Supplementary Reading Read the text and answer the questions. What is goiter?
Is iodine deficiencyone of the most common causes of goiter formation worldwide?
What other causes of goiter formation do you know?
What is the cause of multinodular goiters?
Can genetic defects be the cause of goiter?
The term “goiter” simply refers to the abnormal enlargement of the thyroid gland. It is important to know that the presence of a goiter does not necessarily mean that the thyroid gland is malfunctioning. A goiter can occur in a gland that is producing too much hormone (hyperthyroidism), too little hormone (hypothyroidism), or the correct amount of hormone (euthyroidism). A goiter indicates there is a condition present which is causing the thyroid to grow abnormally.
One of the most common causes of goiter formation worldwide is iodine deficiency. 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). 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.
Hashimoto’s thyroiditis is a common cause of goiter formation in the world. 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 with smaller nodules in the gland, or may show as multiple nodules when first detected. 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 tumours (both cancerous and benign tumours).