Thyroid Function (TFT)

Thyroid gland is a butterfly-shaped endocrine gland that is normally located in the lower front of the neck. The thyroid’s job is to make thyroid hormones, which are secreted into the blood and then carried to every tissue in the body. Thyroid hormones help the body use energy, stay warm and keep the brain, heart, muscles, and other organs working as they should.


The major thyroid hormone secreted by the thyroid gland is thyroxine, also called T4 because it contains four iodine atoms. To exert its effects, T4 is converted to triiodothyronine (T3) by the removal of an iodine atom. This occurs mainly in the liver and in certain tissues where T3 acts, such as in the brain. The amount of T4 produced by the thyroid gland is controlled by another hormone, which is made in the pituitary gland located at the base of the brain, called thyroid stimulating hormone (abbreviated TSH). The amount of TSH that the pituitary sends into the bloodstream depends on the amount of T4 that the pituitary sees. If the pituitary sees very little T4, then it produces more TSH to tell the thyroid gland to produce more T4. Once the T4 in the bloodstream goes above a certain level, the pituitary’s production of TSH is shut off. In fact, the thyroid and pituitary act in many ways like a heater and a thermostat. When the heater is off and it becomes cold, the thermostat reads the temperature and turns on the heater. When the heat rises to an appropriate level, the thermostat senses this and turns off the heater. Thus, the thyroid and the pituitary, like a heater and thermostat, turn on and off. This is illustrated in the figure below.

Drawing blood for thyroid function tests

Before you get a blood draw to check your thyroid levels, talk with your doctor about any medications you’re taking. Also let them know if you’re pregnant. Certain medications and being pregnant may influence your test results.

A blood draw, also known as venipuncture, is a procedure performed at a lab or a doctor’s office. When you arrive for the test, you’ll be asked to sit in a comfortable chair or lie down on a cot or gurney. If you’re wearing long sleeves, you’ll be asked to roll up one sleeve or to remove your arm from the sleeve.

A healthcare professional, like a technician or nurse, will tie a band of rubber tightly around your upper arm to make the veins swell with blood. Once the healthcare professional has found an appropriate vein, they’ll insert a needle under the skin and into the vein.

You may feel a sharp prick when the needle punctures your skin. The healthcare professional will collect your blood in test tubes and send it to a laboratory for analysis.

When the healthcare professional has gathered the amount of blood needed for the tests, they’ll withdraw the needle and place pressure on the puncture wound until the bleeding stops. They will then place a small bandage over the wound.

You should be able to return to your typical daily activities immediately.

Side effects and aftercare


A blood draw is a routine, minimally invasive procedure and doesn’t have many side effects.

During the days immediately after the blood draw, you may notice slight bruising or soreness at the area where the needle was inserted. Placing an ice pack on the affected site or taking an over-the-counter pain reliever can help ease your discomfort.

If you experience a great deal of pain, or if the area around the puncture becomes red and swollen, follow up with your doctor immediately. These could be signs of an infection.


The best way to initially test thyroid function is to measure the TSH level in a blood sample. Changes in TSH can serve as an “early warning system” – often occurring before the actual level of thyroid hormones in the body becomes too high or too low. A high TSH level indicates that the thyroid gland is not making enough thyroid hormone (primary hypothyroidism). The opposite situation, in which the TSH level is low, usually indicates that the thyroid is producing too much thyroid hormone (hyperthyroidism). Occasionally, a low TSH may result from an abnormality in the pituitary gland, which prevents it from making enough TSH to stimulate the thyroid (secondary hypothyroidism). In most healthy individuals, a normal TSH value means that the thyroid is functioning properly.


T4 is the main form of thyroid hormone circulating in the blood. A Total T4 measures the bound and free hormone and can change when binding proteins differ (see above). A Free T4 measures what is not bound and able to enter and affect the body tissues. Tests measuring free T4 – either a free T4 (FT4) or free T4 index (FTI) – more accurately reflect how the thyroid gland is functioning when checked with a TSH.

The T4 test is known as the thyroxine test. A high level of T4 indicates an overactive thyroid (hyperthyroidism). Symptoms include:

  • anxiety
  • unplanned weight loss
  • tremors
  • diarrhea

The finding of an elevated TSH and low FT4 or FTI indicates primary hypothyroidism due to disease in the thyroid gland. A low TSH and low FT4 or FTI indicates hypothyroidism due to a problem involving the pituitary gland. A low TSH with an elevated FT4 or FTI is found in individuals who have hyperthyroidism.

