Free Triiodothyronine (FT3)
Triiodothyronine is one of the thyroid hormones present in serum which regulates metabolism. Determination of this hormone concentration is important for the diagnostic differentiation of euthyroid, hyperthyroid, and hypothyroid states. The major fraction of total triiodothyronine is bound to the transport proteins (TBG, prealbumin, albumin). Free triiodothyronine (FT3) is the physiologically active form of the thyroid hormone triiodothyronine (T3). The determination of free T3 has the advantage of being independent of changes in the concentrations and binding properties of the binding proteins; additional determination of a binding parameter (T-uptake, TBG) is therefore unnecessary, hence the advantage of FT3 testing.
FT3 testing and diagnosis
The test measures the amount of free triiodothyronine, or FT3, in the blood. T3 is one of two major hormones produced by the thyroid gland (the other hormone is called thyroxine, or T4). The thyroid gland is found in the neck, in front of the windpipe. T3 makes up less than 10% of what we call thyroid hormone, while T4 makes up the rest. T3, however, is about four times as strong as T4, and is thought to cause most, if not all, the effects of thyroid hormones.
Thyroid disorders are among the most common endocrine diagnoses. A crucial component for the assessment of thyroid function is thyroid hormone testing, with thyroid-stimulating hormone (TSH) concentration as the primary screening method. Physiologically, secretion of the thyroid hormones triiodothyronine (T3) and thyroxine (T4) is tightly controlled by TSH produced by the pituitary gland. TSH is negatively regulated by T3 and T4 and is inversely proportional to free thyroid hormone activity in normal homeostasis. As TSH secretion is highly sensitive to thyroid hormone change, serum TSH concentration is a suitable indicator of early thyroid dysfunction. As a result, TSH testing has high sensitivity and specificity for diagnosis of hypo and hyperthyroidism. Further investigations of thyroid function, including free T4 (FT4) measurement, is usually performed based on TSH results. Total T3 or free T3 (FT3) measurement is most useful when TSH is suppressed but FT4 is normal or decreased. These laboratory values in a symptomatic patient are referred to as T3 thyrotoxicosis and can occur with Graves' disease, toxic multinodular goiter, or toxic adenoma.
T3/FT3 is often ordered simultaneously with TSH and FT4 tests. While the measurement of T3 is crucial for the diagnosis of T3 thyrotoxicosis, T3 testing without a priori knowledge of thyroid status is likely unnecessary in most cases.
FT3 meaning and test
The Total T3 test measures both the bound (T3) and free forms of T3 (Free T3) levels in the blood to evaluate the thyroid functioning; if it has underactive (Hypothyroidism) or overactive (Hyperthyroidism) performance. The FT3 (free triiodothyronine) test measures the level of free T3 hormone in a blood sample to assess the amount of active thyroid hormone available in the body. FT3 helps evaluating the thyroid function and diagnose and monitor thyroid disorders. FT3 is mainly used for diagnosing hyperthyroidism and to monitor its treatment.
FT3 test if mainly performed when a patient gets an abnormal thyroid stimulating hormone (TSH) result or an abnormal free thyroxine (FT4) result. A blood sample taken from a vein in the arm is required for performing the FT3 testing. No preparation is needed for FT3 testing. However, certain medications can interfere with the FT3 test results. Hence, the doctor should be informed by the patient about any drugs or prescribed medications that are being taken, including any over-the-counter supplements.
The T3 hormone comes in two forms:
- Bound T3, which attaches to protein;
- Free T3, which does not attach to anything.
The vast majority of thyroid hormones are in the bound form, but it is generally more useful to measure the free form, because it is the one that has a clinical effect. In the past, this is where the T3 uptake test was coming into play. This test provided a way to indirectly measure free T4 levels, before modern laboratory technologies arised.
About the T3 uptake test that is no longer used nowadays: The T3 uptake test, also called the T3 resin uptake (T3RU) test, was indirectly measuring the binding capacity of thyroxine binding globulin (TBG). It wasn't measuring T3, despite the name.
Thyroid Function Tests (TSH, FT4, FT3)
- Thyroid function tests (TFTs) include thyroid stimulating hormone (TSH, also known as thyrotropin), free T4 (FT4; thyroxine) and free T3 (FT3; tri-iodothyronine).
T3 Uptake Test
TBG, short for thyroxine binding globulin, is a protein that can bind to T4 and T3. The T3 uptake test reports the percentage of TBG that is bound to thyroid hormones: a higher percentage of T3 uptake means more TBG bound to T4 and T3. This test used to be useful when abnormalities in thyroid-binding proteins made total T4 levels misleading.
But % T3 uptake is not very useful by itself. The T3 uptake test should be done in conjunction with a total T4 test to calculate free T4, also called the free T4 index. A high level of free T4 can indicate hyperthyroidism, while low levels can indicate hypothyroidism.
Difference between T3 and FT3
T3 has 2 forms: bound and free. Bound T3 is attached to a protein and free T3 is not attached to anything. The free T3 test measures only the amount of free T3. The total T3 test measures both free and bound T3 in the blood.
