How is thyroid disease diagnosed?

Thyroid Health

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Emily Condon
BSc (Hons) Biological Sciences

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According to the British Thyroid Foundation, 1 in 20 people in the UK suffer from a thyroid disorder. However, the figure may be much higher than this, as many of those who suffer from a thyroid problem go undiagnosed or are misdiagnosed as symptoms can mimic other conditions. Blood tests measuring hormone and antibody levels are essential to investigate thyroid symptoms and find out whether there's a problem.Thyroid hormones together with thyroid stimulating hormone (TSH) are commonly measured in most thyroid blood tests.

To accurately determine how well your thyroid is functioning, it is important to measure your thyroid hormone levels. Most investigations into a thyroid disorder begin by testing levels of thyroid stimulating hormone (TSH), free thyroxine (FT4) and free triiodothyronine (FT3).

  • Thyroid Stimulating Hormone (TSH) is produced in the pituitary gland and instructs the thyroid to produce the T4 and T3 hormones. High levels of TSH may indicate an underactive thyroid while low levels indicate an overactive thyroid. In primary pituitary failure, a low TSH will be associated with an underactive thyroid.
  • Total thyroxine (T4). T4 (Thyroxine) is the main hormone produced by the thyroid gland. Most of the T4 in the blood is bound to proteins, while some T4 is 'free' or unbound. This test measures the level of total T4 in your blood. Raised T4 can indicate an overactive thyroid while low T4 may indicate that the thyroid is struggling to produce sufficient thyroid hormones. 
  • Free thyroxine (FT4) is the level of T4 travelling in the blood not bound by proteins. High levels of free thyroxine can indicate an overactive thyroid, and low levels can indicate an underactive thyroid.
  • Free triiodothyronine (T3) is a thyroid hormone produced by the thyroid gland. Most T3 is bound to proteins in the blood. Free T3 measures the level of T3 that is free, or unbound to protein, and is available to regulate metabolism. T3 testing is used to see whether an individual has an underactive thyroid, as a raised T3 level can indicate hyperthyroidism.
  • Reverse T3 (rT3). Usually T4 is converted to T3, however, when the body is under stress (due to illness, starvation, extreme cold), it instead converts T4 to reverse T3 (rT3). Reverse T3 inhibits the action of T3 in the cells, which is thought to be a protective mechanism to conserve energy. While low rT3 levels are normal, a high rT3 result can indicate that the body cells are not receiving enough T3, which can lead to hypothyroidism.

A blood test for 2 important thyroid antibodies is an excellent way to see if a thyroid autoimmune disorder is to blame for your symptoms. 

  • Thyroglobulin antibodies (TGAb). Thyroglobulin is a protein used by the thyroid gland to produce T3 and T4. Ordinarily, thyroglobulin does not enter the bloodstream, but if your thyroid is inflamed or under attack from the immune system, then thyroglobulin can be secreted and antibodies detected. Raised levels of TGAb can indicate autoimmune thyroid disease. 
  • Thyroid peroxidase antibodies (TPOAb). The thyroid produces the enzyme thyroid peroxidase, which is required for converting T4 to T3. A TPOAb blood test looks for antibodies in the blood that target the thyroid peroxidase enzyme. High levels the blood indicates that the body's immune system is attacking the thyroid gland and impairing its function. Raised levels of TPOAb are often found in Hashimoto's disease and in many cases of Graves' disease. 

Learn more about thyroid disease:

What is the thyroid?

What is thyroid disease?

What are the risk factors for thyroid disease?

What is hypothyroidism (an underactive thyroid)?

What is hyperthyroidism (an overactive thyroid)?

Reverse T3 - what is it and do I need to test for it?

Reverse T3 - results explained

What are the symptoms of thyroid disease?

Thyroid FAQs

Medicheck your thyroid

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