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Thyroid glossary

Are thyroid-related terms and abbreviations leaving you feeling confused? We explain every thyroid-related medical term in a way that makes sense.

If you have ever wondered what Free T3 or T4 means, or what's the difference between hyperthyroidism and hypothyroidism, then you are in the right place.

Medical terminology can be confusing, especially when it comes to thyroid health. Our thyroid glossary explains the key terms used to describe the thyroid, its diagnosis and management – simply.

Autoimmune disease

Autoimmune disease occurs when the immune system begins attacking its cells. Autoimmune diseases include rheumatoid arthritis, lupus, psoriasis, pernicious anaemia, and multiple sclerosis, as well as Hashimoto's thyroiditis and Graves' disease.


Euthyroid means that levels of thyroid hormones are within the normal range.

Free T3

T3 or triiodothyronine is the more biologically active thyroid hormone produced by the thyroid gland. Some T3 is bound to proteins in our blood and is therefore unavailable to our cells. Free T3 is the unbound or available hormone and is generally a better measure of thyroid status than a T3 blood test.

Free T4

T4 or thyroxine is the main hormone produced by the thyroid gland. Some T4 will attach to proteins in the blood, which means it is not available to be converted to T3, the biologically active thyroid hormone. A blood test to measure Free T4 can be a good measure of available T4.


A goitre is caused by the enlargement of the thyroid gland, which produces swelling in the neck. It occurs when the thyroid is struggling to produce enough thyroid hormone for the body's needs, either because of insufficient dietary iodine or because the thyroid gland is under attack from the immune system. In these circumstances, the pituitary gland stimulates the thyroid to produce more thyroxine, causing it to enlarge. Graves' disease can also cause a goitre as too much thyroid hormone is produced.

Graves' disease

Graves' disease is an autoimmune disease that causes the thyroid to produce excess levels of thyroid hormones. It can cause goitre and bulging eyeballs which are commonly associated with an overactive thyroid. It is named after Robert Graves, who described it in 1835.

Hashimoto's thyroiditis

Hashimoto's thyroiditis or Hashimoto's disease is an autoimmune disease where the immune system attacks cells of the thyroid gland, resulting in lower production of thyroid hormones. Hashimoto's disease is named after Hakaru Hashimoto, who first described it in 1912.


Hyperthyroidism is the name given to an overactive thyroid gland. It causes nervousness, weight loss, fatigue, diarrhoea and a host of other symptoms.


Hypothyroidism is the term given to an underactive thyroid gland, causing tiredness, cramps, a slowed heart rate and weight gain, among many other symptoms.


Levothyroxine is synthetic thyroxine. It is chemically identical to T4 and is prescribed within the NHS to treat hypothyroidism.


Liothyronine is a synthetic form of the active thyroid hormone T3. It is used in some patients to treat suspected hypothyroidism when problems occur in converting T4 to T3 for use by the cells, which may be due to a deficiency in deiodinase enzymes.

Natural desiccated thyroid (NDT)

Natural desiccated thyroid is sometimes known as ‘armour thyroid’ or NDT. The NHS uses the synthetic thyroid replacement hormone, levothyroxine (or T4) to treat hypothyroidism. However, some practitioners believe that certain patients do not respond symptomatically to this approach and so use NDT instead. NDT is actual thyroid tissue (usually acquired from pigs), which has been dried to a powder and contains natural T4 and T3.

Overactive thyroid

An overactive thyroid is a familiar term for hyperthyroidism. Hyperthyroidism means that the thyroid gland produces excess levels of thyroid hormones, resulting in a raised metabolism.

Pernicious anaemia

Pernicious anaemia is an autoimmune disease in which the immune system attacks cells in the stomach leaving it unable to absorb vitamin B12. Vitamin B12 is essential for the production of normal red blood cells. Symptoms include excessive tiredness, numbness in the arms and legs, weakness and unsteadiness.

Pituitary gland

The pituitary gland is a small, round gland located at the base of the brain that releases hormones that control other glands (such as the thyroid) in the body.

