Hyperthyroid Disorders

Lesson 4: Causes of Hyperthyroidism

Pituitary Tumors and Miscellaneous Causes

As we’ve learned in the previous sections, Graves’ disease is the most common cause of hyperthyroidism. Other common causes of hyperthyroidism include thyroid nodules, thyroiditis and genetic mutations. In this lesson we will learn about some of the less common causes of hyperthyroidism, which include TSH-secreting tumors and iodine excess.

In earlier sections we learned that the pituitary hormone thyrotropin, which is commonly known as TSH, stimulates thyroid cells to produce and release thyroid hormone. Normally, TSH secretion is under the influence of the hypothalamus via the hormone TRH or thyrotropin releasing hormone. Thus, any condition that causes abnormally high levels of TRH or TSH will indirectly cause hyperthyroidism. Furthermore, the hormone hCG, which is secreted in pregnancy, is very similar to TSH. When hCG levels are high, they too, can cause the production of excess thyroid hormone.

Tumors In this section we’ll learn about some of the tumors that can cause hyperthyroidism when they secrete the hormones TRH, TSH or hCG. The most common of these rare tumors are TSH secreting pituitary adenomas. These are commonly called TSH-omas. The first of these tumors was identified in 1960, and it was discovered by an investigation of the unusually high TSH level. Up until mid-2004, only 336 cases of TSH-oma have been published and they have occurred in patients ranging from 11 to 84 years. Patients have symptoms of hyperthyroidism and also symptoms related to the pressure effects of pituitary adenoma. These include loss of vision, visual field defects and loss of anterior pituitary function. In some patients, hyperthyroid symptoms are shadowed by the acromegaly or hypothyroidism that accompanies some pituitary tumors. Because TSH stimulates thyroid cell growth, most patients have a goiter.

Tumors causing excess hCG and related hyperthyroidism include hydatiform mole, choriocarcinoma, seminomas and struma ovarii. These conditions can be determined by ultrasound and measurements of hCG. Patients generally have goiter but no other obvious symptoms of thyrotoxicosis. In the condition of hyperemesis gravidarum, which occurs more often and Asian women and is related to pregnancy, high concentrations of hCG can cause a transient hyperthyroidism accompanied by profuse vomiting.

Excess iodine Hyperthyroidism may also be caused by medications with high iodine content or by food and medications contaminated with thyroid hormone derivatives. Hamburger toxicosis is a condition that occurs sporadically and is associated with ground beef contaminated with thyroid hormone. Dietary supplements, especially metabolic boosters, sometimes contain T3 or the derivatives TETRAC and TRIAC. Medications like amiodarone or products containing high iodine content can cause transient hyperthyroidism. In patients with nodules or who are genetically predisposed to Graves’ disease, excess dietary iodine can trigger hyperthyroidism.

Trauma and viruses In injury to the thyroid gland or forceful palpitation of the gland, the immune response can result in hyperthyroidism. Similarly, hyperthyroidism can occur after certain viral infections including infection with the EBV virus.

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