If you show signs of hypothyroidism and have a TSH reading above 4.5 mIU/L, you’re at risk of progressing to hypothyroidism. Symptoms can include:

  • weight gain
  • fatigue
  • depression
  • brittle hair and fingernails

Your doctor may order a T4 test if a thyroid-stimulating hormone (TSH) test has come back with abnormal results. A T4 test will help your doctor determine what type of problem is affecting your thyroid.

Some disorders that affect thyroid function include:

  • hyperthyroidism, or overactive thyroid
  • hypothyroidism, or underactive thyroid
  • hypopituitarism, or underactive pituitary gland

Your doctor may suspect that one of these conditions is present if you have symptoms such as:

  • eye issues, like dryness, irritation, puffiness, and bulging
  • skin dryness or skin puffiness
  • hair loss
  • hand tremors
  • changes in heart rate
  • changes in blood pressure

You may also experience more general symptoms, such as:

  • weight changes
  • difficulty sleeping or insomnia
  • anxiety
  • fatigue and weakness
  • intolerance to cold
  • sensitivity to light
  • menstrual irregularity

Sometimes, your doctor may also order other thyroid tests (like T3 or TSH) after you have a T4 test.

TSH, or thyroid-stimulating hormone, comes from the pituitary gland. It stimulates your thyroid to release both T3 and T4. Performing one or both of these other tests may help your doctor gain a better understanding of your thyroid problem.

In some cases, your doctor may perform one or more of these tests to help assess whether a known thyroid issue is improving.

How do you prepare for a T4 test?


A number of drugs can interfere with your T4 levels, especially total T4, so it’s important to tell your doctor what medications you’re taking. You may need to temporarily stop taking certain drugs before the test to help ensure accurate results.

It’s also important to let your doctor know if you may be pregnant.

Medications that can affect your T4 levels include:

  • drugs that contain hormones, such as androgen, estrogen, and birth control pills
  • drugs designed to affect your thyroid or treat thyroid conditions
  • some drugs designed to treat cancer
  • steroids

These aren’t the only drugs that can affect your results. Make sure to tell your doctor about all medications, as well as any herbal supplements, you use.

What’s the procedure for a T4 test?

A healthcare provider will collect your blood into a tube or vial and send it to a lab for testing.

Typical results for the total T4 test in adults generally range from 5.0 to 12.0 micrograms per deciliter (μg/dL). Results for children vary based on age. Talk to your doctor about the normal ranges expected for your child. There may also be some variation between labs.

Typical results in adults for the free T4 test generally range from 0.8 to 1.8 nanograms per deciliter (ng/dL). Like total T4 in adults, free T4 also varies in children according to age.

As with any test result, if it falls outside the expected range, talk to your doctor about what the results may mean in regards to your own personal health condition.

Because T4 isn’t the only hormone involved in thyroid function, a normal result on this test could still mean there’s a thyroid problem.

For example, your T4 results could fall into a normal range, but your T3 results could be abnormal. This can be particularly true in cases of an overactive thyroid.

You can also order an at-home thyroid test to measure T4 levels.

What do abnormal T4 test results mean?

An abnormal result on the T4 test alone may not give your doctor enough information to fully understand or diagnose your condition. They may also need to consider the results of T3 and TSH levels for a more complete picture.

Pregnancy can also affect your T4 levels. If your T4 levels are abnormal but you’re pregnant, your doctor may order further testing.

Abnormally high test results

Elevated T4 levels may indicate hyperthyroidism. They may also indicate other thyroid problems, such as thyroiditis or toxic multinodular goiter.

Other reasons for abnormal results may include:

  • high levels of protein in the blood
  • too much iodine
  • too much thyroid replacement medication
  • trophoblastic disease, a group of rare pregnancy related tumors
  • germ cell tumors

Too much iodine can raise your T4 levels. Because X-ray dyes can include iodine, a recent X-ray involving dye may also raise your T4 test results.

Abnormally low test results

Abnormally low levels of T4 may indicate:

  • dietary issues, such as fasting, malnutrition, or an iodine deficiency
  • medications that affect protein levels
  • hypothyroidism
  • illness
  • a pituitary problem

Are there any risks associated with a T4 test?

A T4 test has no specific risks. Risks include those present whenever you have your blood drawn.

In rare cases, you may experience a complication, such as:

  • an inflamed vein
  • an infection
  • excessive bleeding

More commonly, you may feel pain or discomfort during the blood draw. You may also bleed slightly after the needle is removed. You may develop a small bruise around the puncture site.


