What is a goiter?
Goiter (goitre) is the term for enlargement of the thyroid gland, the butterfly-shaped gland that lies in the neck and controls the body’s metabolism. It occurs when the thyroid gland is overstimulated and there is an increase in the number of cells in the gland (hyperplasia). This may be diffuse, where the entire gland is enlarged, or nodular, where there are focal areas of overgrowth.
Goiter may be accompanied by changes in the thyroid hormone levels, with an excess (toxic goiter) or deficiency (hypothyroid goiter) of the thyroid hormones. Sometimes there is no change in the thyroid gland activity thereby maintaining normal levels of the thyroid hormones in the body and this is known as a non-toxic goiter.
A goiter may not be present as a visible enlargement in the neck associated with a swollen thyroid gland. However, prominent enlargement will cause associated signs and symptoms due to its compression of the surrounding neck structures. The features may not be significant and is only associated with a thyroid disorder when the symptoms of hypo- or hyperthyroidism are evident.
How does a goiter occur?
Goiter may arise when there is a low intake of iodine and this results in reduced synthesis of the thyroid hormones (endemic goiter). This disrupts the feedback mechanism and the thyrotropin-releasing hormone (TRH) stimulates excessive secretion of thyroid-stimulating hormone (TSH) from the anterior pituitary. The higher levels of TSH acts on the thyroid gland in an attempt to increase the thyroid hormone synthesis and secretion. This increases the number of cells in the thyroid gland and can increase the size of the thyroid follicles by 15 to 20 times its normal size.
However a lack of iodine is not the only stimulus for enlargement of the thyroid gland. Other factors mat also be responsible and this includes :
- congenital defects
- substances that suppress the thyroid gland function (goitrogens)
- dysfunction with thyroid-stimulating hormone (TSH) regulation possibly due to an adenoma in the pituitary gland or hypothalamus
- human chorionic gonadotropin from tumors
At times there is no clear cause for the enlargement of the thyroid gland. This is known as idiopathic goiter and could be indirectly related to thyroid conditions like thyroiditis.
Types of Goiter
Diffuse goiter, also known as a simple goiter, is usually a smooth and symmetrical enlargement of the thyroid gland but most become nodular with time (nodular goiter). Thyroid nodules may be single (solitary) or multiple (multinodular) and causes an irregular swelling of the thyroid gland. The development of thyroid nodules is an indication that the disease process is progressing.
An enlarged thyroid may or may not be accompanied by disturbances in thyroid function. If the enlargement is associated with an excess of thyroid hormones, then it is known as a toxic goiter. In a non-toxic goiter, there is no change in thyroid hormone levels (normal levels) while with a hypothyroid goiter, there are low levels of circulating thyroid hormones.
Causes of Goiter
While there are various known causes of an enlarged thyroid gland, some cases of sporadic diffuse goiter may arise for no known reason.
Iodine is necessary for the production of the thyroid hormones, thyroxine (T4) and triiodothyronine (T3). A deficiency of iodine lowers the levels of circulating hormones. The thyroid is overstimulated by thyroid-stimulating hormone from the anterior pituitary in an attempt to increase thyroid hormone levels. There may be symptoms of hypothyroidism although in mild cases, this may be barely noticeable.
This disease arises when the body’s own immune system attacks the thyroid gland due to the presence of thyroid antibodies. This causes the thyroid gland to produce an excess of thyroid hormones (hyperthyroidism). Symptoms of hyperthyroidism will be apparent.
This form of thyroiditis (thyroid inflammation) is also due to the action of the body’s immune system (autoimmune disease). However, the effect of this autoimmune disease is a reduction in thyroid function leading to a deficiency of circulating thyroid hormones. Symptoms of hypothyroidism will be apparent.
Other Types of Thyroiditis
A goiter may be seen with other forms of thyroiditis, apart from Hashimoto’s thyroiditis. Swelling is a typical feature of inflammation and is more likely to cause diffuse enlargement of the thyroid gland. This can be a painful or painless swelling. The inflammation of the thyroid gland may be due to infections, autoimmune factors, exposure to radiation, certain drugs or unknown factors.
A malignancy may present with a solitary lump or less frequently as multiple nodules on the the thyroid gland. There may be no changes in thyroid hormone levels unless there was a pre-existing thyroid pathology. However, advances stages of thyroid cancer can disrupt thyroid function.
Benign Tumors and Thyroid Nodules
There are various types of growths that may arise in the thyroid gland. These nodules can be solid or fluid-filled (cyst), either solitary (single) or multiple. Some of the benign tumors of the thyroid gland may produce thyroid hormones independent of the thyroid gland. At other times, the growths may be present without causing any change in the thyroid hormone levels. Depending on the location, size and number of nodules, a person may have no other symptoms other than an enlarged thyroid gland or may experiences symptoms of compression – hoarse voice, difficult breathing and difficulty swallowing with or without thyroid pain or a sore throat.
Pituitary and Other Tumors
Pituitary tumors may cause an excessive secretion of thyroid-stimulating hormone (TSH). This overstimulates the thyroid gland resulting in a goiter. At times, other secretory tumors located at sites outside of the thyroid gland or pituitary gland may produce hormones that can also stimulate the thyroid gland thereby resulting in an enlargement.