determine if the thyroid supplement needs to be increased or lowered. Normal TSH is 0.3 to 3.0. Free T4 should be about 0.8 to 2.4 ng/dL.
I'm confused about the dose of hormone you are taking. I think you must mean you went from 175 mcg to 75 mcg. If you will let me know what was 6.4 we can talk some more.