Hyperthyroidism or ?????

I have the symptoms of hyperthyroidism but all tests show that my thyroid is fine. The doctor still says I have 'mild hyperthyroidism' but the hormones are still in the normal range.

What other conditions can resemble hyperthyroidism by having the same symptoms?

My symptoms include: weight loss, thining hair, heart palpitations, tremors, insomnia, and lack of concentration.

I have been prescribed Ambien for the insomnia, Atenolol for the palpitations and Tapazole for the mild hyperthyroidism.

I stopped taking the Tapazole (methimazole) since it made me feel like crap. I would take it and feel sluggish. Toyed with the doses and figured it was the new Rx making me feel crappy.

Being a pharmacist I understand the purpose of meds and know how they work, ect. I also know it isn't suggested to go off medicines without doctors approval - but this specific medicine takes 3-4 months to reach theraputic response levels. Therefore I stopped taking it, one to stop the immediate side effects, and two to research other possible causes for my symptoms.

I can always go back on it if need be. The symptoms aren't life changing nor threatening. My goal is to get to the bottom of the cause of the symptoms and determine another cause of my symptoms....

Without knowing your symptoms I can't offer a specific list, but the differential diagnosis for hyperthyroidism is pretty short and includes some mood disorders (bipolar, depression, dysthymic disorder), a goiter, Crohns Disease, and Graves Disease (Graves would have shown on thyroid tests).
Sounds very much like you may have what is called subclinical hyperthyroidism, in which your serum TSH numbers are low, but your thyroid hormone numbers (T3, T4, FT3, or FT4) are normal. This is a laboratory diagnosis.
If your serum TSH level is high, you should have the test repeated, with a serum FT4 measurement, in no sooner than two weeks but no later than three months to confirm subclinical hypothyroidism.

Well that all depends on what symptoms your actually having. I went to med school for awhile so I know some stuff that might be helpful but before I actually tell you anything I'd rather here some of what your experiencing so that I don't get you panicked or anything. Your doctor is correct though. You can have a mile case of hyperthyroidism and all test show normal. The testing range is quite wide so if your normal is low but then it starts raising because of this then it may still lay in normal range according to the doctors. Like for instance with me, my normal regulated body temp is low around 95 degrees so if my temp is 99 at the doc they dont think anything is wrong but in reality for my body i'm actually on the high side. Get what I'm saying? If you want me to help you out a little feel free to e-mail me and I can check in my med books or give any input i have but again i dont want to jump to any conclusions as to what it might be without knowing what your experiencing.