Hyperthyroidism - Causes

Causes

There are several causes of hyperthyroidism. Most often, the entire gland is overproducing thyroid hormone. Less commonly, a single nodule is responsible for the excess hormone secretion, called a "hot" nodule. Thyroiditis (inflammation of the thyroid) can also cause hyperthyroidism. Functional thyroid tissue producing an excess of thyroid hormone occurs in a number of clinical conditions.

The major causes in humans are:

• Graves' disease. An autoimmune disease (usually, the most common etiology with 50-80% worldwide, although this varies substantially with location- i.e., 47% in Switzerland (Horst et al., 1987) to 90% in the USA (Hamburger et al. 1981). Thought to be due to varying Iodine in the diet.
• Toxic thyroid adenoma (the most common etiology in Switzerland, 53%, thought to be atypical due to a low level of dietary iodine in this country)
• Toxic multinodular goitre

High blood levels of thyroid hormones (most accurately termed hyperthyroxinemia) can occur for a number of other reasons:

• Inflammation of the thyroid is called thyroiditis. There are several different kinds of thyroiditis including Hashimoto's thyroiditis (immune-mediated), and subacute thyroiditis (DeQuervain's). These may be initially associated with secretion of excess thyroid hormone, but usually progress to gland dysfunction and, thus, to hormone deficiency and hypothyroidism.
• Oral consumption of excess thyroid hormone tablets is possible (surreptitious use of thyroid hormone), as is the rare event of consumption of ground beef contaminated with thyroid tissue, and thus thyroid hormone (termed "hamburger hyperthyroidism").
• Amiodarone, an anti-arrhythmic drug, is structurally similar to thyroxine and may cause either under- or overactivity of the thyroid.
• Postpartum thyroiditis (PPT) occurs in about 7% of women during the year after they give birth. PPT typically has several phases, the first of which is hyperthyroidism. This form of hyperthyroidism usually corrects itself within weeks or months without the need for treatment.
• A struma ovarii is a rare form of monodermal teratoma that contains mostly thyroid tissue, which leads to hyperthyroidism.

Hypersecretion of thyroid stimulating hormone (TSH), which in turn is almost always caused by a pituitary adenoma, accounts for much less than 1 percent of hyperthyroidism cases.