What is thyroid cancer?
Thyroid cancer develops when the cells of the thyroid gland grow and divide in an abnormal way.
The thyroid gland
The thyroid is a butterfly-shaped gland at the front of the neck. It is found below the voice box (larynx). The thyroid gland is made up of 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 thyroid gland is part of the endocrine system, which consists of a group of glands responsible for producing the body's hormones. Hormones are chemical messengers that help the body function properly. The thyroid gland makes hormones that control the speed of the body's processes, such as heart rate, digestion, body temperature and weight. This is known as your metabolic rate.
How the thyroid gland works
The thyroid gland produces three hormones that are released into the bloodstream:
One of the hormones that controls the body's metabolism. T4 is converted into another thyroid hormone called T3.
The active form of the thyroid hormone, T3 also helps control metabolism. The thyroid produces only small amounts of T3. The majority of this hormone is created when the liver and kidneys convert T4 into T3.
This hormone plays a role in controlling the body's calcium levels.
The thyroid gland makes T4 and T3 from iodine, which is found in a range of foods such as seafood and iodised salt.
The thyroid gland is made up of two main types of cells:
- follicular cells – produce and store T4 and T3, and make a protein called thyroglobulin (Tg)
- parafollicular cells (C-cells) – produce calcitonin.
Behind the thyroid gland are four additional glands called the parathyroid glands. These glands produce parathyroid hormone (PTH), which controls the amount of calcium in the blood.
To keep the body working properly, it is important that the thyroid gland makes the right amount of hormones. This is controlled by the pituitary gland, which is located at the base of the brain:
- If the levels of T3 and T4 drop below normal, the pituitary gland produces a hormone called thyroid-stimulating hormone (TSH). TSH prompts the thyroid gland to make and release more T3 and T4.
- If the levels of T3 and T4 are too high, the pituitary gland produces less TSH.
Changes in thyroid hormone levels affect your metabolism by altering the speed of 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 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 an increased appetite, feel shaky and anxious, or have rapid, strong heartbeats called palpitations. Over time, untreated hyperthyroidism can result in loss of bone strength and problems with heart rhythm.
What types are there?
There are several 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 H¸rthle cell carcinoma, a less common subtype
- 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 are the signs and symptoms?
Thyroid cancer usually develops slowly, without many obvious signs or symptoms. However, some people experience one or more of the following:
- a painless lump in the neck, which may grow gradually
- trouble swallowing
- difficulty breathing
- changes to the voice, e.g. hoarseness
- swollen lymph glands in the neck, which may slowly grow in size over months or years.
Although a painless lump in the neck is the most common sign, thyroid lumps (or nodules) are common and are benign in 90% of adults.
Having an underactive or overactive thyroid (hypothyroidism or hyperthyroidism) is not typically a sign of thyroid cancer.
How common is thyroid cancer?
About 2700 people are diagnosed with thyroid cancer each year in Australia. 1 Thyroid cancer occurs three times more often in women than men – it is the seventh most common cancer affecting Australian women. 2
Thyroid cancer can occur at any age. It is the most common cancer diagnosed in women aged 25–29, and the third most common cancer in women aged 25–49.
Diagnoses of thyroid cancer in Australia have increased in recent years. Between 1982 and 2017, cases of thyroid cancer more than tripled. 2
A significant portion of this increase is due to the improved quality of ultrasounds and other scans. This has led to the detection of smaller, often insignificant, thyroid cancers that would otherwise not have been found. Researchers are trying to determine if there are any other causes of the increased rates of thyroid cancer.
What are the risk factors?
The exact cause of thyroid cancer is unknown, but several factors are known to increase the risk of developing it. Having some of these risk factors does not necessarily mean that you will develop thyroid cancer. Most people with thyroid cancer have no known risk factors.
Exposure to radiation
A small number of thyroid cancer cases 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 in the environment, such as the site of a nuclear accident. Thyroid cancer usually takes 10–20 years to develop after radiation exposure.
Only around 5% of thyroid cancer runs in families. Having a first-degree relative (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.
Being overweight or obese may also increase the risk of developing thyroid cancer.
Having a thyroid condition, such as thyroid nodules, an enlarged thyroid (known as a goitre) or inflammation of the thyroid (thyroiditis), only slightly increases your chance of developing thyroid cancer.
"Sometimes I felt people were a little dismissive because thyroid cancer has a good outlook. They would say, `If you're going to get cancer, that's the best type to get.' But I didn't find this very helpful. Hearing the word `cancer' made me feel gutted and afraid." – Jenny
Expert content reviewers:
Dr Mark Pace, Endocrinologist, Royal Melbourne Hospital, VIC; Dr Roger Allison, Radiation Oncologist, Royal Brisbane and Women's Hospital, QLD; Dr Gabrielle Cehic, Nuclear Medicine Physician, Flinders Medical Centre and Queen Elizabeth Hospital, SA; Leisa Davey, Consumer; Ilona Lillington, A/Clinical Nurse Consultant Thyroid, Royal Brisbane and Women's Hospital, QLD; A/Prof Julie Miller, Specialist Endocrine Surgeon and Head of Thyroid/Endocrine Tumour Group, Royal Melbourne Hospital, VIC; Chris Sibthorpe, 13 11 20 Consultant, Cancer Council Queensland; Carly Smith, A/Clinical Nurse Consultant Thyroid, Royal Brisbane and Women's Hospital, QLD.
1. Australian Institute of Health and Welfare (AIHW), Australian Cancer Incidence and Mortality (ACIM) books: Thyroid cancer, AIHW, Canberra, December 2017.
2. Australian Institute of Health and Welfare (AIHW), Cancer in Australia 2017, AIHW, Canberra, 2017.