My tsh is 0.48. my free t4 is not readable. i am being treated with synthroid 88 daily. my tsh in october was .90. why does it keep dropping?

0.088 mg is too high: a dose, as judged by your brain/hypothalamus which releases TSH in response to inadequate thyroid hormone. Basically, if your brain is happy with thyroid hormone output, then the TSH will be ~1.0, optimal, not quite as low as it currently is. https://en.wikipedia.org/wiki/Hypothalamic%E2%80%93pituitary%E2%80%93thyroid_axis is a reasonable overview.