T3 (Triiodothyronine) Test: What It Is, Function & Levels

Max Lab

Jan 05, 2024

The thyroid is a small, butterfly-shaped gland located near the throat which mainly releases two hormones, T3 and T4, called triiodothyronine and thyroxine, respectively. These two hormones work together and are often referred to as thyroid hormones. Like all other hormones in the body, T3 and T4 also have their unique functions in the body. Being an important part of the endocrine system, they play an important role in weight regulation, maintaining energy levels and internal body temperature, and promoting skin, hair, and nail growth. Other than this, T3 and T4 also control the metabolism process in the body. Apart from these two, TSH is another important hormone that is released by the pituitary gland. TSH, or thyroid-stimulating hormone, as the name suggests, stimulates the thyroid gland to produce both T3 and T4.

A Closer Look

Triiodothyronine, or T3, is the active form of thyroid hormone, whereas T4 is the inactive form. A large part of T4 released by the thyroid gland gets converted into T3 by the liver and kidneys. As a result, approximately 80% of the T3 present in an individual’s bloodstream is a result of this conversion. Only the remaining 20% is produced by the thyroid gland. High or low levels of the T3 hormone may indicate an overactive or underactive thyroid.

It is important to maintain normal levels of T3, T4, and TSH. Otherwise, it can lead to a number of thyroid conditions and several other complications. To check these levels in an individual’s body, doctors usually suggest a T3 T4 TSH test. This is a panel test that checks the levels of all three hormones, T3, T4, and TSH, combined. However, sometimes the doctor suggests a T3 blood test separately as well. This lab test determines the average t3 levels in one’s body and is an important diagnostic tool for thyroid-related diseases.

What is the T3 Test?

Generally, the T3 hormone is present in two different forms in the human body:

• Bound T3, which attaches to protein
• Free T3, which is not attached to anything and moves freely

Depending on the type of T3 being measured, there are two different tests that are performed. When the test measures the levels of free T3, it is called a free T3 test. On the other hand, when both bound and free T3 are being measured, it is called a total T3 test. Either test can be used to check T3 levels, and if these levels are not within the normal range, this can indicate a thyroid condition.

Types of Thyroid-Related Conditions

There are a number of health conditions that can be caused when t3 and t4 levels are not within the normal range. Here are some of the most common thyroid disorders:

• Hyperthyroidism: This is a condition in which the body starts producing too much thyroid hormone (T3 and T4).
• Hypopituitarism: In this condition, the pituitary gland doesn’t produce normal amounts of pituitary hormones, including TSH.
• Hypothyroidism: This is a condition when the thyroid doesn’t produce sufficient amounts of thyroid hormones.
• Thyrotoxic Periodic Paralysis: This is a condition caused by excessive production of the thyroid hormone. When an individual has higher than normal levels of T3 and T4, it may result in muscle weakness.

What is the Need for a T3 Blood Test?

A T3 blood test is most often used to diagnose hyperthyroidism. This is a condition caused by unusually high production of the T3 hormone. The doctor is likely to suggest this test when an individual experiences symptoms that can be associated with hyperthyroidism.

Here are a few common symptoms of this condition:

• Anxiety
• Unintentional weight loss
• Hand tremors
• Increased heart rate
• Bulging of the eyes
• Trouble sleeping
• Fatigue
• Low heat tolerance
• More frequent bowel movements
• Irregular menstrual cycle

Other than this, pregnant women are also advised to get thyroid testing done. Pregnancy is a time when thyroid levels in the body can fluctuate. However, these changes are temporary, and the levels go back to normal after childbirth.

Reverse T3 Test        

Another common test that a doctor may suggest to diagnose thyroid-related complications is the T3 reverse test. Also known as Triiodothyronine Reverse, this blood test measures the levels of the reverse T3 hormone in an individual’s bloodstream. This is an inactive thyroid hormone that blocks the conversion of T4 to T3, leading to lower levels of T3 hormone in the body. High levels of reverse T3 are often associated with conditions related to low T3 levels. Although reverse T3 does not act like other thyroid hormones (T3 and T4), it plays a major role in brain development. Generally, this test is suggested when other thyroid tests like T3, T4, TSH, or the total/free T3 test show an expected result. In such a scenario, reverse T3 levels help in further diagnosis.

How to Prepare for the Test?

The T3 test is a simple blood test that does not require any specific preparation. Like other thyroid tests, T3 blood doesn’t require overnight fasting. However, certain medicines can alter the value of thyroid hormone and cause inaccurate test results. So, it is important to consult the doctor if an individual is on some kind of medication and take the necessary precautions.

Result Interpretation

When T3 levels are not within the normal range, this may be a sign of some thyroid-related problem. When these levels are high, they indicate a number of conditions, including:

• Graves’ disease
• Hyperthyroidism
• Painless (silent) thyroiditis
• Thyrotoxic periodic paralysis
• Toxic nodular goiter

In rare cases, high T3 levels may indicate thyroid cancer or thyrotoxicosis. Other than this, it can also be caused by high levels of protein in the bloodstream. On the other hand, when T3 levels are low, it may indicate hypothyroidism. Or, these levels can also be dropped because of a long-term illness.

Unlike several other health conditions, a thyroid disorder cannot be diagnosed by performing a single test. It may give an idea, but other additional tests are required to get any definitive answers about the problem. In such a condition, the doctor may suggest a T4 or TSH test to perform a proper diagnosis.