Thyroid Function Tests (2023)


The 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.

(Video) Thyroid Hormones and Thyroid Function Tests

Thyroid Function Tests (1)

T4 and T3 circulate almost entirely bound to specific transport proteins. If the levels of these transport proteins changes, there can be changes in how much bound T4 and T3 is measured. This frequently happens during pregnancy and with the use of birth control pills. The “free” T4 or T3 is the hormone that is unbound and able to enter and affect the body tissues.


Blood tests to measure these hormones are readily available and widely used, but not all are useful in all situations. Tests to evaluate thyroid function include the following:

(Video) TSH and Thyroid Function Tests | UCLA Endocrine Center

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 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.

(Video) 2016: Interpreting Abnormal Thyroid Function Tests & Understanding Indications for Treatment

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.

(Video) Thyroid Function Tests- what your results can mean


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.

(Video) Thyroid Function Tests (TFTs) explained | COMPLETE GUIDE in 4 minutes!


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:

(Video) Thyroid function tests (TFTs) and labs explained in under 7 minutes (ish)

  • 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.


What is included in a thyroid function test? ›

The most common thyroid function tests are: Free T4 (the main thyroid hormone in your blood -- a precursor for T3) TSH (the hormone from the pituitary gland that stimulates the thyroid to produce T4) Total T3 (the active form of the hormone -- T4 is converted to T3)

What is the most important test for thyroid function? ›

Assessment of TSH is the single most useful test of thyroid function in the vast majority of patients. Primary care providers should seldom need to order any other biochemical thyroid test. In most cases the TSH will be within the normal range, and no further testing is indicated.

How many thyroid function tests are there? ›

Thyroid function tests are a series of blood tests used to measure how well your thyroid gland is working. Available tests include the T3, T3RU, T4, and TSH. The thyroid is a small gland located in the lower-front part of your neck.

What are early warning signs of thyroid problems? ›

What Are the Early Warning Signs of Thyroid Problems?
  • Feeling tired or lacking energy even after getting enough sleep.
  • Sudden weight loss or weight gain that cannot be explained by changes in diet or exercise.
  • Mood swings, irritability, anxiety, or depression.
  • Sleep disturbances, insomnia, or excessive sleepiness.
Feb 10, 2023

What is the difference between TSH and thyroid function test? ›

The pituitary gland in your brain makes TSH and sends it to your thyroid gland. TSH controls the production of thyroid hormones. TSH is usually tested first. The combination of a TSH test and tests for the thyroid hormones T3 and T4 is known as a thyroid function test (TFT).

What is a full thyroid test called? ›

A TSH test is used to find out how well your thyroid is working. It can tell if you have hyperthyroidism (too much thyroid hormone) or hypothyroidism (too little thyroid hormone) in your blood.

What is the most sensitive thyroid test? ›

TSH measurement is generally regarded as the most sensitive initial laboratory test for screening individuals for thyroid hormone abnormalities. This is due to the fact that small changes in Free T4 levels result in larger changes in TSH values.

What TSH level indicates Hashimoto's disease? ›

This study illustrates that individuals with a TSH above 2.5 should be screened for Hashimoto's disease as well as cardiovascular risk markers.

How many hours fasting is required for thyroid test? ›

Usually,no special precautions including fasting need to be followed before taking a thyroid test. However, your pathologist can guide you better. For example, if you have to undergo some other health tests along with thyroid hormone levels, you may be asked to fast for 8-10 hours.

What is the most accurate test for hypothyroidism? ›

Thyroid-stimulating hormone (TSH) level is the preferred test for initial evaluation of suspected primary hypothyroidism (strength of recommendation [SOR]: expert opinion). If TSH is abnormal, a free thyroxine (T4) level will further narrow the diagnosis.

What is the perfect thyroid test results? ›

A normal Total T4 level in adults ranges from 5.0 to 12.0μg/dL. A normal Total T3 level in adults ranges from 80-220 ng/dL. Free T3 assays are often unreliable and not routinely used to assess thyroid function.

Why don t doctors test T3 and T4? ›

Matter of fact, the American Thyroid Association guidelines say that, “… T3 is rarely helpful in a hypothyroid patient since it's the last test to become abnormal.” TSH and T4 will become abnormal before T3 will, so doctors just don't run it. They don't see a value in it.

How accurate are thyroid function tests? ›

Thyroid blood tests are usually simple and accurate. A few things can affect your results, though. You may have ongoing fluctuations in thyroid hormone levels. This is a sign you need your medication dose changed.

What time of day is TSH highest? ›

Circulating TSH shows a normal circadian rhythm with a peak between 11 pm and 5 am and a nadir between 5 pm and 8 pm.

What food should be avoided in thyroid? ›

Fatty foods – Fats are known to upset your body's ability to absorb thyroid replacement hormones. Fats can also prevent the thyroid's natural ability to produce hormones. Physicians recommend that you cut out all fried foods and limit your intake of fats from butter, mayonnaise and fatty meats.

