Thyroid cancer

Thyroid cancer

Thyroid cancer develops when the cells of the thyroid grow and divide in an abnormal way. There are several types of thyroid cancer. It is possible to have more than one type at once, although this is unusual.

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The thyroid 

The thyroid is a butterfly-shaped gland found at the front of the neck and just below the voice box (larynx). It has two halves, called lobes, which lie on either side of the windpipe (trachea). The lobes are connected by a small band of thyroid tissue known as the isthmus.

The role of the thyroid

The thyroid is part of the endocrine system, which is a group of glands that makes and controls the body’s hormones. Hormones are chemical messengers that help the body work properly.

The thyroid makes two hormones (T4 and T3) that control the speed of the body’s processes, such as heart rate, digestion, body temperature and weight. This speed is known as your metabolic rate. See below for more information about the role of T4 and T3. The thyroid also produces the hormone calcitonin, which plays a role in controlling the body’s calcium levels.

Cells in the thyroid

There are two main types of cells in the thyroid:

  • follicular cells – produce and store the hormones T4 and T3, and make a protein called thyroglobulin (Tg)
  • parafollicular cells (C-cells) – produce the hormone calcitonin.

Parathyroid glands

Behind the thyroid are four additional glands known as the parathyroid glands. These glands produce parathyroid hormone (PTH), which works with calcitonin to control the amount of calcium in the blood.

The thyroid 

The role of thyroid hormones

The hormones T4 (thyroxine) and T3 (tri-iodothyronine) are known as the thyroid hormones. To make these hormones, the thyroid needs iodine, which is found in a range of foods such as seafood and iodised salt.

T4 is the main hormone made by the thyroid, but it is converted by the liver and kidneys into T3, a much more powerful hormone. Most T3 is created when the liver and kidneys convert T4 into T3, but the thyroid also makes small amounts.

To keep the body working properly, it is important that the thyroid makes the right amounts of T4 and T3. This is controlled by the pituitary gland, which is located at the base of the brain:

  • If the levels of T4 and T3 drop below normal, the pituitary gland produces a hormone called thyroid-stimulating hormone (TSH). TSH prompts the thyroid to make and release more T4 and T3.
  • If the levels of T4 and T3 are too high, the pituitary gland produces less TSH.

Changes in thyroid hormone levels affect your metabolism by slowing down or speeding up the body’s processes:

Underactive thyroid (hypothyroidism) – If you don’t have enough thyroid hormones, your metabolism slows down. As a result, you may feel tired or depressed, and gain weight easily. Other symptoms may include:

  • difficulty concentrating
  • constipation
  • brittle and dry hair and skin
  • sluggishness and fatigue
  • in severe cases, heart problems could occur.

Overactive thyroid (hyperthyroidism) – If you have too many thyroid hormones, your metabolism speeds up. As a result, you may:

  • lose weight
  • have increased appetite
  • feel shaky and anxious
  • have rapid, strong heartbeats (palpitations).

Over time, untreated hyperthyroidism can result in loss of bone strength and problems with heart rhythm.

Types of thyroid cancer

Common types of thyroid cancer:

  • most common type (about 70–80% of all thyroid cancer cases)
  • develops from the follicular cells
  • tends to grow slowly
  • about 15–20% of all thyroid cancer cases
  • develops from the follicular cells
  • includes Hürthle cell carcinoma, a less common subtype

Rare types of thyroid cancer

  • about 4% of all thyroid cancer cases
  • develops from the parafollicular cells (C-cells)
  • can run in families
  • may be associated with tumours in other glands
  • a rare thyroid cancer (about 1% of all thyroid cancer cases)
  • may develop from papillary or follicular thyroid cancer
  • tends to grow quickly
  • usually occurs in people over 60

What causes thyroid cancer?

The cause of thyroid cancer is unknown, but some factors are known to increase the risk of developing it. Having a risk factor does not necessarily mean that you will develop thyroid cancer. Most people with thyroid cancer have no known risk factors.

What are the risk factors?

Exposure to radiation

A small number of thyroid cancers are due to having radiation therapy to the head and neck area as a child, or living in an area with high levels of radiation, such as the site of a nuclear accident. Thyroid cancer usually takes 10–20 years to develop after significant radiation exposure.

Family history

Only around 5% of thyroid cancer runs in families. Having a parent, child or sibling with papillary thyroid cancer may increase your risk. Some inherited genetic conditions, such as familial adenomatous polyposis or Cowden syndrome, may also increase your risk of developing papillary thyroid cancer.

Most cases of medullary thyroid cancer do not run in families. However, some people inherit a faulty gene called the RET gene. This gene can cause familial medullary thyroid cancer (FMTC) or multiple endocrine neoplasia (MEN).

If you are concerned about having a strong family history of thyroid cancer, talk to your doctor. They may refer you to a genetic counsellor or a family cancer clinic to assess your risk.

Other factors

People who are overweight or obese possibly have a higher risk of developing thyroid cancer. Other thyroid conditions, such as thyroid nodules, an enlarged thyroid (known as a goitre) or inflammation of the thyroid (thyroiditis), only slightly increase the chance of developing thyroid cancer.

Who gets thyroid cancer?

About 2900 people are diagnosed with thyroid cancer each year in Australia. Thyroid cancer can occur at any age. It affects almost three times as many women as men – it is the seventh most common cancer affecting Australian women of all ages, and the most common cancer diagnosed in women aged 20–24.

Diagnoses of thyroid cancer in Australia have increased in recent years, with almost four times as many cases estimated in 2019 as there were in 1982. Some of this increase is due to the improved quality and greater use of diagnostic scans, such as ultrasounds. This has led to the detection of smaller, often insignificant, thyroid cancers that would otherwise not have been found. Researchers are trying to work out if there are any other causes of the increased rates of thyroid cancer.

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This information was last reviewed in January 2020
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