Thyroid hormones T3 and T4

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Thyroid hormones T3 and T4

The thyroid hormones triiodothyronine (T3) and thyroxine (T4) are produced by the thyroid, a butterfly-shaped gland at the base of the neck. Two forms of T3 and T4 are found in the blood: free (which is active) and protein-bound (constituting the reserves). Though less abundant than T4, T3 plays an essential role in regulating energy metabolism. Thyroid hormone production is carefully controlled by TSH (thyroid-stimulating hormone) and is dependent on the availability of iodine in the blood.

What is the purpose of this test?	T3 and T4 tests are used for the following reasons: To diagnose and monitor thyroid issues To monitor and adjust treatment for hyperthyroidism or hypothyroidism
How to prepare?	No preparation is required for this test.
Reference values*	T4 (free): 10.0 - 22.0 pmol/L T3 (free): 2.8 - 6.8 pmol/L
Associated tests	Thyroid hormone (T3 and T4) blood tests are generally performed when a patient has an abnormal TSH test result: If blood TSH levels are below the target values, T4 (free) test will automatically be performed by the laboratory. A T3 (free) assay could also be performed. If blood TSH levels exceed the target values, the laboratory will automatically perform a T4 (free) test. If blood TSH levels are within the target range, no T4 (free) or T3 (free) test will be performed by default. Other thyroid tests may also be ordered.
How it is done?	Blood sample, usually taken from a vein in the arm.
How long before I get the results?	Results are usually ready in a day. It may take longer to receive your results.

* These values are presented for information purposes only. As different laboratories may have different target values, the lab report usually specifies the values used for the test. It should also be noted that test results must be interpreted based on factors such as age, sex, and health condition. Values outside of this range may be considered normal for some individuals.

What does an abnormal test result mean?

High thyroid hormone levels (hyperthyroidism):

Disease of the thyroid gland, such as Graves' disease, thyroiditis, or a goiter with one or more abnormal growths (nodules)
Excessive use of medication used to treat hypothyroidism

Low thyroid hormone levels (hypothyroidism):

Thyroid disease, such as thyroiditis
Pituitary disorder
Damage to the thyroid gland caused by surgery or radiation therapy

Medications that may affect test results

The use of certain medications can influence test results. Here are a few examples:

Amiodarone
Lithium
Estrogens

Factors that can interfere with test accuracy

Biotin taken as a dietary supplement may interfere with thyroid hormone and TSH measurements. It is recommended to stop using biotin supplements for at least 2 days before undergoing thyroid-related laboratory tests, such as TSH and T3 and T4 tests.

What you should know before going for this test

When you go in for a blood test or any other medical exam, it's always a good idea to bring a complete list of any prescriptions, over-the-counter medications, or natural health products you're taking. Unless otherwise advised, it's best to take your medications as usual on blood test days. If in doubt, don't hesitate to talk to your healthcare professional.

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The patient information leaflets are provided by Vigilance Santé Inc. This content is for information purposes only and does not in any manner whatsoever replace the opinion or advice of your health care professional. Always consult a health care professional before making a decision about your medication or treatment.