Causes of Hyperthyroidism
Broadly speaking, there are four causes of hyperthyroidism—thyroid overstimulation, thyroid nodules, thyroid disruption, and miscellaneous causes.
Diseases Associated with Thyroid Overstimulation : Graves’ Disease
The most common cause of hyperthyroidism in the United States is Graves’ disease, an autoimmune disease characterized by some or all of the following: hyperthyroidism, goiter, eye changes, and skin changes. Autoimmune diseases are characterized by the presence of autoantibodies circulating in the bloodstream. Antibodies are proteins formed by the immune system to protect the body against foreign chemicals, bacteria, and viruses. In an autoimmune disease, antibodies are formed against the body’s own chemicals. Antibodies may be formed against a receptor, the specific site on a cell that captures a chemical such as a hormone. In Graves’ disease, antibodies are formed against the receptors on thyroid follicular cells that capture thyroid-stimulating hormone (TSH). Remarkably, some of these antibodies duplicate the function of TSH and stimulate the thyroid gland to produce and secrete more thyroid hormone than normal. Autoimmune thyroid diseases, such as Graves’ disease and Hashimoto’s thyroiditis, tend to run in families and are more common in women.
It is not unusual for several generations of women to have either Graves’ disease and hyperthyroidism or Hashimoto’s thyroiditis and hypothyroidism. Patients who have Graves’ disease should tell members of their families—especially their mothers, daughters, sisters, aunts, and nieces—about their condition so that they are aware that there is a hereditary thyroid disease in the family. At least 50% of patients with Graves’ disease will have the antibodies found in Graves’ disease as well as the antibodies characteristic of Hashimoto’s thyroiditis.
A disrupted thyroid gland releases stored thyroid hormone into the bloodstream. Disruption of the thyroid gland is seen in certain types of thyroiditis, which are transient or temporary, including: subacute thyroiditis, postpartum thyroiditis, painless thyroiditis, radiation-induced thyroiditis, and acute suppurative thyroidits
Toxic Thyroid Nodules and Toxic Multinodular Goiters
Autonomous function describes the function of a part of the thyroid gland that acts independently of normal control mechanisms. If a thyroid nodule fails to turn off its production of thyroid hormone, it is referred to as an autonomously functioning thyroid nodule or toxic autonomously functioning nodule. An autonomously functioning thyroid nodule is indicated by an increased uptake of radioactive iodine by the nodule. Since uptake of radioactive iodine is greater in the autonomously functioning thyroid nodule than in the surrounding tissue, it is referred to as a hot nodule.
When a thyroid gland containing more than one nodule has one or more autonomously functioning nodules producing sufficient thyroid hormone to cause hyperthyroidism, it is referred to as a toxic multinodular goiter.
Thyroid Overstimulation:Iodine-Induced Hyperthyroidism
Ingestion of excessive iodine can occasionally induce hyperthyroidism in susceptible patients, particularly ones with long-standing multinodular goiters. Iodine-induced hyperthyroidism may also occur weeks to months after exposure to x-ray dyes with iodine, such as those used in CT scans and heart catheterizations.
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