T3:
The thyroid is a small butterfly-shaped gland present in the neck region and is responsible for various functions of the body, including metabolism and growth. Triiodothyronine (T3) is one of the hormones produced by the thyroid gland. This test is done to assess thyroid function and measures the blood level of the T3 hormone. T3 is four times more powerful than T4. Being secreted by the thyroid gland, T3 and T4 play an important role in the growth of the human body.
Free T3:
Free Triiodothyronine test measures the amount of T3 in the blood. Blood levels of T3 are higher or lower because of inadequate or uncontrolled production of the hormone by the thyroid gland because of thyroid dysfunction or pituitary dysfunction. Abnormal levels of T3 in the blood are related to hyperthyroidism, hypothyroidism, and slow metabolism.
What other tests might be evaluated along with this test?
A patient may also need these tests, along with the FT3 testing:
- T3 uptake (or the equivalent used nowadays in the laboratories)
- Total T3
- Total T4
- Free T4
- Free thyroxine index
- TSH
- Tyroglobulin (Tg)
- Thyroid scan
- Radioactive iodine uptake test (not very used nowadays)
- Thyroid-stimulating immunoglobulin (TSI) test
Why does a patient get tested for the FT3 levels?
- Triiodothyronine is given to detect thyroid dysfuntion;
- This test helps detect infertility in women;
- This test is not affected by protein level and is therefore more accurate;
- An overactive thyroid (overactive thyroid gland function), underactive thyroid (low thyroid gland function) and hypopituitarism (underactive pituitary gland) are some of the disorders that affect the thyroid gland.
When to get tested for FT3?
A person may need a T3 or FT3 test if having symptoms of hyperthyroidism. These include:
- Anxiety- Weightloss
- Tremors in the hands
- Increase of Cardiac frequency
- Bulging eyes
- Sleeping problems
- Fatigue
- Low heat tolerance
- More frequent bowel movements
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Increased Level of Free T3:
1. T3 toxicosis
2. Hyperthyroidism
3. Peripheral resistance syndrome
Decreased Level of Free T3:
1. Hypothyroidism
2. Pregnancy in the third trimester
Limited Utility of Free Triiodothyronine Testing
FT3 testing is most useful when thyroid stimulating hormone (TSH) is suppressed, and free thyroxine (FT4) is normal or decreased. These laboratory values in a symptomatic patient are referred to as T3 thyrotoxicosis.
T3 tyrotoxicosis is a relatively rare diagnosis and FT3 measurement has limited utility in the vast majority of patients.
Measurement of TSH, FT4 and FT3
Clinical guidelines recommend the use of T3 testing in patients with normal or low FT4 and low TSH.
While FT3 has diagnostic value for detecting T3 thyrotoxicosis, it is a relatively rare diagnosis.
- These tests are variably indicated for investigation of possible hypo- or hyper- thyroidism, for monitoring of adequacy of thyroid hormone replacement, for monitoring of anti-thyroid treatment, for the investigation of the hypothalamic-pituitary-thyroid axis and for management of thyroid carcinoma;
- TSH is the frontline investigation of thyroid diseases, with FT4 and FT3 being reflexed by the laboratory as appropriate based on the TSH result, previous results, and available clinical details;
- If there is a possibility of hypothalamic or pituitary disease, FT4 should be assayed alongside TSH in these cases;
- TSH levels may remain suppressed for many months after treatment for thyrotoxicosis has commenced. FT4 and FT3 provide better information for clinical management.
Reference Ranges
Adult Reference Ranges:
TSH: 0.27 - 4.2 mIU/L
FT4: 12.0 - 22.0 pmol/L
FT3: 3.1 - 6.8 pmol/L
Age-related reference ranges for children apply as displayed below.
0 - 6 Days 7 - 14 Days 15 Days - 16 Years
TSH (mIU/L) 0.7 - 15.2 0.72 - 11.0 0.27 - 4.2
Free T4 (pmol/L) 11.0 - 32.0 11.5 - 28.3 12.0 - 22.0
Free T3 (pmol/L) 2.7 - 9.7 3.0 - 9.3 3.1 - 6.8
Please note there are also pregnancy related reference ranges displayed below.
1st trimester 2nd trimester 3rd trimester
TSH (mIU/L) 0.3 - 4.6 0.4 - 4.1 0.2 - 3.2
Free T4 (pmol/L) 12.1 - 19.6 9.6 - 17.0 8.4 - 16.0
Free T3 (pmol/L) 3.8 - 6.0 3.2 - 5.5 3.1 - 5.0
Summary FT3
Thyroid hormones help regulate the body's metabolism (how the body functions).
About 99.7% of T3 in blood is attached to a specific protein, and the rest is unattached (free). Blood tests can measure either the total (both bound and unattached) or free T3 hormone in the blood. These tests are called TT3 (total T3), and FT3 (free T3) respectively.
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