Reverse T3 (rT3)

Triiodothyronine (T3) is the active thyroid hormone that governs metabolism in our cells. It is produced from T4 by the removal of an atom of iodine. At times, the wrong atom is removed, resulting in reverse T3. Reverse T3 can block the action of T3 in our cells. Small amounts of rT3 are normal and regulate T3 uptake in our cells, but in times of stress, more rT3 is produced which limits the action of T3. This can cause symptoms associated with an underactive thyroid even though blood test levels of T4 and T3 may be normal.

Sick euthyroid syndrome

Sick euthyroid syndrome (or euthyroid sick syndrome) is a condition where the measured levels of thyroid hormone (usually T3) are low but without any accompanying symptoms of hypothyroidism. This usually occurs when the patient is very unwell for other reasons. It is seen commonly in patients in intensive care units with multiple medical problems.


Thyroglobulin is an abundant protein within the thyroid gland that thyroxine is made from. It contains tyrosine molecules, which when bound to 4 iodine atoms form thyroxine (T4).

Thyroid antibodies

Thyroid antibodies are immunological weapons that target different proteins in the thyroid gland as part of an autoimmune disorder such as Hashimoto’s or Graves' disease, where the immune system turns itself onto the body’s tissues.

Thyroid gland

The thyroid gland is in the front of the neck below the voice box and plays an important role in metabolism where oxygen and calories are converted to energy. The thyroid gland is responsible for producing the thyroid hormones thyroxine (T4) and triiodothyronine (T3).

Thyroid nodule

Thyroid nodules are very common, especially in people over the age of 60. They are caused when there is an abnormal growth of thyroid cells which cause lumps on the thyroid gland. Most thyroid nodules are benign, but a small proportion is cancerous. Usually, a thyroid nodule will be investigated through a biopsy. Sometimes thyroid nodules can affect swallowing and breathing, but normally they do not produce any symptoms.

Thyroid peroxidase

Thyroid peroxidase is an enzyme found within the thyroid gland which is essential for thyroxine hormone production. It converts iodide (which arrives at the thyroid gland in the bloodstream) to iodine which in turn gets bound to tyrosine to form thyroxine.

Thyroid-stimulating hormone (TSH)

Thyroid-stimulating hormone (TSH) is produced in the pituitary gland at the base of the brain to stimulate thyroid hormone production. If blood levels of thyroid hormones are high, the pituitary signals the thyroid to stop production, whereas if thyroid hormone levels are low, it will signal to increase production. TSH is often measured as a proxy for thyroid hormones.

Thyroid storm

A thyroid storm can be fatal and is caused by extreme or untreated hyperthyroidism. Symptoms include rapid heartbeat, raised blood pressure, exhaustion and fever.


"Itis" simply means inflammation. Thyroiditis is inflammation of the thyroid gland.

Thyroid UK

Thyroid UK is a charitable company working primarily within the United Kingdom. They are committed to providing information and resources to promote effective diagnosis and appropriate treatment for people with thyroid disorders in the UK.

Thyroxine (T4)

Thyroxine is also known as T4 and is the predominant hormone produced by the thyroid gland. It is known as T4 because it uses four atoms of iodine (taken from the food we eat). Thyroxine is not active in our cells. It must convert to the active thyroid hormone, triiodothyronine (T3) before it can influence our metabolism.

Triiodothyronine (T3)

Triiodothyronine (T3) is a potent thyroid hormone that governs metabolism in our cells. The thyroid gland produces some triiodothyronine, but it is mostly derived from thyroxine (T4), which loses an atom of iodine to become T3.

Underactive thyroid

Underactive thyroid is the familiar term for hypothyroidism. It means that the thyroid gland produces insufficient amounts of thyroid hormone to maintain the metabolism at its normal rate.

Blood tests

  1. Thyroid Function Blood Test - a comprehensive test that looks for TSH, FT4 and FT3 - the more biologically active form of FT4. This test helps you to take a more in-depth look at your thyroid hormones.

  2. Thyroid Function with Antibodies Blood Test - see if your thyroid hormones are within the healthy range. You can also detect any abnormalities caused by an autoimmune disease. Even if your thyroid function is currently normal, elevated antibodies may increase your risk of developing a disorder in the future.

  3. Advanced Thyroid Function Blood Test - this test includes the hormones, antibodies, and nutritional markers that can support thyroid function. Low iron (ferritin), low vitamin D and low B vitamins can cause fatigue and low energy.

Unsure which blood test is right for you? Head over to our test buying guide.