Uncommon causes of hyperthyroidism include a rare growth in the uterus, a rare tumor on a woman's ovaries or a tumor on the pituitary gland, according to WebMD. Taking certain substances or medicines with iodized salt, such as amiodarone, interferon alfa, cough syrups, kelp and multivitaimins, in large quantities may also cause hyperthyroidism in rare cases.
In some uncommon instances, taking too much thyroid hormone medicine to treat thyroid cancer, lose weight or for other reasons may result in hyperthyroidism, notes WebMD. If a tumor develops on the pituitary gland, which is located in the brain, this causes the pituitary gland to release excess amounts of thyroid-stimulating hormone, which in turn causes the thyroid to produce more hormones than usual.
Radioactive iodine therapy, stress, surgery and undergoing a computerized tomography scan with contrast materials containing iodine may also result in hyperthyroidism in uncommon cases, explains WebMD. After treatment, the thyroid gland typically returns to functioning normally, and the hyperthyroidism resolves itself.
Hyperthyroidism may also occur as a result of thyroiditis, which is inflammation of the thyroid gland, states Mayo Clinic. Excess thyroid hormone located in the thyroid gland may escape into the bloodstream as a result of the inflammation, and in rare cases may cause pain, as is the case with subacute granulomatous thyroiditis. Women may also experience postpartum thyroiditis after pregnancy.