There are several possible causes for high thyroid levels, including Graves' disease, toxic adenoma, Plummer's disease and thyroiditis, according to Mayo Clinic. The general term used when a patient has an overactive thyroid is hyperthyroidism. Women are five to 10 times more likely to get hyperthyroidism than men, as stated by WebMD.
Graves' disease is the most common cause of hyperthroidism, according to Mayo Clinic. Graves' disease is an autoimmune disorder that occurs when the antibodies of the patients' thyroid stimulate the thyroid to produce excessive amounts of T-4. Rather than protecting against viruses, the antibodies attack the thyroid. The cause of Graves' disease is unknown, though a genetic factor is likely.
Toxic adenoma and Plummer's disease are examples of hyperfunctioning thyroid nodules. These occur when one or more adenomas of a patient's thyroid produce an excessive amount of T-4, according to Mayo Clinic. Doctors do not know why some adenomas begin producing too many hormones.
Thyroiditis is the inflammation of the thyroid, states WebMD. It occurs when a virus or another problem with the immune system results in the swelling of the gland, which then leaks thyroid hormone into the bloodstream. There are three types of thyroiditis: subacute, postpartum and silent.