What Is a Thyroid Cyst?

Thyroid cysts are thyroid nodules that contain liquid. It is estimated that about half of Americans will have a thyroid nodule—solid, fluid-filled, or mixed—by the time they’re 60 years old.

Thyroid cysts are usually always benign, or non-cancerous. In some cases, your doctor might recommend further testing or intervention, but oftentimes you’ll just need to monitor the cyst.

What Is a Cyst?

A cyst is a closed pocket of tissue, often filled with liquid or air. Cysts can appear anywhere on the body, including:

• On or under the skin
• On internal organs like the thyroid

When it comes to thyroid cysts, there are two types, defined by what is inside the cyst. 

Fluid-Filled Cysts

These thyroid cysts are filled entirely with liquid, like pus. These are almost always benign, or non-cancerous. 

Complex Cysts

Complex cysts, also known as mixed echogenic nodules, have both liquid and solid components. They are more likely than fluid-filled cysts to be cancerous.

The likelihood of cancer depends on the composition of the cyst.

• If it is mostly liquid, the risk of cancer is less than 5%.
• If it is is more than 50% solid, it has about a 10% risk of being cancerous.

Causes

Doctors aren’t sure why so many people have thyroid cysts. In general, cysts can develop when ducts are blocked and fluid gets trapped in one area. Cyst growth is also related to hormone activity, and some thyroid cysts contain the hormone colloid, which is produced in the thyroid.

Symptoms

In most cases, people with a thyroid cyst do not experience any symptoms. In fact, this is true of most kinds of thyroid nodules. 

However, you might experience symptoms if your cyst grows so large that it presses on the other organs in your neck. These symptoms can include:

• Difficulty breathing
• Difficulty swallowing
• Vocal changes

Some patients also experience pain in the neck or feel the lump.  

Diagnosis

After your doctor has detected a thyroid cyst or suspects that you may have one, he or she will likely order further testing. This will help give you both a better understanding of the cyst, so that you can choose the treatment option that is right for you.

Testing will determine whether the cyst is fluid-filled or complex, and whether it’s benign or cancerous. 

Further testing for thyroid cysts includes:

Ultrasound

Thyroid cysts are often detected during ultrasounds. An ultrasound can provide important information on the composition of a cyst. This is because materials of different density—like solids and liquids—show up as different shades on the ultrasound.

A radiologist will get an idea of the cyst’s composition just by reading the image. The doctor will also determine the size and location of the cyst. That’s important because larger cysts are more likely to be cancerous or to be causing symptoms.

Biopsy

If your doctor sees something concerning on the ultrasound, they may opt to do a fine-needle aspiration biopsy. During this procedure:

1. A very small needle is inserted through your neck into the thyroid cyst, using an ultrasound image to guide the needle.
2. The needle removes a sample of the material of the cyst.
3. The cells from that sample are then analyzed under a microscope to determine whether any of the cells are cancerous. 

A fine-needle aspiration biopsy is most often used for cysts:

• Over 1.5 centimeters
• That are complex
• That raise the concern of your doctor

Thyroid Scan

If your doctor finds that the thyroid cyst is complex, they may want to determine whether or not it’s producing thyroid hormones. A nodule that produces thyroid hormones, known as an active or toxic nodule, can cause you to experience hyperthyroidism.

If you need a thyroid scan, you’ll take a radioactive iodine pill. Doctors will take a picture of your thyroid function by tracking the iodine as it moves through your thyroid.

Treatment

About 15% of thyroid cysts will resolve on their own. Others will not need any treatment, but if your cyst is found to be cancerous or is causing symptoms, your doctor might decide to address it. The common treatments for thyroid cysts are:

Monitoring

In most cases, if your cyst is less than 3 centimeters large and not cancerous, your doctor will leave it alone. You'll likely be monitored by ultrasound once or twice a year to ensure it’s not changing or getting larger.

Aspiration

Your doctor may be able to drain the cysts through aspiration, similar to the fine-need aspiration biopsy. However, if the cyst is drained it will reoccur 60 to 90% of the time.

Doctors may combine aspiration with percutaneous ethanol injection (PEI), which can permanently reduce the size of the cyst without surgery.  

Surgery

In rare cases, your doctor might recommend surgery to remove the thyroid or cyst. This is most often used for large cysts or those causing symptoms. The options for thyroid surgery are:

• Thyroidectomy: Removes all of the thyroid, or just a part, like the cyst. 
• Thyroid lobectomy: Removes the half of the thyroid that the cyst is on.
• Isthmusectomy: Removes just the thyroid isthmus.

Outlook

The prognosis for people with thyroid cysts is extremely good. The chances that your cyst is cancerous are less than 10%. If you do have cancer, the 5-year survival rates for the most common types of thyroid cancer are 98 to 100%.

A Word From Verywell

Having an unexpected health diagnosis can be stressful and scary. If you’ve recently found out that you have a thyroid cyst, try to relax: in most cases, thyroid cysts are harmless and will just need on-going monitoring. 

Almost all thyroid cysts are benign. However, occasionally complex thyroid cysts might be cancerous. Because of that, your doctor will want to do additional testing to determine whether you have thyroid cancer, a very treatable form of cancer. 

Before meeting with your doctor to discuss your thyroid cyst, write down any questions that you may have, and be sure that your doctor takes the time to answer them thoroughly.