All You Need To Know About The Thyroxine (T4) Test

Overview

Thyroxine, also called T4, is a hormone produced by your thyroid gland, a small gland that sits in your neck. This hormone, along with T3, is involved in metabolism, body temperature, mood, and heart rate control.

A doctor usually orders a T4 test to monitor how your thyroid is working. Thus, it can help diagnose a thyroid condition or monitor the effectiveness of thyroid medication.  

What is a thyroxine (T4) test?

The thyroxine test is a blood test that measures the level of the thyroid hormone T4 in your blood. The test involves drawing blood, usually from a vein in your elbow. This test usually only takes a few minutes. 

What is it used for?

This test is used to monitor the thyroid gland. Information from the test can be used to diagnose a problem with the thyroid gland or see how effective different thyroid medication is. 

Hyperthyroidism- This condition is characterized by high T4 blood values, indicating that the thyroid gland is overactive.

Hypothyroidism- In this condition, little T4 is produced, which may indicate an underactive thyroid gland.  

Pituitary gland problems- Low levels of T4 may indicate a problem with the pituitary gland.

Why do I need a thyroxine test?

A doctor will likely recommend a thyroxine test if they suspect there is a problem with your thyroid gland. This will be the case if you are experiencing symptoms such as unexpected changes in your weight, mood changes, fast heart rate, intolerance to heat or cold, a puffy face, or dry skin.

A doctor may also request a thyroxine test if you are on thyroid medication to see whether the medication dose needs to be adjusted. Symptoms of thyroid problems include:

• Anxiety and irritability

• Problems getting to sleep

• Hot flushes

• Unexpected weight loss

• Muscle weakness

• Diarrhea

• Fast heart rate

• Puffy skin

• Dry eyes 

How is the thyroxine test performed?

The thyroxine test is a simple blood test performed by a trained phlebotomist or nurse. The test involves taking blood from a vein in your elbow and will likely last around five minutes. 

How to prepare for the test

Before your test, it is important to let your doctor know if you are on any medications, including taking any supplements or prescribed drugs, or if you are pregnant, as these factors may influence the levels of T4 in your blood. 

Fasting is usually not required before a thyroxine test. 

How the test will feel

The thyroxine test will feel like any other blood test. You will likely feel a pinch as the needle is inserted to get a blood sample. This pain should only last a few seconds and will subside within minutes.  

Are there any risks to the test?

The thyroxine test is a simple blood test with minimal risks. However, if you have a bleeding disorder, you may experience bleeding or bruising at the blood test site. You should tell the nurse if you have a history of heavy bleeding after blood tests or if you are on any blood thinning medication. 

Understanding T4 blood test results

Normal values

Normal T4 levels usually sit between 4.5 to 12.0μg/dL in adults. However, this number can be influenced based on age, diet, and other medications. 

What abnormal results mean

Abnormal levels of T4 may indicate a problem with your thyroid gland. Your doctor will decide if you need another test to confirm these results or will start you on medication to restore the levels of T4 to normal. 

High levels of T4 may indicate hyperthyroidism, Graves’ disease, or a non-cancerous tumor which occurs when your thyroid gland is overactive and produces too much T4. 

Low levels of T4 may indicate that your thyroid gland is underactive, called hypothyroidism. 

Is there anything else I need to know about a thyroxine test?

Several medications can influence levels of T4 in the blood. It is important to discuss what medications and supplements you are taking with your doctor before getting a thyroxine test. Medications that influence the level of T4 in your blood include: 

• Amiodarone - Used to treat heart palpitations

• Lithium - Used to treat bipolar disorder

• Interferons (IFN) -Used to treat hepatitis

• Iodine

• Contraceptive pills 

The lowdown

Thyroxine, also known as T4, is a hormone produced by your thyroid gland that controls your metabolism, heart rate, and mood. Your doctor will request a T4 test if they suspect you have a problem with your thyroid gland or to monitor how well your thyroid medication is working. The test involves a simple blood test measuring how much T4 is in your blood. 

Frequently asked questions:

How do I prepare for a T4 test?

Your doctor will talk to you about stopping medication that may interfere with your T4 test results. Fasting is not usually required before a T4 test.

What is a normal thyroxine T4 level?

Normal levels of total T4 usually sit between 5.0 to 12.0μg/dL in adults. 

Do I need to fast for a T4 test?

No, you usually do not need to fast for a T4 test. However, your doctor will let you know what is needed for your thyroxine test. 

What if my thyroxine T4 is high?

If T4 levels in the blood are high, this may indicate that your thyroid gland is overactive. Your doctor will discuss this with you and may send you for further testing before starting you on medication to reduce the number of thyroid hormones produced.