FT3 (Free T3)

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;

- It help diagnose hyperthyroidism, a condition in which the thyroid produces excess hormones;

- 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

TSH	Total and Free T4	Total and Free T3	MOST LIKELY DIAGNOSIS
Normal	Normal	Normal	Normal thyroid function (e.g., “euthyroid”)
Normal or decreased	Normal or decreased	Decreased	Normal adjustment in thyroid function due to illness (nonthyroidal illness or sick euthyroid syndrome)
Increased	Normal	Normal	Subclinical hypothyroidism; in a person with hypothyroidism on treatment, not enough thyroid hormone is being given
Increased	Decreased	Normal or decreased	Hypothyroidism resulting from a problem with the thyroid gland itself (primary hypothyroidism)
Normal or increased	Increased	Increased	Hyperthyroidism resulting from a problem with the pituitary gland signals (central hyperthyroidism) or from a problem with the thyroid hormone receptor (thyroid hormone resistance)
Decreased	Normal	Normal	Subclinical Hyperthyroidism; in a person with hypothyroidism, too much thyroid hormone is being given
Decreased	Normal	Increased	Hyperthyroidism resulting from the thyroid gland making too much active thyroid hormone T3 (uncommon, also known as T3 toxicosis)
Decreased	Increased	Increased	Hyperthyroidism resulting from the gland making too much thyroid hormones (primary hyperthyroidism)
Decreased	Decreased	Decreased	Hypothyroidism resulting from a problem with the hypothalamus or pituitary signals that govern the thyroid gland (central hypothyroidism)

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. 

Total T3

Triiodothyronine 

- is an important thyroid hormone that helps maintain muscle control, brain function and development, heart and digestive functions. High or low T3 levels may indicate an overactive or underactive thyroid.

The Total T3 test is primarily used to help diagnose thyroid disorders like hyperthyroidism (overactive thyroid), hypothyroidism and to evaluate the severity of thyroid disorders. Generally speaking, T3 blood tests are most useful for diagnosing hyperthyroidism. They are less helpful in diagnosing hypothyroidism, because T3 levels often remain normal until later stages of underactive thyroid disease. Doctors may order this test alongside TSH and T4 tests, to get a complete picture of thyroid function. 

Triiodothyronine, commonly called T3, is one of two main hormones produced by the thyroid gland and the more biologically active of those two. The other one is T4 (thyroxine).While the thyroid releases mostly thyroxine (T4), it is T3 that does the "heavy lifting": regulating how fast the body burns calories, how quickly the heart beats, and how warm a person feels. 

The thyroid is a small butterfly-shaped gland in the front of the throat. It produces hormones and regulates how the body stores and uses energy. It also controls body temperature, weight, and mood. The main hormone that the thyroid makes is thyroxine (T4). Cells in the body then convert T4 into T3. Together with T4, T3 is responsible for many bodily functions, including heart rate, body temperature, and metabolism, which influences body weight. 

T3 Role in The Body

T3 influences nearly every organ system. It controls the baseline speed of the metabolism, determining how efficiently the cells convert food into energy. It stimulates both the creation and breakdown of fatty acids, playing a dual role in how the body stores and burns fat. It also affects insulin secretion, which ties it directly to blood sugar regulation.

Beyond metabolism, T3 helps maintain the body temperature, supports normal heart rhythm and contraction strength, and plays a role in brain development and nervous system function. It's involved in bone growth, reproductive health, and even how the skin and hair renew themselves. T3 levels naturally fluctuate throughout the day, peaking between 7 a.m. and 1 p.m. and dropping to their lowest between 11 p.m. and 3 a.m., a pattern closely tied to the daily cycles of energy and alertness.

There are two forms of T3 in the blood:

- Bound T3: this form of T3 is the most abundant; bound T3 is attached to proteins in the blood, that help circulate the hormone around the body (so bound T3 is being transported);

- Free T3: this type of T3 is not bound to proteins, and it moves freely around the body; it is less abundant than bound T3 but is the active form of the hormone.

