Can a putitiary cyst cause hyperthyroidism?

Not likely: Hyperthyroidism is almost always caused by the thyroid, usually either graves disease or a toxic multinodular goiter. It is possible for a pituitary tumor to make excessive tsh, which stimulates the thyroid, but this is very rare. Pituitary mris are very sensitive and pick up a lot of small stuff that may not be significant. See your endocrinologist to sort this out.