What is thyroid cancer?

Thyroid cancer is when abnormal cells in the thyroid gland start to divide and grow in an uncontrolled way. Without treatment, cancer cells can eventually grow into surrounding healthy tissues. They may spread to other areas of the body.

 The thyroid gland sits in the throat. This 2 minute video describes the different parts of the throat.

The thyroid gland

The thyroid is a small butterfly shaped gland that makes and releases hormones. It’s found at the front of your neck in the lowest part, just behind the small hollow where your collar bones meet. You can’t usually feel a thyroid gland that is normal.

The thyroid gland is in 2 halves called lobes. They connect together with a thinner bridge of thyroid tissue. The bridge is called the isthmus.

A thyroid cancer is usually found in just one lobe. But some types of thyroid cancer can be found in both lobes. 

Diagram showing the thyroid gland

What does the thyroid gland do?

The thyroid gland is part of the endocrine system. It makes hormones which affect the way your body works. It makes 3 hormones:

  • T3 (tri iodothyronine)
  • T4 (thyroxine)
  • Calcitonin

T3 and T4

T3 and T4 help to control how fast your body works. This is your metabolic rate. 

If you don’t make enough T3 and T4 hormones

Your body cells work slower than usual. You might put on weight, feel very tired and lack in energy. This is called hypothyroidism or having an underactive thyroid.

If you make too many T3 and T4 hormones

Your body cells work faster than usual. You can lose weight, despite an increased appetite. You might also feel anxious and find it difficult to relax. This is called hyperthyroidism or an overactive thyroid.

What is thyroid stimulating hormone (TSH)?

The pituitary gland is at the base of your brain and is also part of the endocrine system. It senses when the levels of T3 and T4 are too low and releases thyroid stimulating hormone (TSH). TSH stimulates the thyroid gland to produce more T3 and T4. If the pituitary gland senses that levels of T3 and T4 are too high, it stops making TSH. So the thyroid makes less T3 and T4.


The thyroid gland produces another hormone called calcitonin. It helps to control the amount of calcium circulating in the blood. Calcitonin works with a hormone called parathyroid hormone to do this.

The parathyroid glands make parathyroid hormone. These sit behind and are attached to the thyroid gland.

Diagram showing the position of the thyroid and parathyroid glands

Too much calcium can make you feel sick and drowsy. Too little can cause nerve problems, such as pins and needles, making muscles twitch and jerk.

Your surgeon leaves the parathyroid glands behind if they remove your thyroid gland. But the operation can affect them and they may take a while to get back to normal. Your doctor will check your calcium levels regularly and might give you calcium supplements.

Where does thyroid cancer start?

There are 2 main types of cell in the thyroid gland. These are:

  • follicular cells
  • C cells, that make calcitonin

Less common cells include immune system cells (lymphocytes) and supportive cells (stromal cells).

There are different types of thyroid cancer. The type you have depends on what type of cell the cancer starts in. Some growths and tumours in the thyroid gland can be non cancerous (benign), and others are cancerous (malignant).

Most thyroid cancers start in follicular cells. This includes:

  • papillary thyroid cancer
  • follicular thyroid cancer
  • oncocytic (hurthle cell) thyroid cancer

A small number of thyroid cancers start in the C cells. These are called medullary thyroid cancers. 

How common is thyroid cancer?

Thyroid cancer is quite rare. Around 3,900 people are diagnosed in the UK each year. It’s more common in women than in men.

There are factors that can increase the risk of developing thyroid cancer. These include radiation and some types of non cancerous thyroid conditions.

Related links