Our glossary is designed to help you understand some of the medical terms used to describe thyroid disease and thyroid blood tests so that you can learn more about how to manage your condition.

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Autoimmune disease

Autoimmune disease occurs when an individual's immune system begins attacking their own cells. Autoimmune diseases include rheumatoid arthritis, lupus, psoriasis, pernicious anaemia, multiple sclerosis as well as thyroid disorders Hashimoto's thyroiditis and Graves' disease.


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

Free T3

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

Free T4

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


A goitre is caused by the enlargement of the thyroid gland, which produces a swelling on 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 body's 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, although in this instance too much thyroid hormone is being 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 body's 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, causing 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, weight gain and a host of other symptoms.


Levothyroxine is the synthetic thyroxine (which is chemically identical to T4) that 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 body's cells. This may be due to a deficiency in deiodinase enzymes.

Natural desiccated thyroid (NDT)

Natural desiccated thyroid is sometimes known as ‘Armour thyroid’ or NDT. In the UK the NHS uses the synthetic thyroid replacement hormone, levothyroxine (or lT4) to treat hypothyroidism. However some practitioners believe that certain patients do not respond symptomatically to this approach and use NDT instead. This is actual thyroid tissue (usually acquired from pigs), which has been dried to a powder and contains natural T4 and T3.

Overactive thyroid

Overactive thyroid is the familiar term for hyperthyroidism which 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 body's 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 which 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 actually 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 and is seen commonly in patients being treated in intensive care units with multiple medical problems.


Thyroglobulin is a protein contained in abundance within the thyroid gland from which the hormone thyroxine is manufactured. 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 disease such as Hashimoto’s disease or Graves' disease, where the immune system turns itself onto the body’s own tissues.

Thyroid gland

They thyroid gland is located in the front of the neck below the voice box and plays an important role in metabolism whereby oxygen and calories are converted to energy. The thyroid gland is responsible for producing 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 abnormal growth of thyroid cells which cause lumps on the thyroid gland. The vast majority of thyroid nodules are benign but a small proportion are cancerous. Normally 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 our brains 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 is caused by extreme hyperthyroidism. Symptoms include rapid heartbeat, raised blood pressure, exhaustion and fever. It is caused by under-treated or untreated hyperthyroidism and can often be fatal.


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

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 4 atoms of iodine (from the food we eat). Thyroxine isn't active in our cells and must convert to Triiodothyronine (T3), the active thyroid hormone, before it can influence our body's metabolism.

Triiodothyronine (T3)

Triiodothyronine (T3) is a potent thyroid hormone which governs metabolism in our body's cells. The thyroid gland produces some triiodothyronine, but most of is 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 levels of thyroid hormone to maintain the body's metabolism at its normal rate.

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