What is a nodule on the thyroid gland?
Thyroid nodules are focal enlargements on the thyroid gland commonly referred to as lumps. It may be present in an enlarged thyroid gland (goiter) or a normal sized gland. Nodules may be due to various causes – it may be due to swelling often associated with inflammation, overgrowth of tissue or a cyst.
The concern often surrounds the presence of cancer but most thyroid nodules are not cancerous. Thyroid nodules may appear as the single nodule (solitary) or multiple nodules. Sometimes with multiple nodules, there may be a single nodule that is larger than the rest and this is known as the dominant nodule.
Symptoms and Dangers of Thyroid Nodules
Most thyroid nodules are silent – it does not cause any symptoms and is often not even visible. Many people who have a thyroid nodule or nodules are not even aware of it until it is discovered during a clinical examination. A larger nodule or nodules may be more of a problem symptomatically.
Apart from the cosmetic effect of thyroid nodules, a large nodule or nodules may press on surrounding structures like the airway or esophagus and cause a variety of symptoms including difficulty breathing or difficulty swallowing. A nodule that impinges on the large blood vessels in the neck or upper thorax can cause more serious and severe symptoms.
Types of Thyroid Nodules
Single Thyroid Nodule
The solitary nodule tends to be seen in the normal sized thyroid gland. It is most often a solid mass but can be a fluid-filled cyst. Most thyroid tumors present as the solitary thyroid nodule but most of these growths are benign. It is due to an overgrowth of follicular cells of the thyroid gland and tends to cause an excess of thyroid hormones, hence the name toxic adenoma. A small percentage of these solitary nodules may be malignant (cancerous) and can be classified according to the the type of cells – follicular cells, parafollicular C cells or lymphocytes.
It is not uncommon for the dominant nodule of multinodular goiter to be mistaken for a solitary thyroid nodule. Other possible causes of solitary thyroid nodule is a localized area of inflammation (foci of thyroiditis), a simple cyst or a solid tissue growth.
Multiple Thyroid Nodules
More than one thyroid nodule is usually seen in the enlarged thyroid gland (goiter), hence the term multinodular goiter. This must be differentiated from the diffuse simple goiter where the entire gland is swollen without any lumps. Multinodular goiter tends to cause an irregular enlargement of the thyroid gland (asymetrical) and most diffuse simple goiters become multinodular over time.
These lumps are more often due to an overgrowth of tissue and are almost always benign. Nevertheless the necessary investigations need to be conducted to rule out a malignancy (cancer). Most cases of multinodular goiter do not cause an excess of thyroid hormones (hyperthyroidism) or if it does so, it is usually mild. Only a minority of cases causes hyperthyroidism (Plummer syndrome) and is more often seen in long-standing goiter.
Causes of a Thyroid Nodule
A thyroid nodule may occur for any number of reasons. Multinodular goiter is often associated with the same causes as a simple diffuse goiter. This includes :
- Iodine deficiency
- Thyroid inflammation and/or infection – thyroiditis
- Birth defects (congenital)
- Antibodies that causes the immune system to target the thyroid gland
- Excessive TSH secretion from the pituitary gland which stimulates the thyroid gland and eventually causes enlargement
- Other hormones like human chorionic gondaotropin which may be secreted from tumors elsewhere in the body or in pregnancy
- Certain substances that reduce thyroid function (goitrogens) leading to the pituitary gland secreting more TSH to stimulate the thyroid gland. This causes thyroid gland enlargement.
- Radiation to the head and neck appears to be associated with a greater risk of developing thyroid nodules. This may include repeated x-rays even for dental treatment and has supported the use of radiation shielding thyroid guards.
- Family history of goiter or thyroid nodules
- Hyperthyroidism or hypothyroidism may precede the onset of nodules and therefore teh causes of these ocnditions also needs to be considered.
It is important to note that a significant number of cases of thyroid nodules occurs for no known reason – idiopathic.
Meaning of a Thyroid Nodule
Is a thyroid nodule cancer?
Most patients are concerned whether the thyroid nodule is cancerous or not. This can only be diagnosed by a doctor after conducting the relevant diagnostic investigations. Here are a few points to consider when assessing whether a thyroid nodule is cancerous :
- Solitary nodules are more likely to be cancerous than multiple nodules.
- Thyroid nodules are more frequently seen in females but are more likely to be cancerous in males.
- Age may be a consideration as the thyroid nodules in younger patients is more likely to be cancerous than in older patients.
- Firm, hard and fixed nodules are more likey to be cancerous than soft, smooth and movable nodules.
- Functional nodules (takes up radioactive iodine ~ hot nodules) that is associated with hyper- or hypothyroidism is more likely to be benign while cold nodules (do not take up iodine) with fairly normal thyroid function is more likely to be malignant (cancerous).
- A history of thyroid cancer, radiations exposure to the head and neck or malignancies elsewhere in the body increases the risk of the nodule being cancerous.
- Painful or tender nodules are more likely to be benign but severe pain may be due to a rupture of a cyst with hemorrhage which is a medical emergency.