Total T3 measures both protein-bound and free triiodothyronine in the bloodstream. A pregnant one performing a T3 test should inform the healthcare provider, ever since medication and pregnancy can both affect the test results.

Only about 20% of T3 comes directly from the thyroid itself. The rest is created when organs like the liver and the kidneys strip one iodine atom from T4, converting it into the more potent T3 form. 

Most T3 in the blood is produced by the conversion of T4 into T3. Free T3 is the active form that can enter tissues, whereas bound T3 is attached to proteins and serves as a circulating reservoir. The 3 and 4 in the names of T3 and T4 represent the number of iodine atoms it has.

T3 testing

A doctor may recommend a T3 test for anyone who has symptoms of a thyroid condition. Thyroid conditions can cause a wide variety of symptoms, including:

- sweating

- tremors

- sensitivity to heat or cold

- tiredness

- irritability 

- mood swings

- unexplained weight loss

- difficulty sleeping

- bulging or irritated eyes

- hair loss

- irregular or slower heartbeat

- shortness of breath

- high blood sugar

- increased frequency of urination

A doctor may also recommend the test for people who have a higher risk of thyroid disease. These people include those with:

- type 1 diabetes

- pernicious anemia

- primary adrenal insufficiency, or Addison's disease

- a family history of thyroid conditions

Older adults and women who are pregnant or have recently given birth are also more at risk.

What happens during the test

T3 tests usually require no preparation. However, some medications, such as birth control pills, can affect the results. Therefore, a person should inform a doctor of any medications that they are taking before undergoing T3 testing. They may need to stop taking them to ensure the accuracy of the test.

Analyzing Total T3

The total T3 test looks at free T3 and bound T3 forms. The healthcare provider may choose to perform a separate test for free T3.

Total T3 tests are generally ordered when a patient has symptoms that are consistent with thyroid disease, especially if the patient has elevated risk (family members diagnosed with thyroid disorders) and age (as well as other factors). Hypothyroidism is more common in females over 60, while hyperthyroidism is more common in female age 20 to 40.

The healthcare provider will likely order other measurements of thyroid function at the same time as T3 test, including tests of thyroid stimulating hormone (TSH) and T4. Changes in TSH level can be an early warning sign of a thyroid condition, even if other thyroid hormone levels are still normal. T4 is the other major hormone that the thyroid produces alongside T3. High or low T4, especially in conjunction with unusual TSH levels, can indicate a thyroid condition.

There may also be ordered tests to look for antibodies that indicate autoimmune thyroid disease, including Hashimoto's thyroiditis and Graves' disease. Thyroid antibody tests: antibodies are part of the body's immune defenses. However, some people develop thyroid conditions when the body mistakenly attacks the thyroid. Antibody tests can help a doctor diagnose this issue.

The normal ranges for adults are generally considered to be:

Total T3: 60-180 ng/dL / 0.92-2.76 nmol/L;

Free T3: 130-450 pg/dL / 2-7 pmol/L.

Different laboratories may use different ranges for the T3 measurements.

Other laboratories may have as normal T3 levels, values between 80 and 200 ng/dL. Free T3 could range from 2.3 to 4.2 pg/mL. 

Hence, the ranges can vary slightly between laboratories, so the results must always be interpreted according to the specific range listed on the lab report.

High T3 levels

High T3 levels can indicate several conditions, including hyperthyroidism and thyrotoxicosis (excess circulating hormones). An abnormally high T3 level isn't definitively a sign of thyroid disease by itself, but it is considered along with TSH and T4 levels. Excess T3 has the opposite effect as that explained for low T3, pushing the body into overdrive. Symptoms of hyperthyroidism include unintentional weight loss, a rapid or irregular heartbeat, heart palpitations, increased appetite, anxiety, irritability, trembling hands, sensitivity to heat, more frequent bowel movements, muscle weakness, and difficulty sleeping. Some people develop a visible enlargement of the thyroid gland (goiter) at the base of the neck. In severe cases, elevated T3 can trigger a dangerous heart rhythm disturbance called atrial fibrillation.

