Thyroid cancer is a cancerous growth of the thyroid gland. Most of them are benign but others are malignant and can spread into nearby tissues and to other parts of the body.

Prevalence and incidence

Classification of thyroid cancer

Anaplastic thyroid cancer

Anaplastic (undifferentiated) carcinomas are highly malignant cancers of the thyroid. They may be subclassified as small cell or large cell carcinomas. Both small cell and large cell carcinomas grow rapidly and are present as hard, ill-defined masses, often with extension into the structures surrounding the thyroid.⁵
Small cell anaplastic thyroid carcinoma must be carefully distinguished from lymphoma. This tumor usually occurs in an older age group and is characterized by extensive local invasion and rapid progression.⁵ The mean survival time is usually less than 6 months from the time of diagnosis and, unfortunately, this outcome is not fundamentally altered by available treatments.⁶

Medullary thyroid cancer⁵

Medullary carcinoma is usually present as a hard mass and is often accompanied by blood vessel invasion. Medullary thyroid cancer occurs in two forms, sporadic and familial. In the sporadic form, the tumor is usually unilateral. In the familial form, the tumor is almost always bilateral. In addition, the familial form may be associated with benign or malignant tumors of other endocrine organs, commonly referred to as the multiple endocrine neoplasia syndromes.
Medullary carcinoma usually secretes calcitonin, a hormonal marker for the tumor, and may be detectable in blood even when the tumor is clinically occult. Metastases to regional lymph nodes are found in about 50% of cases.

Papillary thyroid cancer¹

About 8 of 10 thyroid cancers are papillary (differentiated) carcinomas. These carcinomas typically grow very slowly and usually develop in only one lobe of the thyroid gland, but sometimes occur in both the lobes. Even though they grow slowly, papillary carcinomas often spread to the lymph nodes in the neck. But most of the time, this can be successfully treated and is rarely fatal.

Follicular thyroid cancer¹

Follicular carcinoma is the next most common type of thyroid cancer, making up about 1 out of 10 thyroid cancers. It is also sometimes called follicular cancer or follicular adenocarcinoma. It is more common in countries where people don't get enough iodine in their diet. These cancers usually remain in the thyroid gland.
Unlike papillary carcinoma, follicular carcinomas usually don't spread to lymph nodes, but some can spread to other parts of the body, such as the lungs or bones. The prognosis for follicular carcinoma is probably not quite as good as that of papillary carcinoma, although it is still very good in most cases. .

Etiology⁷

Signs and symptoms⁸

Most commonly, thyroid cancers in the early stage produce no symptoms. As the cancer grows, a small lump or nodule can be felt in the neck. The vast majority of thyroid nodules are caused by benign conditions, but about one per cent of these lumps represent early stages of thyroid cancer. If the cancer spreads, it can cause symptoms like:

Staging and grading¹

Staging is based on the results of the physical exam, biopsy, and imaging tests (ultrasound, CT scan, MRI, chest x-ray, and/or nuclear medicine scans)

The most common system used to describe the stages of cancers is the American Joint Committee on Cancer (AJCC) TNM system. The TNM system describes 3 key pieces of information:

Numbers or letters appear after T, N, and M to provide more details about each of these factors. The numbers 0 through 4 indicate increasing severity. The letter X means "cannot be assessed because the information is not available."

Staging recommended by the American Joint Committee on Cancer (AJCC) for papillary and follicular thyroid cancer are listed below.

Papillary thyroid cancer⁵

Follicular thyroid cancer5

Screening and testing

Physical examination⁹
In a physical examination, the neck, thyroid, voice box and lymph nodes in the neck are examined for nodules or swelling.

Blood tests¹
Utrasonography¹⁰
In the preoperative diagnosis, thyroid ultrasonography has been proven to be quite useful in the detection of thyroid lesions. There are two major reasons to perform thyroid ultrasonography before fine needle aspiration cytology (FNAC): to detect deep-seated small nodules, and to realize the nature of the clinically palpable nodules.

Thyroid scan¹
The computed tomography (CT or CAT) scan can help determine the location and size of thyroid cancers and whether they have spread to nearby areas, although ultrasound is usually the test of choice. A CT scan can also be used to look for spread into distant organs such as the lungs. In some cases, a CT scan can be used to guide a biopsy needle precisely into a suspected area of cancer spread. Like CT scans, magnetic resonance imaging (MRI) scans can be used to look for cancer in the thyroid, or cancer that has spread to nearby or distant parts of the body.

Biopsy¹¹
Fine needle aspiration biopsy (FNAB) of the thyroid gland is the best technique for detecting or ruling out the presence of cancer. It has now become the method of choice for obtaining samples of thyroid tissue. The procedure is technically quite simple and when performed properly, the testing has a false negative rate of less than 5%. This means that a positive finding, such as cancer, will be missed fewer than five times out of 100.

Management

Surgery
Surgery is the main treatment for thyroid cancer and is used in nearly every case, except some anaplastic thyroid cancers. Thyroidectomy is surgery to remove the thyroid gland. If the entire thyroid is removed, it is called a total thyroidectomy. It is the most common surgery for thyroid cancer.¹

Some patients with papillary or follicular thyroid cancer may be treated with lobectomy wherein the lobe with the cancerous nodule is removed. Nearly all patients who have part or the entire thyroid removed will have to take thyroid hormone pills to replace the natural hormone.⁹

Thyroid Hormone Treatment
Hormone treatment after surgery is usually part of the treatment plan for papillary and follicular cancer.⁹ Taking daily pills of thyroid hormone (thyroid hormone therapy) can serve 2 purposes:

Thyroid hormone may also help prevent some thyroid cancers from recurring.¹

Radioactive Iodine Therapy (Iodine-131)
Radioactive iodine therapy uses radioactive iodine (I-131) to destroy thyroid cancer cells anywhere in the body. The therapy usually is given by mouth (liquid or capsules) in a small dose that causes no problems for people who are allergic to iodine. The intestine absorbs the I-131, which flows through the bloodstream and collects in thyroid cells.⁹

This therapy has been shown to improve the survival rate of patients with papillary or follicular thyroid cancer that has spread to the neck or other body parts, and is now a standard practice in such cases. However it cannot be used to treat anaplastic (undifferentiated) and medullary thyroid carcinomas because these types of cancer do not take up iodine.¹

External Radiation Therapy¹
External radiation therapy uses high-energy rays to destroy cancer cells or slow their rate of growth. It is more often used as part of the treatment for medullary thyroid cancer and anaplastic thyroid cancer. Generally, this type of radiation treatment is not used for differentiated thyroid cancers that take up iodine. They are better treated with radioiodine therapy.

Chemotherapy¹
Chemotherapy, a systemic therapy, uses anti-cancer drugs that are injected into a vein, injected into a muscle, or taken by mouth. It is seldom helpful for most types of thyroid cancer. It is combined with external beam radiation therapy for anaplastic thyroid cancer and is sometimes used for other advanced cancers that are no longer responding to other treatments.