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What is Thyroxine test?

Thyroxine or T4 is a hormone produced by the thyroid gland in our body. Along with T3 (another thyroid hormone), T4 plays an important role in various metabolic functions. Most of the T4 is bound to a protein called protein-binding globulin. However, the active form is present in a free state. 

T4 production is regulated through a feedback system mediated by thyroid-stimulating hormone (TSH), which is released from the pituitary gland in the brain.

Overproduction of T4 will result in the onset of hyperthyroidism symptoms, while its underproduction will manifest the symptoms of hypothyroidism.

A thyroxine test helps measures the levels of free or active T4 in the bloodstream to assess thyroid function.

  1. Why is a Thyroxine test performed?
  2. How do you prepare for a Thyroxine test?
  3. How is a Thyroxine test performed?
  4. What do Thyroxine test results indicate?

A thyroxine test is performed in the following conditions:

Common symptoms of hyperthyroidism (overactive thyroid) are 

Common symptoms of hypothyroidism (underactive thyroid) include:

No specific preparation is usually needed for thyroxine test. It can be done at any time during the day, and no fasting is required before undergoing this test. However, some medicines can alter the results of this test; so, make sure to inform your doctor if you are on any medications. He/she can advise you to stop certain medications that can alter test results.

If you are on any thyroid medication, it is best to take the test before taking them. Also, the test should be avoided in case the person is suffering from an acute illness.

For this test, a sample of blood will be collected from a vein of your arm using a needle. The needle injection site will first be cleaned with alcohol. A blood sample will then be withdrawn in a tube and the injection site will be pressed firmly with a cotton or gauze pad to stop bleeding. The collected sample will be sent to a lab for examination.

The whole process is painless and noninvasive, but a few patients experience slight redness in the area of needle insertion. The redness can be minimised by placing an ice pack over the affected area. If you notice persistent discomfort, check in with your doctor at the earliest.

Results of a thyroxin test can vary depending upon the method used and the age, gender, and past health history of the patient. 

Normal results: The normal range of free thyroxine is 10-26 picomoles per litre (pmol/L).

Abnormal results: Higher than normal T4 level could be due to these conditions:

  • Thyroiditis (inflammation of thyroid gland)
  • Graves disease (an autoimmune disorder)
  • Toxic goitre
  • Toxic thyroid nodules
  • Increased intake of thyroid hormone medicines
  • Consuming foods that contain iodine (it is rare and occurs in cases of thyroid problems)
  • Certain tumours of the testes or ovaries (rare)

If T4 level appears to be lower than normal, it could be due to these conditions:

  • Hypothyroidism
  • Malnutrition or fasting (lack of sufficient food)
  • Certain medicines
  • Severe acute illness

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

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  1. Health Link. Thyroid Hormone Tests. British Columbia [internet].
  2. University of Rochester Medical Center. Free T4. Rochester, New York [internet].
  3. Ross DS. American Thyroid Association Guidelines for Diagnosis and Management of Hyperthyroidism and Other Causes of Thyrotoxicosis. Thyroid. 2016 Oct;26(10):1343-1421. PMID: 27521067
  4. Alexander EK. 2017 Guidelines of the American Thyroid Association for the Diagnosis and Management of Thyroid Disease During Pregnancy and the Postpartum.. Thyroid. 2017 Mar;27(3):315-389. PMID: 28056690
  5. Penn State Health. Free T4 test. Milton S. Hershey Medical Center; Pennsylvania