What does thyroid fatigue feel like? ›

With thyroid fatigue, you may feel like you can't get through a day without a nap. You may sleep more than usual but still feel completely exhausted. You may not even have the energy to exercise. At times, you may fall asleep during the day or very quickly at night.

What do your hands look like if you have hypothyroidism? ›

A swollen fingertip, curved nail, and thickening skin above a nail are often signs of thyroid disease.

How can I improve my thyroid function? ›

  1. Maintain Healthy Iodine Levels.
  2. Include Selenium-Rich Foods in Your Diet.
  3. Increase Your Probiotics.
  4. Cut Back on Your Sugar Intake.
  5. Exercise Regularly.
  6. Manage Your Stress.

Why do doctors only order TSH? ›

For most doctors, TSH is the only diagnostic test for hypothyroidism that they use and the most sensitive marker of peripheral tissue availability of thyroid hormone. If TSH is within a normal range, most doctors will “rule out” thyroid problems as a cause of symptoms.

Which is more important T4 or TSH? ›

If values are outside the range of 0.4 to 4.5 milliunits per liter (mU/L), measuring T3 and T4 should follow. However, TSH is always the best first test because it is more reliable than plasma T3/T4 levels, which tend to fluctuate.

Can I test my own thyroid levels? ›

According to Steven D. Shapiro MD, at-home thyroid tests are very easy to use. “They are sent directly to your house, require an easy finger prick, and are sent back in the mail. Most kit providers can turn results around within just a few days, similar to the wait-time for in-person lab results.”

What are the signs and symptoms of thyroid problems in females? ›

Common symptoms are fatigue, cold intolerance, weight gain, constipation, hair loss, "brain fog," skin dryness, nail changes, and menstrual cycle changes (typically irregular cycling). Symptoms of hyperthyroidism include unintentional weight loss and a rapid or irregular heartbeat.

Is a full thyroid panel necessary? ›

It's also a good idea to ask for a full thyroid panel from the beginning, before any diagnosis is made or medication is used, since it will provide the most accurate information about your thyroid health. In addition, thyroid disorders, including hypo and hyperthyroidism, come in two versions: primary and secondary.

What is the #1 blood test to uncover the root cause of most thyroid disorders? ›

TSH test. Health care professionals usually check the amount of TSH in your blood first. TSH is a hormone made in the pituitary gland that tells the thyroid how much T4 and T3 to make. A high TSH level most often means you have hypothyroidism, or an underactive thyroid.

What is the best first screening test for a functional thyroid disease? ›

Thyroid function tests are used in the initial evaluation of thyroid disease. The recommended first test is the measurement of thyroid stimulating hormone (TSH, or thyrotropin), which is generally followed by a thyroxine (T4) test. In limited cases, triiodothyronine (T3) testing may be useful.

What is a good TSH level for age? ›

Low TSH levels usually mean that you're producing too much thyroid hormone.
Age rangeNormalHigh
18–30 years0.5–4.1 mU/L> 4.1 mU/L
31–50 years0.5–4.1 mU/L> 4.1 mU/L
51–70 years0.5–4.5 mU/L> 4.5 mU/L
71–90 years0.4–5.2 mU/L> 5.2 mU/L

What does a Hashimoto's flare feel like? ›

Symptoms of a Hashimoto's Flare-Up

Constipation. Dry skin. Puffy face. Muscle aches.

How to tell the difference between Hashimoto's and hypothyroidism? ›

The main difference between Hashimoto's and hypothyroidism is the cause of each condition. Hashimoto's disease happens when your immune system attacks your thyroid gland. Hypothyroidism occurs when your thyroid is underactive and not producing enough thyroid hormone. And this can happen for many different reasons.

What test confirms Hashimoto's disease? ›

Anti-thyroid antibodies (ATA) tests, such as the microsomal antibody test (also known as thyroid peroxidase antibody test) and the anti-thyroglobulin antibody test, are commonly used to detect the presence of Hashimoto's thyroiditis.

What medications affect TSH levels? ›

Glucocorticoids, dopamine agonists, somatostatin analogs, and retinoids inhibit TSH secretion. Lithium, tricyclic antidepressants and selective serotonin reuptake inhibitors (SSRIs), antiepileptics, rifampin, metformin, and amiodarone mainly affect directly thyroid function.

What supplements affect thyroid tests? ›

Biotin supplements, for example, can interfere with thyroid function tests by showing falsely low TSH levels. The ATA recommends you stop taking biotin for at least two days before a thyroid test to avoid misleading results. You get sick. “If you become very sick, your TSH can be suppressed,” Davies says.

