Hi & Welcome. I can understand your concern. Mirena is a long acting Progesterone containing reversible intrauterine device (IUD) which is one of the most effective forms of birth control. Its most common side effects include menstrual irregularities, weight gain, headache, acne, pelvic inflammatory disease (infection), etc. It is unlikely to be the cause of hyperthyroidism. The most important and common cause of hyperthyroidism is the autoimmune disorder (Graves disease). In this disorder, the body makes an antibody called thyroid-stimulating immunoglobulin that causes the thyroid gland to make too much thyroid hormones. This type of hyperthyroidism tends to run in families and it occurs more often in young women Another cause is development of goiter (lumps in the thyroid) that may gradually grow and increase their activity so that the total output of thyroid hormone into the blood is greater than normal. Reversible hyperthyroidsim can also result from viral infection of the thyroid gland (thyroiditis) that causes the gland to to leak the stored thyroid hormones. You are advised to consult a physician/endocrinologist for proper clinical examination and to repeat the thyroid function tests, if found to hyperthyroid you need to take antithyroid medications. Hope this answers your query.