What are Goiters and Thyroid Nodules?

Any enlargement of the thyroid gland may be called a goiter. A goiter may be diffusely enlarged, irregular, lumpy and bumpy, symmetrical or asymmetrical, and may be associated either with hyperthyroidism, hypothyroidism, or normal thyroid function in various sequences and combinations. Enlargement caused by a solitary thyroid nodule (a lump in or on the thyroid gland) or multiple nodules is still called a goiter; a goiter with many nodules is called a multinodular goiter.

Some people become anxious when they find out that they have goiters with or without thyroid nodules. Their first thoughts may be that they will need surgery, that they have cancer, that their necks will become enormous, or all of the above. The following observations may provide perspective and alleviate undue concern:

• Thyroid surgery for a goiter is uncommon.
• Goiters that contain no thyroid nodules are very
  unlikely to be cancerous.
• The majority of thyroid cancers appear as a single
  thyroid lump (the solitary thyroid nodule).
• Approximately 90% of all solitary thyroid nodules are
  noncancerous.
• Enormous goiters are uncommon in iodine-sufficient
  areas, such as the United States.
  

Causes of Thyroid Gland Enlargement

Five general causes of thyroid gland enlargement are:

• tumors
• cysts
• inflammation/infection
• growth factors
• obscure or unknown (idiopathic) causes

Tumors
A thyroid tumor is an abnormal growth of thyroid cells. Although the exact cause of most thyroid tumors is unknown, certain types of radiation will promote their development. Unfortunately, the 1986 Chernobyl nuclear reactor explosion confirmed this effect of radiation. Children exposed to radiation from the Chernobyl explosion have developed more thyroid tumors, both benign (noncancerous) and malignant (cancerous), than unexposed children have.

Thyroid tumors are the most common cause of thyroid nodules. All tumors are nodules, but not all nodules are tumors-for example, nodules can also be cysts.

Cyst Formation
Cysts are fluid-filled nodules. Thyroid cysts most commonly result from degeneration of benign tumors into liquid, although malignant tumors can occasionally do the same thing. When the tumors degenerate, the residual nodules may be completely fluid-filled (cystic nodule) or partially fluid-filled (mixed nodule). Most nodules containing fluid are mixed nodules. Cystic nodules are less likely to be malignant than mixed and solid nodules are.

It is difficult for a doctor to determine from a physical examina­tion whether a thyroid nodule is cystic, mixed, or solid. Ultrasound can readily distinguish between these different types of nodules. A nodule will sometimes degenerate by abruptly hemorrhaging, causing pain and swelling in the neck that may last a few days. An ultrasound cannot reliably distinguish between blood and other fluid in a cyst.

Inflammation / Infection
Inflammation of the thyroid gland with an accumulation of white blood cells is seen in many types of thyroiditis, as discussed in Chapter 8. The most common type of inflammation, and the most common cause of hypothyroidism in the United States, is Hashimoto's thyroiditis. Although very uncommon, infection of the thyroid gland also causes white blood cells to accumulate in the thyroid. Either inflammation or infection can cause the thyroid gland to enlarge.

Growth Factors
Three common growth factors cause thyroid gland enlargement. The most common cause is increased TSH (thyroid-stimulating hor­mone) production from the pituitary gland. When thyroid hormone is low, the pituitary gland secretes TSH, which stimulates growth of the thyroid gland and the release of more thyroid hormone. For example, since iodine is an essential component of thyroid hormones, iodine deficiency sets off a chain reaction that ultimately leads to thyroid enlargement. The sequence of events leading to goiter formation in these patients is:

iodine deficiency
thyroid hormone deficiency
increased TSH production
goiter

Although iodine deficiency is very uncommon in the United States, there are approximately 655 million people worldwide with goiters caused by iodine deficiency. In iodine-sufficient areas, lithium, a drug used to treat patients with manic depressive illness, is a more common cause of elevated TSH and goiter formation.

A second growth factor is an antibody associated with Graves' disease that duplicates the function of TSH. This antibody is called thyroid-stimulating antibody (TSAb) or thyroid-stimulating immunoglobulin (TSI).

The third growth factor that will cause the thyroid gland to enlarge is a hormone called human chorionic gonadotropin (hCG). This hormone is produced during normal pregnancies and, ordinarily, does not cause any thyroid enlargement. In some uncommon cases, however, a pregnant woman may develop a growth in the uterus, such as a hydatidiform mole or a choriocarcinoma, which may lead to massive overproduction of hCG, a goiter, and hyperthyroidism.

Obscure or Unknown (Idiopathic) Causes
Sometimes a patient is frustrated and bewildered when there is no identifiable cause of a goiter. There could be an obscure or uncommon explanation for the goiter, but the process of identifying it may be too lengthy and costly to pursue. Identifying an obscure cause will not necessarily change the patient's treatment. At the end of the day, the cause of some goiters will remain unknown (idiopathic).