Recently diagnosed with itp (not stable) treated with eltrombopag (mmf didn't work) & have hypothyroidism. what tests can i do to find/rule out cause?

Cause for which one?: The ITP or the hypothyroidism? First I wish you get better and both conditions become controlled. The ITP as its name implies, has no specific reason that we know of, several theories, and is considered an auto-immune disease. Likewise is the hypothyroid disorder can be primary or secondary, but again often times no cause can be elicited. Keep the F/U with your hematologist and endocrinologist