If I don't have graves' disease or hot nodules on my thyroid, yet it's producing 2x what is normal, what is the cause?

Hyperthyroidis/BT. This can be primary hyperthyriodism, hashimoto's thyroiditis or brain tumor. This needs urgent eval by gland specialist, because it can lead to hypertension or heart attack if not treated.
Thyroid. Depending on what lab is actually 2x as normal will help point to the root of the problem, however, i believe in treating the patient and not the labs. So if your signs and symptoms point to dysfunction in thyroid supplementation with iodine and l-tyrosine will help support and normalize your thyroid numbers. These 2 are building blocks for the hormone which we normally are deficient in.