Uncommon causes of hyperthyroidism or too much thyroid hormone in the body include:

• Taking too much thyroid hormone medicine. This can happen if you are taking thyroid hormone to treat thyroid cancer. It can also happen if you take too much thyroid hormone in an attempt to lose weight.
• Taking large amounts of substances or medications that contain iodine, such as iodized salt, kelp, cough syrups, multivitamins, or certain medications, such as amiodarone, lithium, and interferon alfa.
• Having too much thyroid hormone after pregnancy.
• A growth in the uterus that releases hormones into a woman's body, causing the thyroid to produce too much thyroid hormone.
• A rare tumor that grows on a woman's ovaries. The tumor contains thyroid tissue, which releases thyroid hormone into the body.
• A rare tumor on the pituitary gland located in your brain. This tumor causes the pituitary gland to make too much thyroid-stimulating hormone (TSH). TSH stimulates the thyroid gland to make more thyroid hormones.
• Stress, surgery, or using contrast materials containing iodine for a CT scan or radioactive iodine therapy.

The thyroid gland usually returns to normal after treatment. For example, once you quit taking substances that contain large amounts of iodine or a hormone-producing tumor is removed, the thyroid gland again works normally.