Hyperthyroid Disorders

Lesson 4: Causes of Hyperthyroidism

Nodules and Adenomas

Nodules and adenomas are thyroid growths capable of trapping excess iodine and producing excess thyroid hormone. Nodules and adenomas are referred to as toxic when they cause hyperthyroidism. Nodules and adenomas occur when clusters of thyroid cells cluster together forming lumps or masses. Cysts, which are usually benign, are composed of fluid rather than thyroid cells. Nodules and adenomas are frequently composed of more than one type of thyroid cell. The type of cells present determines if nodules are benign or cancerous. Fewer than 10 percent of nodules are cancerous. Of those that cause thyroid cancer, the majority of cancers are easily treated.

Thyroid nodules Like most other thyroid disorders, nodules are more likely to occur in women. In toxic multinodular goiter, patients are generally older and may have recently been exposed to iodine-containing medications such as the heart medication amiodarone or iodine contrast dyes used in imaging procedures. Depending on iodine concentrations in the area, as many as three to seven percent of adults, have been found to have palpable thyroid nodules, which means they can be felt during an external examination of the thyroid gland.

Multiple nodules frequently occur in patients with a family history of benign thyroid nodular goiter or Hashimoto’s thyroiditis. Multiple nodules without a dominant or unusually large nodule, are usually benign although this is not always the case. Risk factors for thyroid cancer include: a family history of thyroid cancer; prior history of external neck irradiation during childhood; symptoms of hoarseness or difficulty swallowing in the presence of a thyroid nodule; and the presence of a firm, fixed nodule with or without swollen cervical lymph nodes.

Adenomas Nodules that take up large amounts of radioiodine on uptake tests are known as hot nodules or adenomas. Until hyperthyroidism occurs hot nodules are usually observed to see if they increase in size. Over time, approximately 5 percent of patients with adenomas will progress to hyperthyroidism each year. If the nodule is larger than 3cm, the progression to hyperthyroidism is usually faster. Patients with Graves’ disease who develop hyperfunctioning nodules are said to have Marine-Lenhart syndrome. This is a rare condition, occurring in about 2.7 percent of patients with Graves’ disease.

Most solitary thyroid nodules are colloid adenomas or benign follicular adenomas. A benign follicular adenoma cannot be differentiated from a well-differentiated follicular carcinoma with fine-needle aspiration as this distinction on the presence or absence of capsular invasion by the tumor.

Patients with toxic adenomas usually present with a lump in the neck and they may have symptoms of hyperthyroidism. Toxic adenomas frequently cause a low TSH with or without an elevated FT4 and or FT3. In toxic adenoma, FT3 levels may also be elevated in the presence of a normal FT4. A hyperfunctioning solitary adenoma is suggested on physical examination by atrophy or shrinkage of the remainder of the thyroid gland. Toxic nodules typically are diagnosed in patients with a gradual onset of hyperthyroidism and a nodule larger than 3 cm in size. In children, these nodules may be low-grade papillary cancers although in adults toxic nodules are very rarely malignant. Rarely, functioning thyroid carcinomas produce thyrotoxicosis. A limited course of thyrotoxicosis can occur in euthyroid patients with nodules and adenomas who are given excess iodine in the form of iodine contrast dyes or other medications containing iodine. A careful history is needed to determine if the thyrotoxicosis is a transient or self-limited event.

Thyroid lymphoma Lymphomas are tumors that occur in lymphoid tissue. Lymphoma primary to the thyroid gland is rare, comprising less than 5 percent of all malignant thyroid tumors. Secondary involvement of the thyroid gland by malignant lymphoma that originated elsewhere in the body is more common and is seen in 20 percent of patients dying from generalized lymphoma. Thyroid lymphoma which invades the thyroid gland can cause thyrotoxicosis. Thyroid lymphoma is more prevalent in patients with long-term chronic autoimmune hypothyroidism. Thyroid lymphoma should be suspected in patients with chronic thyroiditis who experience sudden thyroid gland enlargement.

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