The most common cause of elevated thyroid levels, referred to as overactive thyroid or hyperthyroidism, is Grave's disease, notes WebMD. Other conditions, such a thyroid nodule, thyroiditis and excess iodine, can cause hyperthyroidism as well. Taking too much thyroid medication can also cause the thyroid to overproduce hormones.
Graves disease is an autoimmune disease. It causes the body to make an antibody, which in turn causes the overproduction of thyroid hormones, states WebMD. A thyroid nodule, or in some cases multiple nodules, that grow in the thyroid gland can gradually increase the production of hormones over time. Thyroiditis is inflammation of the thyroid that is caused by a virus, an immune system issue or an unknown cause. This occurs in 20 percent of women after pregnancy.
The body uses iodine to make thyroid hormone, according to WebMD. People who eat or drink excess amounts or who are exposed to toxic levels of iodine can develop hyperthyroidism. Supplements and foods such as kelp and seaweed as well as the medication amiodarone contain high amounts of iodine.
A person who takes medication for hypothyroidism, which is a condition in which thyroid levels are too low, must never take an extra dose to make up for a missed dose, warns WebMD. Doing so can cause excess levels of thyroid levels in the body leading to hyperthyroid symptoms.