constant headache, dizzyness, hoshimotos disease?

Hi. every since i was young i have had this constant headache and light-headedness, and doctors say i have low thyroid levels. though it is only off by .1 ... I have had every lab test done that you can imagine, an MRI, amd they all came out normal except for a low amount of vitam D, and the thyroid level. They put me on some medication (synthroid, i think) to get my thyroid level up, but frankly, i feel my symptoms are worsening. I can barely keep my eyes open in class, and it's hard to do/concentrate on anything. My speech is kinda messed up, and after doing something as simple as lifting up my mattress to clean it, im incredibly shakey (shaky). My heart pounds even when im resting, and i feel like someone put a brick inside my head ... (i know, wierd description .... haha) im also involved in the school play, (which takes a TON of energy and one of my doctors thought i shouldn't perform, but its the one thing i love) and now-a-days i just have no will power ... it's so strange, and one by one i see my doctors give up. even my parents are starting to stop believing that im actually sick.

Sorry for the long post, im just feeling very lost and alone right now ... tips and/or encouragement please! Thanks for anything you type!