Is it possible to have nothing wrong your thyroid, but still have an autoimmune disease?

Yes: Autoimmune thyroid disease can manifest as an enlarged thyroid, abnormal thyroid antibodies on lab testing, sometimes nodular thyroid, and yet thyroid function can be normal, low, or high. Thyroid functions would be indicated by tsh, t4, T3 (liothyronine) levels. Thyroid ultrasound would look for nodules. There is no treatment for the autoimmune part, but abnormal thyroid function or nodules can be treated.