Isthmus Thyroid Nodules and Cancer Risk

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The isthmus is a bridge of tissues that connect the two lobes of the thyroid gland, located on either side of the windpipe at the front of the neck. Just 2 to 9% of cancers are found in the isthmus, but cancers in this area are more likely to spread outside of the thyroid.

A thyroid nodule is a lump found in or on the thyroid. Nodules are found in roughly 6% of women and 1 to 2% of men.

When a nodule is discovered in the thyroid, physicians must consider the possibility of cancer (malignancy). In 95% of cases, thyroid nodules are found to be benign (non-cancerous).

woman holding throat

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What Is the Thyroid Isthmus?

The thyroid is located at the front of the lower part of the neck. The gland is shaped like a butterfly, with "wings" or lobes located on each side of the windpipe. These lobes are joined by a bridge of tissue known as the isthmus, which spans across the windpipe.

The thyroid makes hormones that travel in the blood to tissues throughout the body.

The hormones produced by the thyroid help the body with energy, staying warm, and keeping vital organs—like the heart and brain—working effectively.

Thyroid Cancer

Cancer risk can differ based on whether nodules are found in the isthmus or lobar areas of the thyroid.

Recent research found that nodules in the thyroid isthmus pose a greater risk of malignancy compared with nodules in the lobes. Nodules in the lower portion of the lobes are believed to be at least risk for cancer.

Malignant nodules occur in roughly 10% of all nodules.

The location where nodules are found is significant in determining cancer risk. Thyroid cancer risk is highest for the isthmus, followed by the upper, then middle thyroid.

Cancers in the Isthmus Are More Likely to Spread

Whilst the majority of thyroid cancers are found in the lobes, those that are found in the isthmus are more likely to spread to other parts of the body. The prognosis for cancer that spreads beyond the thyroid is less favorable than cancer that remains in the thyroid.

Risk Factors

There are a number of risk factors for thyroid cancer. Some of these—like weight—are modifiable risk factors, while others—like age—can not be changed.

Non-modifiable risk factors include:

Age and Gender

Although thyroid cancer can happen at any age, the risk of thyroid cancer occurs earlier for women, often between the ages of 40 to 50 at the time of diagnosis. For men, diagnosis often occurs between the ages 60 to 70.

Nodules occur ten times as more in older individuals than younger people. Thyroid cancer also tends to be more aggressive in elderly people. 

Thyroid cancer occurs more commonly in women than men. The rate of thyroid cancer in women is about three times that of men. The reasons for this are unknown.

Family history

Having a member of your immediate family (parent, sibling, or child) with thyroid cancer raises your risk. The reason for this genetic link is currently unknown.

Modifiable risk factors for thyroid cancer include:

Radiation exposure

Exposure to radiation is a proven risk factor for cancer in the thyroid. Radiation exposure can come from power plant accidents, fallout from nuclear weapons use, or some medical treatments.

Those who had radiation treatments involving the neck or head as children are at higher risk for thyroid cancer. This risk typically increases the larger the dose or the younger the person at the time of treatment.

Radiation treatments for cancer in childhood also increases risk.

Thyroid cancer is more common in those who have been exposed to radioactive fallout from power plant accidents. This was seen in children and adults who lived near Chernobyl. In 1986 an accident at the power plant at Chernobyl exposed millions to radioactivity.

Although tests like x-rays and CT scans expose people to radiation at a low dose, it is not known how significantly this raises the risk of thyroid cancer. It is believed the dose is so low the risk is likely to also be low, however, given the slight risk, such tests should be avoided in children unless absolutely necessary.

Exposure to radiation as an adult poses less risk for thyroid cancer than in children.

Weight

Those who are overweight or obese have a higher risk of developing thyroid cancer than those who are not. This risk increases as BMI does.

Iodine in diet

A diet low in iodine is a risk factor for thyroid cancer. However, a diet too high in iodine can also increase your risk of thyroid cancer.

Iodine deficiency is typically not an issue in the US as iodine is added to things like table salt.

Symptoms

Symptoms and signs of thyroid cancer may include:

  • A lump in the neck that could be fast-growing
  • Pain at the front of the neck that extends towards the ears
  • Swelling of the neck
  • Voice changes or hoarseness that persists
  • Difficulty with swallowing
  • Difficulty with breathing
  • A persistent cough that is not because of a cold

Lumps in the thyroid can be common and are often not cancerous. Any of these symptoms could also be caused by conditions other than thyroid cancer. But if you have any of these symptoms, you should speak with your doctor.

In many cases, thyroid nodules do not cause symptoms, and the nodules are found during a routine exam or during scans for unrelated conditions.

Sometimes those with nodules in the thyroid may see or feel a lump in their neck. In rare cases, there may also be pain in the neck, ears or jaw. If a nodule is large, it can cause problems with swallowing or breathing and may give a feeling of a "tickle in the throat".

Rarely, thyroid nodules may cause a hoarse voice, but this is typically due to thyroid cancer.

thyroid cancer symptoms

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Diagnosis

Once a thyroid nodule is found, tests will be undertaken to determine whether nodules are malignant or benign.

