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What are Thyroid Function tests?

Thyroid function tests (TFTs) are tests that are used to measure the working capacity of thyroid gland. Thyroid gland is a small endocrine gland that is located just above the collarbone. It produces two hormones: triiodothyronine (free T3) and thyroxine (free T4).

TFTs are used to diagnose various thyroid disorders such as hyperthyroidism (increase in thyroxine hormone) and hypothyroidism (decrease in thyroid hormones).

Hormones that are included in TFTs are thyroid-stimulating hormone (TSH), triiodothyronine (T3) and thyroxine (T4) along with a few antibodies.

  • TSH test: It is done to check the amount of TSH in blood. TSH is a hormone produced in the pituitary gland, which instructs the thyroid gland to produce the right amount of T3 and T4 hormones. TSH is continuously produced and released in blood by pituitary gland.
  • T4 test: This test checks the amount of T4 in blood. Pregnancy and oral contraceptives can result in an increase in the levels of this hormone. Some illnesses and medications (for the treatment of arthritis, asthma or other health problems) can cause a drop in T4 levels.
  • T3 test: If an individual has hyperthyroidism despite a normal level of T4, then they need to undergo a T3 test to confirm the diagnosis.
  • Thyroid antibody test: Thyroid antibodies are produced when the immune system starts to attack the thyroid gland. Measuring thyroid antibodies can be useful for diagnosing autoimmune disorders such as Graves’ disease and Hashimoto’s disease.
  1. Why are Thyroid Function tests performed?
  2. How do you prepare for Thyroid Function test?
  3. How are Thyroid Function tests performed?
  4. Thyroid Function test results and normal values

A healthcare provider may advise TFTs to diagnose various thyroid conditions such as:

  • Hyperthyroidism (overactive thyroid): Symptoms of hyperthyroidism are:
    • Anxiety
    • Weight loss
    • Increased heart rate
    • Weakness
    • Diarrhoea
    • Difficulty in sleeping
    • Tremors
    • Puffiness around the eyes
    • Hypothyroidism (underactive thyroid):
  • Hypothyroidism (underactive thyroid): Symptoms of hypothyroidism are:
  • Graves’ disease
    • An autoimmune disorder that causes hyperthyroidism
  • Hashimoto’s disease
    • An autoimmune disorder in which the thyroid gland is destroyed gradually
  • Thyroid Nodules
  • Thyroid cancer
  • TFTs may also need to be done under the following conditions:
    • If an individual is on thyroid hormone replacement treatment
    • For women who test positive for infertility

No specific preparation is usually needed for this test. It can be done at any time of the day, and no fasting is required to take this test. However, some medicines can alter the result of thyroid function tests, so it’s better to inform the doctor if you are taking any prescription or non-prescription medication.

To perform this test, a blood sample is drawn from a vein in the arm with the help of a needle. The process is painless and non-invasive, but some patients may experience slight redness in the area of needle puncture. Redness can be minimised by placing an ice pack over the affected area.

Normal results:

Following is the normal range for thyroid hormone levels:

  • TSH: 0.4-4.0 mU/L (milliunits per litre)
  • Free T4: 9.0-25.0 pmol/L (picomoles per litre)
  • Free T3: 3.5-7.8 pmol/L 

Abnormal results:

If thyroid gland is not functioning properly, it can result in an increased or decreased level of thyroid hormones. Increased TSH concentration can be a sign of hyperthyroidism and decreased TSH concentration could be a sign of hypothyroidism.

TSH will probably be tested first. If it is abnormal, other tests will be done to get further clarity about your condition.

High TSH level can be due to the following conditions:

  • Hypothyroidism
  • Dysfunction of the pituitary gland
  • Inadequate dose of thyroid hormone medication for the treatment of hypothyroidism.

Low TSH level can be due to the following conditions:

  • Hyperthyroidism
  • The dose of thyroid hormone medication for the treatment of hypothyroidism is more than required.

Other than TFTs, the following tests can be used to diagnose thyroid disorders:

  • Ultrasound
  • Thyroid scan
  • Radioactive iodine uptake test

Disclaimer: All results must be clinically correlated with the patient’s complaints to make a complete and accurate diagnosis. This information is purely from an educational point of view and is in no way a substitute for medical advice by a qualified doctor.


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  1. Thyroid Tests. NIDDK [Internet]. National Institute of Diabetes and Digestive and Kidney Diseases. [cited 2019 May 10].
  2. MedlinePlus Medical Encyclopedia: US National Library of Medicine; Thyroid Tests
  3. Australia H. Thyroid function tests . [Internet]. 2019 [cited 2019 May 10].
  4. Lab Tests Online. Washington D.C.: American Association for Clinical Chemistry; Thyroid Panel
  5. British Thyroid Foundation [internet]. British Association of Endocrine and Thyroid Surgeons; Thyroid Function Tests

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