The thyroid and parathyroid glands

Learn about the hormones that can affect your metabolism and overall wellbeing.

The thyroid and the parathyroid glands are located next to each other in the neck but, despite their name, that’s all they have in common.

What is the thyroid gland and what does it do?

The thyroid gland is a butterfly-shaped gland that wraps around the windpipe in the throat [1]. It is responsible for producing the hormones thyroxine (T4) and triiodothyronine (T3) which affect the metabolic rate of the body.

Thyroid hormones impact almost all bodily processes from how fast your heart beats, how much energy you have, how quickly you digest food, to how sensitive you are to heat and cold. Elevated thyroid hormones cause your metabolism to run too fast (hyperthyroidism) and insufficient levels cause your metabolism to run too slowly (hypothyroidism).

What hormones does the thyroid gland produce?

  • Thyroxine (T4) -Thyroxine is the primary hormone produced by the thyroid gland but it has little direct influence on the body’s cells. It is a prohormone for triiodothyronine (T3) which is the more biologically active thyroid hormone. Thyroxine is converted to T3 in the liver and other organs of the body [2].
  • Triiodothyronine (T3) -Only around 20% of T3 is produced by the thyroid gland itself – the rest is converted from T4 by the deiodinase enzymes. T3 is the more biologically active thyroid hormone that directly affects the body’s metabolism [3].

What can go wrong with the thyroid gland?

  • Autoimmune thyroid diseaseAutoimmune conditions cause the majority of thyroid disorders. In an autoimmune condition, the body mistakenly identifies its cells as “foreign” and begins attacking them. In the case of a thyroid disorder, this autoimmune attack can cause the thyroid gland to over-produce thyroid hormone (such as Graves’ disease) which speeds up the metabolism. Or it can destroy thyroid tissue which reduces its ability to produce thyroid hormones causing the metabolism to slow down (such as Hashimoto’s thyroiditis).
  • Secondary thyroid disease - In secondary thyroid disease, the problem does not lie with the thyroid gland itself, but with the signalling gland – the pituitary. A tumour in, or surrounding, the pituitary gland can cause thyroid-stimulating hormone (TSH) to be over-produced which in turn stimulates the over-production of thyroid hormones (secondary hyperthyroidism). TSH can also be under-produced causing the under-production of thyroid hormones (secondary hypothyroidism).

What are the symptoms of thyroid disease?

Thyroid hormones affect almost every cell in the body and have wide-reaching effects on all the body’s processes.

Classic symptoms of an overactive thyroid include a faster heartbeat, sensitivity to heat, losing weight and diarrhoea, while an underactive thyroid causes the opposite – a slower heartbeat, sensitivity to cold, weight gain and constipation.

Find out more about thyroid disease symptoms.

How is thyroid disease diagnosed?

Thyroid disease is normally diagnosed through an assessment of symptoms alongside a thyroid function blood test. A thyroid function blood test normally tests for thyroid-stimulating hormone (TSH), free thyroxine (FT4) and sometimes, but not always, free triiodothyronine (FT3).

For more information on thyroid blood tests, head over to our thyroid test buying guide.

How is thyroid disease treated?

An overactive thyroid is normally treated by anti-thyroid medication such as carbimazole – or by removing or destroying all or part of the thyroid gland through surgery or radioactive iodine treatment. After the thyroid gland has been destroyed or removed, thyroid replacement hormones will need to be taken for life.

An underactive thyroid is treated by replacing the hormones that are no longer being produced by the thyroid gland. In most cases, this is achieved by replacing T4 (levothyroxine) which then converts into T3 as normal. In some people who don’t convert T4 to T3 effectively, a combination of T4 and T3 (liothyronine) will be prescribed.

For more information on thyroid health, visit our thyroid health hub.

What are the parathyroid glands and what do they do?

Despite their name, the parathyroid glands have nothing to do with producing thyroid hormones; their name refers to the fact that they are located behind the thyroid gland in the neck. There are four parathyroid glands, each the size of a grain of rice, that are responsible for regulating calcium in the body [4]. Calcium is a very important element in the human body and is necessary not only for strong bones and teeth but also for the functioning of the nervous and muscular systems by providing electrical energy.

What hormone does the parathyroid gland produce?

Parathyroid hormone (PTH) - The sole job of the parathyroid gland is to regulate calcium within a tight range by the release of parathyroid hormone. If the glands sense that the level of calcium in the blood is too low they will release parathyroid hormone which extracts calcium from the bones. If they detect excess calcium in the blood, they will reduce the production of PTH.

What can go wrong with the parathyroid glands?

Hyperparathyroidism is the most common condition of the parathyroid glands and is normally caused by a benign tumour affecting one or more of the glands. This causes them to over-produce parathyroid hormones, which means the body extracts more calcium from the bones than it needs and blood levels of calcium rise. The condition affects mostly women and is likely to be diagnosed between the ages of 50 and 60.

Hypoparathyroidism, where the body produces insufficient amounts of parathyroid hormone, is rare. It can develop if the parathyroid glands are damaged during surgery on the thyroid gland.

What are the symptoms of hyperparathyroidism?

Symptoms of high levels of calcium in the blood can come on suddenly or can develop over several years and are not always related to calcium levels. Some people can experience debilitating symptoms with only slightly raised blood calcium levels. Many of the symptoms are non-specific and may be confused with other conditions, especially if they are mild.

Symptoms of hyperparathyroidism, include [5]:

  • Fatigue
  • Aches and pains
  • Aching bones
  • Depression
  • Headaches
  • Heart palpitations
  • Irritability
  • Difficulty sleeping
  • Low libido
  • Memory loss and confusion
  • Kidney stones
  • Osteoporosis (thinning of the bones)
  • High blood pressure
  • Feeling run down

How is hyperparathyroidism diagnosed?

The effects of elevated calcium levels are progressive meaning that symptoms like osteoporosis will get worse the longer the condition remains untreated. A diagnosis is made through the combination of an assessment of symptoms and a test to measure the amount of parathyroid hormone (PTH) and calcium in the blood.

How is hyperparathyroidism treated?

Treatment may begin with medication to lower calcium levels, but normally surgery is required to remove the affected parathyroid gland.

Where next?

Learn more about other glands and their role in hormone health:

  1. The hypothalamus
  2. The pituitary gland
  3. The adrenals
  4. The gonads
  5. The pancreas, pineal and thymus
  6. Hormone blood test buying guide


  1. 2022. Overactive thyroid (hyperthyroidism). [online] Available at:,problems%20that%20may%20need%20treatment. [Accessed 12 July 2022].
  2. 2022. Free T4 - Health Encyclopedia - University of Rochester Medical Center. [online] Available at: <> [Accessed 12 July 2022].
  3. 2022. Triiodothyronine | You and Your Hormones from the Society for Endocrinology. [online] Available at: <> [Accessed 12 July 2022].
  4. National Cancer Institute. 2022. NCI Dictionary of Cancer Terms. [online] Available at: <> [Accessed 12 July 2022].
  5. 2022. Hyperparathyroidism. [online] Available at: <> [Accessed 12 July 2022].

Related tests