Thyroid Function Tests Explained

Last updated: 4 December 2017

What is a thyroid function test?

Thyroid function tests (TFT) are a biochemical measure of the amount of thyroid hormones in the bloodstream, to determine whether the thyroid gland is overactive or underactive.

How is a thyroid function test performed

A few millilitres of blood from a vein are required to perform a thyroid function test.

When would you need a thyroid function test?

A thyroid function test may be requested by your doctor if you have symptoms suggestive of an underactive thyroid gland, known as hypothyroidism.

Symptoms of hypothyroidism may include

  • Weight gain
  • Greasy skin and hair
  • Low mood (depression)
  • A general slowing down of body metabolism and mental processes

The other obvious reason to check thyroid function would be when there is a suspicion of an over-active thyroid, known as hyperthyroidism.

Symptoms of hyperthyroidism include:

  • Anxiety
  • Sweatiness
  • Tremors
  • Weight loss
  • Weakness of the proximal muscles – shoulders and hips
  • Increased heart rate (tachycardia) and rarely, atrial fibrillation (an irregular heartbeat) can also occur with hyperthyroidism

A common cause of excessive thyroid hormone in young women, known as Graves disease, causes bulging eyes, with retraction of the eyelids. Pain over the thyroid gland, at the front of the neck, may be associated with thyroiditis, an inflammation of the gland usually due to an autoimmune problem.

A lump in the thyroid gland may or may not be associated with an alteration in thyroid gland function, and should always be investigated.

Thyroid function test results explained

Thyroid function tests may be misleading in that they can be abnormal when a person is unwell from another cause, the so-called “sick euthyroid” situation.

The most commonly performed test is the Thyroid Stimulating Hormone (TSH), an indirect measure of thyroid hormone levels.

TSH is a hormone produced by the hypothalamus, a gland in the brain.

The secretion of TSH is regulated by thyroid hormone levels, thus a high TSH usually means that thyroid hormone levels are low – TSH is trying to stimulate the underactive thyroid gland.

Low TSH usually means that high thyroid hormone levels from an overactive gland are suppressing TSH production.

The thyroid hormones – Thyroxine (T4) and Tri-iodothyronine (T3) – can also be directly measured, both in their total blood concentration and the “free” portions (FT3 and FT4) of that total, in other words the portion not bound to blood proteins.

Most T4 and T3 are carried in the blood by attaching to proteins such as albumin and Thyroxine Binding Globulin (TBG), whose levels can be affected by other medical conditions, including pregnancy.

Related specialists

Related procedures

  • Venesection (Blood Sample)

Also known as

  • TFT’s
  • Thyroid Function



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This article is for informational purposes only and should not be taken as medical advice. If in doubt, HealthEngine recommends consulting with a registered health practitioner.