What Is A Triiodothyronine Test?

The thyroid gland is a small gland found in your neck. This gland makes some hormones that play a role in several bodily functions, including metabolism regulation, mood management, growth, and heart rate control.

The thyroid gland produces two main hormones: triiodothyronine (T3) and thyroxine (T4). In hyperthyroidism, these hormones are produced excessively, which causes symptoms including changes in weight, anxiety, and sleeping problems. 

Hypothyroidism, on the other hand, is characterized by low production and release of these hormones and causes other issues such as weight gain, cold intolerance, and fatigue.

A triiodothyronine test measures the levels of T3 in your blood, indicating how the thyroid gland is working or checking the effectiveness of thyroid medication. 

What is a triiodothyronine (T3) test?

If your doctor suspects a problem with your thyroid gland or is monitoring it, they may order a triiodothyronine (T3) test. In this test, a healthcare professional, such as a nurse, collects a small sample of your blood. Then, this sample is sent to a lab, where its T3 levels will be assessed.

The T3 hormone is either free or bound to a protein in the blood. A total T3 test is the most commonly performed test and looks at both free and bound levels of T3. 

What is it used for?

The triiodothyronine (T3) test is used to see the amount of this hormone in your blood. As the thyroid gland produces this hormone, it is a relevant test to gauge how your thyroid gland is functioning. 

This test is used to support the diagnosis of some conditions, such as: 

Hyperthyroidism

The diagnosis of this condition is the primary clinical reason for recommending a triiodothyronine test. In this disorder, T3 levels are elevated, indicating that the thyroid gland is overactive.

Hypothyroidism

This test can show if there is too little T3 produced, which may indicate an underactive thyroid gland. 

Thyrotoxic periodic paralysis

In this disorder, elevated T3 levels are accompanied by severe muscle weakness.

Hypopituitarism

Low T3 levels can indicate an underactive pituitary gland.

Monitoring thyroid medication

The levels of T3 in your blood will give a supporting indication of the effectiveness of thyroid medication and whether the dose needs to be adjusted. 

Why do I need a T3 test?

Your doctor will often ask for a T3 test if you experience symptoms of an undiagnosed problem with your pituitary gland. The test can also be used to monitor the effectiveness of thyroid medication and ensure that adequate thyroid hormone levels are produced.

Symptoms of thyroid problems that may make your doctor suspect a problem with your thyroid gland include the following: 

• Anxiety and irritability 

• Problems getting to sleep 

• Hot flushes

• Unexpected weight loss 

• Muscle weakness 

• Diarrhea 

• Fast heart rate 

• Puffy skin 

• Dry eyes 

What happens during a T3 test?

The T3 test is a simple blood test performed by a trained phlebotomist or nurse. The test involves having your blood drawn from a vein in your elbow, lasting around 5 minutes. 

Will I need to do anything to prepare for the test?

Before a T3 test, it is essential to let your doctor know what medications you are taking, as some medications will influence the T3 levels in your blood. Your doctor will advise you whether you need to stop taking any of these medications before your test. 

Drugs that change the levels of T3 in your blood include: 

• Amiodarone, a medication used to treat heart palpitations

• Lithium, a medication commonly used to treat bipolar disorder

• Interferons (IFN), a medicine used to treat hepatitis

• Iodine 

• Contraceptive pills 

Fasting is usually not required before a T3 test, meaning you can eat as you usually would.

Are there any risks of the test?

There are usually very minimal risks involved in the T3 test. As the test is a simple blood test, the only risks involved occur if you have a history of bleeding disorders which may cause you to bleed where the needle was used to collect blood. You may experience bruising at the site and a bit of pain.

However, these are minimal. It is important to let the nurse know if you have a history of heavy bleeding after blood tests. 

What do the results mean?

Once the lab has analyzed your blood, your doctor will discuss your results. Normal levels of T3 hormone in your blood are:

• 60 to 180 ng/dL total T3

• 130 to 450 pg/dL free (unbound) T3

What does it mean when your triiodothyronine is high?

If the levels of triiodothyronine or T3 in your blood are too high, you may be experiencing several symptoms, including unexpected weight loss, hyperactivity or nervousness, intolerance to heat, and diarrhea. High T3 levels can be caused mainly by hyperthyroidism, non-cancerous tumors, and Grave’s disease. 

What happens if your T3 is too low?

If your levels of T3 are too low, you may be experiencing symptoms of hypothyroidism. These include weight gain, muscle weakness, fatigue, constipation, and cold intolerance. 

What could abnormal results mean?

T3 levels above or below these ranges 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 your levels of T3 to normal.

The lowdown

Triiodothyronine test is a blood test that measures the levels of T3, a hormone produced by the thyroid gland in your neck. A doctor will order this test if they suspect a problem with your thyroid gland. This test is used to diagnose problems with your thyroid gland, along with monitoring the effectiveness of thyroid medication.