In the condition called T3 toxicosis, where T3 is elevated but T4 remains normal, a standard T4 test alone would miss the problem entirely, which is why T3 testing is particularly useful for diagnosing and gauging the severity.

In some cases, high T3 levels can signify thyroid cancer, but this disease does not often cause abnormal T3 levels.

Thyrotoxicosis and thyroid cancer are rare, so hyperthyroidism is more commonly the cause. In more than 70% of cases, hyperthyroidism is due to Graves' disease, which occurs most often in females and can run in families. Nodules and lumps that grow on the thyroid can also cause hyperthyroidism. Doctors refer to this as toxic nodular goiter. 

A high T3 level may indicate:

           - Hyperthyroidism if accompanied by a low TSH level

          - Graves' disease if accompanied by a low TSH level and positive tests to detect certain antibodies

           - Toxic nodular goiter

            - Liver disease

            - A rare condition called T3 thyrotoxicosis

    - High T3 can also be due to pregnancy or the use of medications containing estrogen.

Low T3 levels

Low T3 test results can indicate an underactive thyroid, or hypothyroidism. Sometimes, low T3 can also indicate starvation.

A person can develop an underactive thyroid due to autoimmune disease. Sometimes, getting too much or too little iodine also causes symptoms.

Low T3 is part of the broader picture of hypothyroidism. Because T3 sets the pace for so many body processes, a deficiency tends to slow everything down. Common signs include fatigue, unexplained weight gain, feeling unusually cold, a slower heart rate, dry skin, thinning hair, and joint or muscle pain. Many people also experience depression, heavier or irregular menstrual periods, and difficulty concentrating. These symptoms develop gradually, which is one reason hypothyroidism often goes undiagnosed for months or years.

An abnormally low T3 level may indicate:

            - Hypothyroidism, if accompanied by a high TSH level

           - Hashimoto's thyroiditis, if accompanied by a high TSH level and positive tests to detect certain antibodies

            - Malnutrition/starvation

            - A severe short-term illness or some long-term illnesses

Limitations of this test:

            - T3 is largely bound to blood proteins (over 90%)

            - Only unbound T3 is active

Measuring total T3 (both bound and unbound) may give misleading information if the binding proteins are increased or decreased.

Therefore, most healthcare providers today rely on the free T3 blood test instead of the total T3 blood test.

Many factors influence T3 levels. Some of the factors are age, sex, pregnancy, preexisting health conditions such as liver disease, and some medications. Pregnancy can increase total T3 levels. All of these can influence T3 levels, so results are interpreted in the context of other thyroid tests and clinical symptoms. An abnormal level of T3 does not always mean that a person has a thyroid condition.

Summary Total T3

How T3 works in the body 

T3 levels are kept in check through a feedback loop between three structures: the hypothalamus in the brain, the pituitary gland just below it, and the thyroid gland in the neck. When T3 (and T4) levels drop, the hypothalamus releases a signaling hormone called TRH, which tells the pituitary to release TSH (thyroid-stimulating hormone). TSH then prompts the thyroid to produce more T4 and T3.

When levels are sufficient, the process reverses. T3 directly suppresses the production of both TRH and TSH, slowing down the thyroid hormone output. This feedback in the brain depends heavily on circulating T4. That locally produced T3 binds to receptors on the neurons that control TRH, dialing them down. This is why both T4 and T3 matter for the system to work properly.

Free T3 vs. Total T3

Most of the T3 in the bloodstream isn't actually available for the cells to use. About 80% is bound to a transport protein called thyroxine-binding globulin. The remainder attaches to albumin and transthyretin, two other blood proteins. Only roughly 0.04% of total T3 circulates freely, unattached to any protein. This tiny fraction, called free T3, is the biologically active form that can enter cell and influence metabolism.

This distinction matters for blood testing. A total T3 test measures both the bound and unbound hormone, while a free T3 test measures only the active portion. Conditions that change protein levels in the blood (pregnancy, estrogen therapy, liver disease) can shift total T3 without actually changing how much active hormone the tissues receive. That's why doctors sometimes order free T3 specifically when total T3 results don't match the clinical picture.