T3 tests are often useful to diagnosis hyperthyroidism or to determine the severity of the hyperthyroidism. Patients who are hyperthyroid will have an elevated T3 level. In some individuals with a low TSH, only the T3 is elevated and the FT4 or FTI is normal. T3 testing rarely is helpful in the hypothyroid patient, since it is the last test to become abnormal. Patients can be severely hypothyroid with a high TSH and low FT4 or FTI, but have a normal T3.

Measurement of free T3 is possible, but is often not reliable and therefore not typically helpful.

Reverse T3 is a biologically inactive protein that is structurally very similar to T3, but the iodine atoms are placed in different locations, which makes it inactive. Some reverse T3 is produced normally in the body, but is then rapidly degraded. In healthy, non-hospitalized people, measurement of reverse T3 does not help determine whether hypothyroidism exists or not, and is not clinically useful.



If your blood work suggests your thyroid gland is overactive, your doctor may order a thyroid uptake test or an ultrasound.

If scan results are normal, your doctor will likely prescribe medication to regulate your thyroid activity. They will follow up with additional thyroid function tests to make sure the medication is working. These tests will check for:

  • structural problems with the thyroid gland
  • thyroid gland activity
  • any tumors that may be causing problems

Your doctor may also order an ultrasound if they discover abnormal findings during a physical exam of your neck. If ultrasound results are abnormal, your doctor may order a tissue sample of the thyroid.

Thyroid cancer is not related Trusted Source to hyperthyroid or hypothyroid. Keep in mind that blood tests are not used to find thyroid cancer.

Why doctors perform T3 tests

Your doctor will typically order a T3 test if they suspect a problem with your thyroid.

Potential thyroid disorders include:

  • hyperthyroidism: when your thyroid produces too much thyroid hormone
  • hypopituitarism: when your pituitary gland doesn’t produce normal amounts of pituitary hormones
  • primary or secondary hypothyroidism: when your thyroid doesn’t produce normal amounts of thyroid hormones
  • thyrotoxic periodic paralysis: when your thyroid produces high levels of thyroid hormones, resulting in muscle weakness

A thyroid disorder can cause a wide range of symptoms. For example, you might have mental issues such as anxiety, or physical problems such as constipation and menstrual irregularity.

Other possible symptoms include:

  • weakness and fatigue
  • difficulty sleeping
  • increased sensitivity to heat or cold
  • weight loss or gain
  • dry or puffy skin
  • dry, irritated, puffy, or bulging eyes
  • hair loss
  • hand tremors
  • increased heart rate

If you already have confirmation of a thyroid problem, your doctor might use a T3 test to see whether there have been any changes in your condition.

Sometimes, your doctor might also order a T4 test or a TSH test. TSH, or thyroid-stimulating hormone, is the hormone that stimulates your thyroid to produce T3 and T4. Testing the levels of either or both of these other hormones can help give your doctor a more complete picture of what’s going on.

Preparing for a T3 test

It’s important to tell your doctor about all of the medications you’re currently taking, as some may affect your T3 test results. If your doctor knows about your medications in advance, they can advise you to temporarily stop using them or consider their effect when interpreting your results.

Some medications that can affect your T3 levels include:

  • thyroid-related drugs
  • steroids
  • birth control pills or other medications containing hormones, such as androgens and estrogens

Procedure for a T3 test

The T3 test simply involves having your blood drawn. The blood will then be tested in a laboratory.

Typically, normal results range from 100 to 200 nanograms per deciliter (ng/dL).

A normal T3 test result doesn’t necessarily mean that your thyroid is functioning perfectly. Measuring your T4 and TSH can help your doctor figure out if you have a thyroid problem despite a normal T3 result.

What do abnormal T3 test results mean?

Because the thyroid’s functions are complicated, this single test may not give your doctor any definitive answers about what is wrong. However, abnormal results can help point them in the right direction. Your doctor may also choose to perform a T4 or TSH test to gain a clearer picture of your thyroid function.

Abnormally high levels of T3 are common in pregnant women and those with liver disease. If your T3 test also measured the free T3 level, your doctor may be able to rule out these conditions.

High T3 levels

If you’re not pregnant or suffering from liver disease, elevated T3 levels might indicate thyroid issues, such as:

  • Graves’ disease
  • hyperthyroidism
  • painless (silent) thyroiditis
  • thyrotoxic periodic paralysis
  • toxic nodular goiter

High T3 levels might also indicate high levels of protein in the blood. In rare cases, these elevated levels could indicate thyroid cancer or thyrotoxicosis.

