One of the most common causes of goiter formation worldwide is iodine deficiency. While this was a very frequent cause of goiter in the United States many years ago, it is no longer commonly observed. The primary activity of the thyroid gland is to concentrate iodine from the blood to make thyroid hormone. The gland cannot make enough thyroid hormone if it does not have enough iodine. Therefore, with iodine deficiency the individual will become hypothyroid. Consequently, the pituitary gland in the brain senses the thyroid hormone level is too low and sends a signal to the thyroid. This signal is called thyroid stimulating hormone (TSH). As the name implies, this hormone stimulates the thyroid to produce thyroid hormone and to grow in size. This abnormal growth in size produces what is termed a “goiter.” Thus, iodine deficiency is one cause of goiter development. Wherever iodine deficiency is common, goiter will be common. It remains a common cause of goiters in other parts of the world.
Hashimoto’s thyroiditis is a more common cause of goiter formation in the US. This is an autoimmune condition in which there is destruction of the thyroid gland by one’s own immune system. As the gland becomes more damaged, it is less able to make adequate supplies of thyroid hormone. The pituitary gland senses a low thyroid hormone level and secretes more TSH to stimulate the thyroid. This stimulation causes the thyroid to grow, which may produce a goiter.
Another common cause of goiter is Graves’ disease. In this case, one’s immune system produces a protein, called thyroid stimulating immunoglobulin (TSI). As with TSH, TSI stimulates the thyroid gland to enlarge producing a goiter. However, TSI also stimulates the thyroid to make too much thyroid hormone (causes hyperthyroidism). Since the pituitary senses too much thyroid hormone, it stops secreting TSH. In spite of this the thyroid gland continues to grow and make thyroid hormone. Therefore, Graves’ disease produces a goiter and hyperthyroidism.
Multinodular goiters are another common cause of goiters. Individuals with this disorder have one or more nodules within the gland which cause thyroid enlargement. This is often detected as a nodular feeling gland on physical exam. Patients can present with a single large nodule or with multiple smaller nodules in the gland when first detected (see Thyroid Nodule brochure). Thus, in early stages of a multinodular goiter with many small nodules, the overall size of the thyroid may not be enlarged yet. Unlike the other goiters discussed, the cause of this type of goiter is not well understood.
In addition to the common causes of goiter, there are many other less common causes. Some of these are due to genetic defects, others are related to injury or infections in the thyroid, and some are due to tumors (both cancerous and benign tumors).