What Is a Colloid Cyst on the Thyroid?

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A thyroid colloid cyst is one type of noncancerous (benign) thyroid nodule. Thyroid nodules are very common, with about 60% of adults having at least one. Fortunately, the vast majority, about 95% of these, are noncancerous. Here's what you should know about diagnosis and treatment for a thyroid colloid cyst.

Doctor feeling female patient's thyroid gland

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What Is a Thyroid Colloid Cyst?

The thyroid is located at the base of the neck, below the Adam’s apple or larynx. It has a right and left lobe, connected by the isthmus, which is like a bridge. The thyroid gland secretes thyroid hormones. These hormones affect various body systems, influencing heart function, metabolism, temperature regulation, and more.

Sometimes, thyroid tissue grows abnormally large, producing a nodule. In a colloid thyroid nodule, the overgrowth may multiply. However, colloid thyroid nodules remain in the thyroid and do not spread to other areas of the body.

If a thyroid nodule is filled with fluid, it is considered "cystic." Colloid nodules may be completely fluid-filled or partially-fluid filled with some solid areas. Purely cystic nodules (meaning 100% fluid-filled) are rarely cancerous.

It's possible to develop other thyroid nodules, most of which are also benign, such as hyperplastic nodules and follicular adenomas. Some patients have more than one type of nodule at the same time.

A goiter describes an enlarged thyroid that either encompasses the entire gland or appears as bumpy nodules. Hormonal imbalances (such as those caused by an iodine deficiency) are a common cause of goiters.

Cancer Risk

For every 20 palpable thyroid nodules, one is cancerous. Examples of cancerous thyroid nodules include papillary or follicular carcinomas, medullary cancer, or anaplastic cancer.

Exposure to ionizing radiation (from cancer treatments, occupational exposure, or nuclear fallout) increases thyroid cancer risk, especially if the exposure happened during childhood. Nodules that grow rapidly or cause a hoarse-sounding voice tend to be more concerning.

You may suspect that a painful thyroid nodule is cancerous. However, that's not necessarily the case. Chronic inflammation of the thyroid gland can produce inflammatory nodules or a swollen thyroid gland.

Like many cancers, thyroid cancer can run in families. Your doctor will take your family history and lifestyle factors into account when assessing your risk of thyroid cancer.

Nonfunctioning nodules (meaning they don't produce thyroid hormone) have a 14% to 22% risk of being cancerous compared to hyperfunctioning nodules (nodules that produce thyroid hormone), which are rarely cancerous.

Although hyperfunctioning thyroid nodules don't tend to cause cancer, they can result in other unpleasant side effects. By overproducing thyroid hormones, hyperfunctioning thyroid nodules can lead to symptoms of hyperthyroidism (including osteoporosis and abnormal heartbeats).

Ongoing monitoring and treatment can help keep hyperfunctioning thyroid nodules under control.

Testing and monitoring are required to determine whether a nodule is cancerous. If you have multiple nodules, each one should be evaluated individually.

Diagnosis

Nodules are more common in women and the incidence increases with age. Depending on the size and location of a nodule, it may be palpable (able to be felt) during a physical exam. About 5% of women and 1% of men living in iodine-sufficient areas have palpable nodules.

Nonpalpable nodules are usually discovered through imaging studies such as an ultrasound, computed tomography (CT) scan, or magnetic resonance imaging (MRI) of the neck or surrounding areas. High-resolution ultrasounds are particularly effective and detect thyroid nodules in up to 68% of healthy individuals.

If your doctor finds a thyroid nodule, the next step is ordering a blood test to measure your levels of thyroid-stimulating hormone (TSH) and assess your thyroid function. Performing an ultrasound will help evaluate the type of nodule and check the surrounding lymph nodes in the neck.

Based on the results of these tests and other risk factors, further testing may be required. It can be helpful to undergo the following tests.

Radioactive Iodine Uptake Test

This test is reserved for patients who have a thyroid nodule and have lower than normal TSH. A radionuclide (radioactive iodine) is taken by mouth, in the form of a pill.

Imaging studies are performed six hours later, or even the next day. Waiting provides enough time to allow the radioactive iodine to reach the thyroid. The results of this test can determine if the nodule is hyperfunctioning (unlikely to be cancerous) or nonfunctioning.

Fine-Needle Aspiration (FNA)

For nonfunctioning nodules, fine-needle aspiration may be required (depending on the ultrasound findings and the size of the nodule). During this procedure, the doctor inserts a thin needle through the skin and into the thyroid nodule to collect cells for testing.

Performing FNA while using an ultrasound machine helps ensure the correct placement of the needle. You may receive medication to numb the area of the neck to prevent discomfort. Your doctor will take multiple cell samples from different areas of the nodule to analyze for cancerous characteristics.

Molecular Testing

Sometimes the results of the FNA come back as “indeterminate” meaning the results are inconclusive as to whether the cells are cancerous or benign. Your doctor may send the sample for molecular testing. Molecular testing analyzes the genetic sequence of the cells' DNA or RNA to check for mutations.

Outlook

Benign thyroid nodules are monitored with a follow-up ultrasound one to two months after the initial discovery. If there has been no significant growth, repeat ultrasounds are typically performed every three to five years.

If your doctor is concerned about the nodules' growth rate, you may need more ultrasounds or FNA testing to rule out cancer. These test results will help guide further treatment and follow-up recommendations.

Cysts are usually noncancerous but occasionally contain cancerous solid components. Even benign cysts may require surgical removal.

A Word From Verywell

Noticing a thyroid nodule on your neck or having one discovered during a routine examination can be alarming. There are several tests available to help your doctor determine whether the nodule is concerning or harmless.

Most of the time, the only recommended treatment is ongoing monitoring to keep an eye out for any changes. Let your doctor know if you suspect a growth on your thyroid so they can rule out potential dangers and you can learn more about the best next steps to take.

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