Low T3 levels

Abnormally low levels of T3 may indicate hypothyroidism or starvation. It could also indicate that you have a long-term illness since T3 levels decrease when you’re sick. If you’re sick enough to be hospitalized, your T3 levels are likely to be low.

This is one reason that doctors don’t routinely use only the T3 test as a thyroid test. Instead, they often use it along with the T4 and TSH test to get a more complete picture of how your thyroid is working.

Risks of the T3 test

When you have your blood drawn, you can expect to have a bit of discomfort during the procedure. You may also have minor bleeding or bruising afterward. In some cases, you may feel light-headed.

Serious symptoms, though rare, can include fainting, infection, excessive bleeding, and inflammation of the vein.


The immune system of the body normally protects us from foreign invaders such as bacteria and viruses by destroying these invaders with substances called antibodies produced by blood cells known as lymphocytes. In many patients with hypothyroidism or hyperthyroidism, lymphocytes react against the thyroid (thyroid autoimmunity) and make antibodies against thyroid cell proteins. Two common antibodies are thyroid peroxidase antibody and thyroglobulin antibody. Measuring levels of thyroid antibodies may help diagnose the cause of the thyroid problem. For example, positive anti-thyroid peroxidase and/or anti-thyroglobulin antibodies in a patient with hypothyroidism result in a diagnosis of Hashimoto’s thyroiditis. While detecting antibodies is helpful in the initial diagnosis of hypothyroidism due to autoimmune thyroiditis, following their levels over time is not helpful in detecting the development of hypothyroidism or response to therapy. TSH and FT4 are what tell us about the actual thyroid function or levels.

A different antibody that may be positive in a patient with hyperthyroidism is the stimulatory TSH receptor antibody (TSI). This antibody causes the thyroid to be overactive in Graves’ Disease. If you have Graves’ disease, your doctor might also order a thyrotropin receptor antibody test (TSHR or TRAb), which detects both stimulating and blocking antibodies. Following antibody levels in Graves’ patients may help to assess response to treatment of hyperthyroidism, to determine when it is appropriate to discontinue antithyroid medication, and to assess the risk of passing antibodies to the fetus during pregnancy.

Thyroglobulin (Tg) is a protein produced by normal thyroid cells and thyroid cancer cells. It is not a measure of thyroid function and it does not diagnose thyroid cancer when the thyroid gland is still present. It is used most often in patients who have had surgery for thyroid cancer in order to monitor them after treatment. Tg is included in this brochure of thyroid function tests to communicate that, although measured frequently in certain scenarios and individuals, Tg is not a primary measure of thyroid hormone function.


Because T4 contains iodine, the thyroid gland must pull a large amount of iodine from the bloodstream in order to make an appropriate amount of T4. The thyroid has developed a very active mechanism for doing this. Therefore, this activity can be measured by having an individual swallow a small amount of iodine, which is radioactive. The radioactivity allows the doctor to track where the iodine goes. By measuring the amount of radioactivity that is taken up by the thyroid gland (radioactive iodine uptake, RAIU), doctors may determine whether the gland is functioning normally. A very high RAIU is seen in individuals whose thyroid gland is overactive (hyperthyroidism), while a low RAIU is seen when the thyroid gland is underactive (hypothyroidism). In addition to the radioactive iodine uptake, a thyroid scan may be obtained, which shows a picture of the thyroid gland and reveals what parts of the thyroid have taken up the iodine (see Thyroid Nodules brochure).


There are many medications that can affect thyroid function testing. Some common examples include:

  • Estrogens, such as in birth control pills, or in pregnancy, cause high levels of total T4 and T3. This is because estrogens increase the level of the binding proteins. In these situations, it is better to ask both for TSH and free T4 for thyroid evaluation, which will typically be in the normal range.
  • Biotin, a commonly taken over-the-counter supplement, can cause the measurement of several thyroid function tests to appear abnormal, when they are in fact normal in the blood. Biotin should not be taken for 2 days before blood is drawn for thyroid function testing to avoid this effect

By Mehfooz Ali

Explore the fascinating journey of Mehfooz Ali, a renowned website developer diving into the world of blogging. Discover insights, tips, and inspirations for your blogging endeavors. Click now for an enriching experience.

28 thoughts on “Thyroid Function Tests”
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