How T3 Levels Are Tested

Thyroid evaluation usually starts with a TSH test, since TSH is the most sensitive early indicator that something is off. If TSH comes back abnormal, doctors typically follow up with free T4 and T3 tests to pinpoint the issue. A T3 test is especially valuable when TSH is low (suggesting an overactive thyroid) but T4 looks normal, because the excess thyroid activity may be driven entirely by T3.

Factors like time of day, recent meals, illness, and certain medications can all influence T3 readings. Serious non-thyroid illnesses can temporarily suppress T3 levels (a phenomenon sometimes called "sick euthyroid syndrome"), making interpretation trickier in hospitalized patients.

Total T3 - final outcome

Normal T3 levels indicate that someone's thyroid is producing an adequate amount of T3 hormone. High levels can indicate hyperthyroidism, and low levels can be a sign of hypothyroidism.

However, other factors can also influence T3 levels, including certain medications and pregnancy. Due to this, a doctor may need to run additional tests to determine whether someone has a thyroid condition for real.

Treatment for thyroid conditions may involve medication and, in some cases, surgery. 

Total T4 (Total Thyroxine)

Tetraiodothyronine/T4 (Total Thyroxine)

T4 is a thyroid hormone, and too much or too little of it can indicate an issue with the thyroid. A T4 (thyroxine) test is a blood test that helps diagnose thyroid conditions.

Thyroxine, also known as T4 and tetraiodothyronine, is the main hormone secreted by the thyroid gland and released into the bloodstream. The thyroid also releases small amounts of triiodothyronine (T3). T4 and T3 work together and are commonly referred to as "thyroid hormone". Hormones are chemicals that coordinate different functions in the body by carrying messages through the blood to the organs, muscles and other tissues. These signals tell the body what to do and when to do it. 

Thyroxine (T4) plays an important role in the hypothalamic-pituitary system of thyroid regulation and has an influence on general metabolism. T4 results from the coupling of two molecules of 3,5-diiodotyrosine, is bound to thyroglobulin and remains in the cells of the thyroid follicles, from where it is excreted under the action of TSH.

Other names when referring to a T4 test may include: free thyroxine, total T4 concentration, thyroxine screen, free T4 concentration, free T4 index (FTI).

The thyroid is a small, butterfly-shaped gland located at the front of the lower neck, above the clavicle. It's a part of the endocrine system and it makes hormones that control the way the body uses energy. These hormones affect nearly every organ in the body and control many of the body's most important functions. For example, they affect the breathing, heart rate, weight, digestion, and mood. In children, thyroid hormones affect growth, too. The thyroid gland makes and releases thyroid hormones in the blood, which then travel to the organs to exert their effect.

Most thyroxine circulates in the blood bound to proteins. Since the concentration of serum carrier proteins is subject to exogenous and endogenous influences (for example, it increases during pregnancy and after oral contraceptives and decreases in nephrotic syndrome), their status should be taken into account when assessing the T4 level. 

There are two forms of T4 in the blood:

- Free T4 is the active form of thyroxine hormone that enters the body tissues where it is needed;

- Bound T4 is thyroxine that attaches or binds to certain proteins which prevent it from entering the body tissues. It stays in the bloodstream as a "backup supply" until the tissues need it.

T4 is the main hormone that the thyroid makes. The T4 that the thyroid releases is inactive, meaning that it doesn't affect the body's cells. However, the liver and kidneys convert most of this thyroxine into triiodothyronine (T3), which is an active hormone that impacts the cells in the body. 

Together, T4 and T3 play vital roles in regulating various bodily functions, such as:

- Metabolic rate (the rate at which the body transforms the food that is eaten into energy);

- Heart and digestive functions;

- Muscle control;

- Brain development;

- Bone maintenance.

T4 levels can be measured with either a free T4 test or a total T4 test:

- A free T4 test measures the amount of free T4 in the blood. This form "freely" enters the body's tissues where it's needed. Medical experts believe this test is more accurate than a total T4 test, so it's used more often;

- A total T4 test measures free and bound T4 together; Bound T4 attaches to proteins, which prevents it from entering the body's tissues.

