TSH level is normal but my T4 level is low?

Doctor said my TSH level is normal but my T4 is the exact same as it was 4 months ago when my synthroid was increased from .075 mcg to .1 mcg. Has anyone ever heard of this or have any ideas why the synthroid isn't raising my T4 levels? I am scheduled to have bloodwork to check prolactin levels and an MRI/MRA to check for possible tumor on pituitary gland. I just feel lost, confused, and like I am fighting a losing battle with hypothyroidism and migraines (which I am not sure if they could be associated). Any help is appreciated!!