Initially, a physical exam of the thyroid will be done, when a physician will feel the neck to see whether the entire thyroid gland is enlarged, and to determine if there are multiple nodules.

how to do a thyroid neck check

Emily Roberts / Verywell

Blood tests may be ordered to determine whether the thyroid is functioning normally.

It is often not possible to determine if a nodule is cancerous only through physical exams and blood tests, so further tests are typically needed.

Thyroid Scan

A thyroid scan involves taking a picture of the thyroid gland after a small amount of radioactive isotope has been either swallowed or injected. This test can determine whether a nodule is considered hyperfunctioning (also referred to as a "hot nodule").

Cancer is rarely found in hot nodules, so a thyroid scan that gives a result of a hot nodule may eliminate the need for a fine needle aspiration biopsy. A thyroid scan should not be performed on women who are pregnant.

Ultrasound

Ultrasounds of the thyroid use high-frequency sound waves to get a picture of the thyroid gland.

Using ultrasounds, physicians can determine the exact size of a nodule, as well as if it is solid or full of fluid.

Ultrasounds can also be used to identify suspicious characteristics of nodules that are more common in thyroid cancer than in benign nodules. A thyroid ultrasound is painless.

Fine Needle Aspiration

Also referred to as a fine needle aspiration biopsy (FNA or FNAB), this test involves using a fine needle to take a biopsy from the thyroid nodule.

This procedure can be done in a doctor's office. During this test, a physician uses a fine needle to take a sample of cells from the nodule. Typically, multiple cell samples are taken from different parts of the nodule to increase the chances of finding cancer cells if they are present in the nodule. These cells are then examined using a microscope.

Thyroid biopsies can return various results:

  • In 80% of thyroid biopsies, the nodule is non-cancerous (benign).
  • In 15 to 20% of cases, the nodule is indeterminate.
  • In about 5% of cases, the nodule is malignant.

Benign nodules can include:

  • Multinodular goiter (also referred to as nontoxic goiter): This refers to the thyroid gland growing too big. This usually occurs when the brain produces too much thyroid-stimulating hormone. If the goiter is large, surgery may still be needed though the nodule is benign.
  • Benign follicular adenomas: Follicular refers to cells that appear in small circular groups under a microscope. If such cells stay within the nodule, it is considered non-cancerous, but if they spread to surrounding areas, it is cancerous.
  • Thyroid cysts: These are fluid-filled nodules. Nodules that have both fluid and solid areas are referred to as complex nodules. Though benign, these nodules need to be removed through surgery if they cause pain in the neck or difficulties with swallowing.

Treatment

Treatment options will vary depending on whether thyroid nodules are found to be benign or malignant.

Benign Nodules

Most cases of benign modules don't require specific treatments.

Thyroid nodules that are found to be benign by a fine needle aspiration biopsy, or for nodules that are too small to biopsy are monitored closely using thyroid ultrasound every six to 12 months. Annual physical examination should also occur.

In some cases, like multinodular goiter, thyroid medications like the hormone levothyroxine may be advised to stop the goiter from growing. Medications like radioiodine may also be prescribed to treat "hot nodules."

For benign modules that continue to grow, or develop concerning features discovered during a follow-up ultrasound, surgery may be recommended, even in the absence of cancer.

Malignant nodules

All nodules that are found to be malignant (cancerous), or are highly suspicious of being cancerous require surgical removal.

Depending on the nodule characteristics, sometimes the entire thyroid is removed during surgery. This procedure is called a total thyroidectomy. Lymph nodes may also be surgically removed in order to determine cancer spread.

The therapies following surgery will be dependent on what is found during surgical procedures. In some cases, treatment will involve thyroid hormones and monitoring through blood tests and ultrasounds. In other cases, radioactive iodine is used to destroy any residual concerns, which is then followed up with blood tests and ultrasounds.

Using these therapies will cure or control the majority of cancers. Less than 20% will recur.

Outlook

Annually, it is estimated 52,890 adults in the United States will be diagnosed with thyroid cancer. Just over 40,000 of those will be women and just under 13,000 will be men.

Every year, it is estimated 2,180 deaths will occur from thyroid cancer. Although women are three times more likely than men to have thyroid cancer, men and women die from the disease at a similar rate. This suggests the prognosis for men is worse than the prognosis for women.

Overall, the 5-year survival rate for thyroid cancer is 98%. But this varies based on the type and stage of thyroid cancer. If the cancer is localized to the thyroid only, the 5-year survival is nearly 100%. Roughly two-thirds of thyroid cancers are diagnosed at the localized stage.

A Word From Verywell

Nodules can grow in the thyroid for a variety of reasons. Cancer is the primary concern when nodules appear, but in the vast majority of cases, nodules in the thyroid are benign. Only a small percentage of nodules in the isthmus are found to be cancerous. But cancerous nodules in this area are more likely to spread outside the thyroid. Just because you have a nodule in the thyroid, doesn't mean you have cancer. But if you experience any unusual symptoms, or notice any lumps in your neck, you should speak with your doctor.

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