Because of this, there are a few different tests that measure T4 levels. A T4 test alone can't provide enough information to diagnose thyroid problems. A blood test that measures both free and bound T4 is called a total T4 test. Some blood tests measure just free T4. Healthcare providers most often use a free T4 test to assess thyroid function because it's more accurate than a total T4 test. Aditionally, T4 analysis is usually done with a TSH blood test. TSH stands for thyroid stimulating hormone. It's a hormone made by the pituitary gland, a small gland at the base of the brain. TSH tells the thyroid how much hormone to make. Normally, if the T4 levels are too low, the pituitary makes more TSH to make the thyroid work harder. If the T4 levels are too high, the pituitary stops making TSH.

A TSH test is the best way to initially assessing the thyroid function. In fact, T4 tests more accurately reflect thyroid function, when combined with a TSH test. Measuring T4 levels might not be necessary in all thyroid conditions.

Recommendations for T4 determination:

- diagnosis of hyper- and hypothyroidism (primary or secondary);

- monitoring of TSH suppressive treatment.

Patient preparation: fasting (without food); if the patient is on treatment with hypolipidemic drugs containing thyroxine, blood sampling for T4 determination will be done 4-6 weeks after its discontinuation. Specimen collected: venous blood.  

 

Why is it a T4 (thyroxine) test needed?

Healthcare providers may use T4 tests to assess how well the thyroid is working. The provider may order a T4 test (in addition to a TSH test) for any of the following reasons:

- To follow up on an abnormal thyroid-stimulating hormone (TSH) test result;

- To diagnose hyperthyroidism (overactive thyroid) or hypothyroidism (underactive thyroid);

- To monitor T4 levels if taking thyroid hormone replacement therapy (medication);

- To screen for an underactive thyroid in newborns;

- To evaluate other conditions, such as goiter, thyroid nodules and issues with the pituitary gland or hypothalamus;

- To evaluate for low thyroid hormone levels from a pituitary cause (central hypothyroidism).

Again, providers more commonly order free T4 tests than total T4 tests. In certain situations, such as pregnancy, a total T4 test might be necessary rather than a free T4 test.

T4 (Total Thyroxine) Reference Values:

- are based on age:

Age:                             Value (nmol/L):                         Value (µg/dL):

0 - 6 days                     64.9 - 239                                   5.04 - 18.5

6 days - 3 months        69.6 - 219                                   5.41 - 17

3 - 12 months              73 - 206                                      5.67 - 16

1 - 6 years                   76.6 - 189                                   5.95 - 14.7

6 - 11 years                  77.1 - 178                                  5.99 - 13.8

11 - 20 years                76.1 - 170                                  5.91 - 13.2

> 20 years                    66.0 - 181                                  5.13 - 14.1

In pregnancy, T4 level increases by 40-60%, starting with weeks 11-12, due to the increase in TGB:

1st trimester: 94.4 - 191 nmol/L (7.33 - 14.8 µg/dL);

2nd trimester: 102 - 208 nmol/L (7.93 - 16.1 µg/dL);

3rd trimester: 89.5 - 202 nmol/L (6.95 - 15.7 µg/dL).

Detection limit - 5.40 nmol/L (0.42 µg/dL).

Clinical alert values - low level: <26 nmol/L (possibility of myxedema coma); high level: >258 nmol/L (possibility of "Thyroid storm").

Limitations and interferences

In the first 2 months of life, T4 shows values much higher than in normal adults.

Elevated T4 values can also be found in dysalbuminemic familial hyperthyroxinemia - albumins bind T4 more avidly than normal, but not T3, causing laboratory changes similar to those in thyrotoxicosis, but the patients are not clinically thyrotoxic.

Normal T4 values can be found in hyperthyroid patients who present T3 thyrotoxicosis or artificial hyperthyroidism due to T3.

What are normal free T4 levels?

