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What Are the Different Types of Thyroid Tests and Which Do You Need?

You may have heard your friends or even your doctor discussing thyroid tests. It can be a confusing term: what exactly is the thyroid and why is testing a good idea? 

Let’s start at the beginning. The thyroid is a two-chambered gland found in the neck, not far from the windpipe. Glands are organs that produce biologically active substances, and the thyroid is part of a particularly significant network of these called the endocrine system. This system transmits signalling chemicals called hormones around the body via the bloodstream. These hormones keep our bodies functioning correctly. 

The thyroid produces three specific hormones: triiodothyronine, thyroxine and calcitonin. The first two are synthesised from the mineral iodine (which is also a chemical element). They’re involved in multiple fundamental cellular functions within the human body. These include:

  • Regulation of our metabolic rate
  • The growth of bone
  • The growth of brain cells (neurons)
  • Our response to neurotransmitters (messaging chemicals within the brain)
  • The digestion of key nutrients like carbohydrates and protein
  • The control of muscles and the generation of heat
  • Energy metabolisation

Triiodothyronine and thyroxine are more commonly referred to as T3 and T4 respectively. These names are references to the number of iodine atoms within each molecule. 

Since the body uses iodine to create T3 and T4, it’s clearly important to consume a healthy level. Good dietary sources of iodine include fish and other seafood – for example, cod, tuna or shrimp. Milk, cheese and other dairy foods are also abundant in iodine.

If you have an iodine deficiency you may feel unusually tired, put on disproportionate amounts of weight, experience the cold more intensely, or just feel physically weak. Talk to your doctor if you regularly experience any of these. In severe cases, the thyroid gland may swell: this is called a ‘goiter’.

T4 is by far the most common of the three thyroid hormones – constituting around 95% of these substances in the bloodstream of the average person. The body can produce T3 directly from T4.

Calcitonin is deployed by the body in the digestion of minerals like calcium and phosphorus. 

What are thyroid blood tests?

Thyroid tests are conducted to assess whether this key gland is working as well as it should. Samples are taken and assessed by laboratory testing equipment. Measuring varying levels of thyroid hormone reveals the current health of this underappreciated gland.

Having an overactive and underactive thyroid is surprisingly common. It is more common in adults than children, and women are no less than 20 times more likely than men to experience such problems.

An underactive thyroid is known as hypothyroidism. Symptoms vary between individuals but typically they will include:

  • Feeling excessively cold
  • Feeling disproportionately tired
  • Aches and pains

By contrast, an overactive thyroid gland is referred to as hyperthyroidism. People with this condition can, to varying degrees, experience:

  • Feeling uncomfortably hot
  • Heart palpitations
  • Muscular tremors

What are the different tests and what do they do?

There are four principal types of thyroid testing. They may be conducted individually but it is more common to carry them out in combination for a fuller picture of the patient’s health.

T3 tests

As you might expect, an abnormally high level of T3 normally suggests an overactive thyroid, or hyperthyroidism. Meanwhile, an unusually low level can indicate illness or overuse of medication.

FT3 test

There are actually two forms of T3 (and T4): it is either bound to proteins or circulating freely through the bloodstream. The latter form, known as ‘free triiodothyronine’ or FT3, is the only form accessible to cells during metabolism. Tests are most commonly conducted for suspected hyperthyroidism, often following the detection of abnormal T4 or TSH levels (see below for more on these).

Similarly, FT4 refers to freely circulating thyroxine.

T4 tests

Levels of T4 that are higher or lower than normal are another indicator of thyroid health. T4 levels may also be affected by pregnancy and certain illnesses – for example hepatitis. Medication containing steroids and contraceptives also alters T4 levels.

TSH test

TSH stands for thyroid stimulating hormone. As you might have guessed from the name, this is not produced in the thyroid. Instead it is secreted by the pituitary gland. This is located in the centre of the brain roughly level with our eye sockets.

TSH triggers the thyroid into producing T3, T4 and calcitonin. Low levels of the thyroid hormones will trigger the release of TSH, so levels of the latter can be used to infer the overall health of a person’s thyroid. 

Hormone levels in the following ranges normally indicate health:

  • T3: 80-220 nanograms per decilitre (ng/dL)
  • T4: 5.0 to 12.0 micrograms per decilitre (μg/dL)
  • TSH: 0.5 to 5 milli-international units per litre (mIU/L)

Home-to-lab thyroid tests

Your GP may refer you for testing if your symptoms suggest either an overactive or underactive thyroid. Typically, these will be to assess your TSH and T4 levels. But as we all know, NHS waiting times can be lengthy. Why not jump the queue with a home-to-lab thyroid test? Home testing kits – like the full thyroid function test from Health Hub – are inexpensive and easy to administer: just take a finger prick blood sample and send this in for a full laboratory analysis. 

And what if the results indicate you do have a thyroid problem? Don’t worry too much: both hyperthyroidism and hypothyroidism can be successfully treated via prescription medication.

Your doctor will regularly conduct blood tests to monitor the effectiveness of the doses taken by the patient. But even when the condition has cleared up, it’s sensible to continue monitoring thyroid hormone levels, perhaps on an annual basis. That way you will be able to stay on top of your health and seek appropriate treatment if your hormone levels begin to move in the wrong direction again.

Sian Baker

Medically reviewed by Sian Baker, Dip ION mBANT mCNHC – Written by Beth Giddings.

Updated on 1st December 2021