Normal levels of free T4 vary based on the age. In general, normal ranges of free T4 for healthy people include:

- Children up to 5 years old: 0.8 - 2.8 nanograms per deciliter (ng/dL);

- Children 6 to 15 years old: 0.8 - 2.1 ng/dL;

- Male adolescents 16 to 17 years old: 0.8 - 2.8 ng/dL;

- Female adolescents 16 to 17 years old: 0.8 - 1.5 ng/dL;

- Adults over 18 years old: 0.9 - 1.7 ng/dL.

Normal value ranges for free T4 may vary slightly among different laboratories. One should check the lab report's reference range on the lab results. If there are questions about the results, the patient should ask the healthcare provider.

What happens when T4 (thyroxine) levels are too high?

If the patient has higher-than-normal T4 or free T4 levels, it could indicate thyrotoxicosis. This can result from several situations and conditions, including hyperthyroidism (overactive thyroid), thyroid inflammation (thyroiditis) and taking excessive amounts of thyroid medication.

Thyrotoxicosis speeds up the metabolism, which can be dangerous to the patient's health. Symptoms of thyrotoxicosis include:

- Unexplained weight loss;

- Increased bowel movements;

- Rapid or irregular heartbeat (arrhythmia).

If one is experiencing symtoms of thyrotoxicosis, it's important to contact the healthcare provider.

Other conditions that could cause elevated total T4 levels with normal free T4 levels include pregnancy and estrogen-containing birth control pills. This is because estrogen levels are high in those two scenarios. Estrogen increases the proteins bound to T4 and causes the total T4 (which is free T4+ binding proteins) to be high.

What happens when T4 (thyroxine) levels are too low?

If a patient has lower-than-normal T4 levels, it usually indicates hypothyroidism (underactive thyroid). Hypothyroidism has several causes, including certain autoimmune diseases, poor iodine intake in the diet and the use of certain medications.

Hypothyroidism slows down the metabolism. Symptoms include:

- Fatigue;

- Intolerance of cold temperatures;

- Low heart rate;

- Weight gain.

If symptoms of hypothyroidism are experienced, it's important to talk to the healthcare provider.

If the T4 test results are irregular, the provider may order more thyroid tests to help make a diagnosis. These tests may potentially include:

- T3 (triiodothyronine) test (another thyroid hormone);

- A TSH (thyroid-stimulating hormone) test;

- Tests to diagnose Graves disease, an autoimmune disease that causes hyperthyroidism;

- Tests to diagnose Hashimoto disease, an autoimmune disease that causes hypothyroidism.

How are TSH (thyroid-stimulating hormone) and T4 (thyroxine) levels related?

TSH and T4 are related, since thyroid-stimulating hormone (TSH) triggers the production of thyroxine (T4) and temporarily elevated levels of T4 prevent the release of TSH. TSH and T4 levels directly affect each other. 

When one gets their blood tests that check the thyroid function and TSH level, different levels of each hormone can indicate different conditions. 

Low TSH and normal T4

Having low TSH levels and normal T4 levels is usually considered subclinical hyperthyroidism. "Subclinical" means the condition doesn't cause any symptoms or symptoms haven't yet started. Studies estimate that subclinical hyperthyroidism affects up to 16% of the population. If the blood test report reveals these results, the healthcare provider will likely continue to monitor the levels to see if they change and result in clinical hyperthyroidism.

Low TSH and high T4

Having low TSH levels and high T4 levels typically indicate hyperthyroidism. This is because excess T4, due to an issue with the thyroid, is preventing the hypothalamus from releasing TSH.

Low TSH and low T4

Having low TSH and low T4 levels may indicate that one is having an issue with the pituitary gland, such as large pituitary adenoma, that's preventing it from releasing enough TSH to trigger T4 production. This is a less common result combination. 

High TSH normal T4

When a patient is having high TSH levels and normal T4 levels, it's usually considered subclinical hypothyroidism (also called mild thyroid failure). This condition occurs in 3% to 8% of the population.

If the blood test report reveals these results, the healthcare provider will likely continue to monitor the levels to see if they change and result in clinical hypothyroidism.

High TSH and low T4

Having high TSH and low T4 levels usually indicates hypothyroidism, an underactive thyroid due to a primary thyroid problem. This is a lack of T4 production due to an issue with the thyroid that's causing the pituitary gland to release excess TSH to try to stimulate the thyroid into making more T4.

High TSH and high T4

Having high TSH and high T4 levels may indicate that the patient has an issue with the pituitary gland that's causing it to release too much TSH, and thus triggering the thyroid to make excess T4. This is an extremely rare result combination.

Patients should know that thyroid conditions are fairly common, especially hypothyroidism, and are treatable. The healthcare provider will let the patient know if there is a need to undergo further tests to determine the cause of the abnormal T4 level. Patients should ask more questions if there are, to their healthcare provider.

What a T4 test is used for? Summary

A T4 is usually used with other thyroid tests to help diagnose and monitor thyroid disease and to gather more information about other conditions that may affect the thyroid. It may be used to:

- Diagnose: 

                - Hypothyroidism;

                - Hyperthyroidism.

- Help learn more about:

                     - Other thyroid conditions, such as if a patient has thyroid nodules (growth on the thyroid that aren't cancer) or a goiter (an enlarged thyroid that may make the neck look swollen). Sometimes these conditions can cause high T4 levels;

              - Disorders of the pituitary gland; abnormal T4 levels are usually caused by thyroid problems, but sometimes they are a sign of a pituitary problem that causes too much or too little TSH;

                     - Disorders of the hypothalamus, an area of the brain that controls the pituitary gland and other body functions.

- Check a newborn for congenital hypothyroidism, which is hypothyroidism that is present at birth;

- Check the T4 levels, if the patient is taking thyroid hormone medicine to treat hypothyroidism.

In certain cases, a T4 test may be done as part of a group of thyroid tests called a thyroid panel.

Why does a patient need a thyroxine (T4) test? Summary

A patient may need a T4 test if:

          - the patient had abnormal results on a TSH test;

         - the patient has symptoms of hypothyroidism. Not having enough thyroid hormone slows down the body functions. The symptoms can vary from person to person and may include:

                    - Fatigue

                    - Weight gain

                    - Being very sensitive to cold

                    - Joint and muscle pain

                    - Dry skin

                    - Dry, thinning hair

                    - Heavy or irregular menstrual periods

                    - Fertility problems in women

                    - Slow heart rate

                    - Depression

                    - Constipation

            - the patient has symptoms of hyperthyroidism. Having too much thyroid hormone speeds up the body's functions. The symptoms can vary from person to person and may include:

                    - Weight loss, even though the patient may be eating more than usual

                    - Rapid or irregular heartbeat

                    - Feeling nervous or irritable

                    - Trouble sleeping

                    - Fatigue

                    - Shaky hands, muscle weakness

                    - Sweating or being very sensitive to heat

                    - Frequent bowel movements (pooping a lot) or diarrhea

                    - Goiter

         - a member of the patient's family has had thyroid disease. Thyroid disease tends to run in families;

             - the patient is taking thyroid hormone medicine for hypothyroidism. A T4 test may be used to check how well the treatment is working;

             - the patient has symptoms that could be caused by another thyroid condition or a problem with the pituitary gland.

What do the results mean?

If the patient had a free T4 test, the test results may be reported as "free T4". If the patient had a total T4 test, the results may be reported as "free T4 index (FTI)". FTI is the amount of free T4 in the blood, based on a calculation using the total T4 test results.

To fully understand the results, the healthcare provider will usually need to compare them with the results of other thyroid tests.

If the T4 results are abnormal, the patient may need more testing to find the cause. But abnormal T4 levels don't always mean that the patient has a severe medical condition.

T4 and pregnancy

Thyroid disease can develop during pregnancy, but it's not common. If this happens, the healthcare provider will treat the patient, if necessary. After the woman gives birth, she would usually have the thyroid checked again.

If a patient had thyroid disease, she should tell her healthcare provider if she is pregnant or if she is thinking about becoming pregnant.