Can I drink coffee before a thyroid blood test? ›

Even if you drink it black, coffee can interfere with blood test results. That's because it contains caffeine and soluble plant matter, which might skew your test results. Coffee is also a diuretic, which means that it will increase how much you pee. This can have a dehydrating effect.

What is the most sensitive test for primary hypothyroidism? ›

The most common blood test for hypothyroidism is thyroid-stimulating hormone (TSH). TSH is the most sensitive test because it can be elevated even with small decreases in thyroid function. Thyroxine (T4), the main product of the thyroid gland, may also be measured to confirm and assess the degree of hypothyroidism.

What is the first and best indicator of hypothyroidism? ›

The first blood test typically done to diagnose hypothyroidism measures the level of thyroid-stimulating hormone (TSH) in the blood. If it's high, the test is done again, along with a blood test for the thyroid hormone T-4 . If the results show that TSH is high and T-4 is low, then the diagnosis is hypothyroidism.

When should I see an endocrinologist for hypothyroidism? ›

When you are first diagnosed: Some experts recommend, and some patients prefer, seeing an endocrinologist at least once when you're first diagnosed with hypothyroidism to review your condition, your test results and your treatment plan.

What is included in a full thyroid panel? ›

The Thyroid #4 Comprehensive Blood Test Panel includes a Thyroid Profile with TSH, Tri-iodothyronine (T3) Free Serum, Thyroxine (T4) Free Direct Serum, Thyroid Peroxidase (TPO) Antibodies, Thyroid Antithyroglobulin Antibody (TAA) plus Reverse T3, Thyroxine-binding Globulin (TBG), Thyroid-stimulating Immunoglobulin (TSI ...

Can I drink water before thyroid test? ›

Can I drink water before the thyroid test? Yes, you can have water before the thyroid test.

What is a bad number for thyroid? ›

Ultimately, the standard of care is to treat into the goal range for TSH, which is typically between 0.5 and 4.5 or 5. A TSH level of 10 mIU/L or higher is typically indicative of hypothyroidism. A TSH level of 4.5 to 10 mIU/L is considered indicative of subclinical hypothyroidism.

What is the gold standard test for thyroid function? ›

Assessment of TSH is the single most useful test of thyroid function in the vast majority of patients. Primary care providers should seldom need to order any other biochemical thyroid test. In most cases the TSH will be within the normal range, and no further testing is indicated.

Why endocrinologists do not test T3? ›

Lower-than-normal T3 levels may indicate you have hypothyroidism (underactive thyroid). However, healthcare providers don't typically rely on T3 tests to diagnose hypothyroidism because it's usually the last of the thyroid function tests to come back abnormal.

What mimics thyroid problems? ›

Blood Disorders

Disorders of red or white blood cells can mimic thyroid disorders with symptoms like fatigue, weakness, feeling cold, excessive sweating, pale skin, easy bruising, shortness of breath, leg cramps, difficulty concentrating, dizziness and insomnia.

What is the most sensitive thyroid function test? ›

The blood test for TSH, which is the most sensitive marker of your thyroid status, is used as a biochemical marker to ensure that your thyroid hormone replacement is adequate. It is recommended that patients on thyroid hormone replacement should keep their TSH within the reference range.

Is it better to fast for thyroid blood test? ›

Most providers will suggest you do not fast before your thyroid function test. Research shows that fasting, especially early in the morning, may impact TSH levels. A fasting test typically results in higher TSH levels versus one done in the afternoon.

Can lack of sleep cause high TSH? ›

Jauch-Chara et al. indicated that an up-regulation of pituitary-thyroid activity, after short-term total sleep deprivation, led to an increase in TSH levels. Kuetting et al. found that sleep deprivation significantly increases cardiac contractility, blood pressure, and stress hormone secretion.

How quickly can thyroid levels change? ›

Daily circadian rhythms cause TSH to fluctuate by as much as 0.95 mIU/mL to 2.0 mIU/mL throughout the day. TSH levels are highest in the early morning and decrease late afternoon to mid-evening. TSH levels even fluctuate on average 0.75 mIU/mL on a monthly basis in healthy adults.

Is thyroid function test T3 or T4? ›

A T3 test is most often used to diagnose hyperthyroidism, a condition in which the body makes too much thyroid hormone. T3 tests are frequently ordered with T4 and TSH (thyroid stimulating hormone) tests. A T3 test may also be used to monitor treatment for thyroid disease.

What should I avoid before a thyroid test? ›

Thyroid scan

Your health care professional may ask you to avoid foods high in iodine, such as kelp, or medicines containing iodine for a week before the test. For the scan, a technician injects a small amount of radioactive iodine or a similar substance into your vein.

What is the best time of day to have a thyroid test? ›

TSH levels measured in blood samples drawn in the afternoon should be viewed with caution, as they may be falsely low and appear normal. Samples for all thyroid tests should be collected first thing in the morning.

What happens if I eat before a thyroid test? ›

Is fasting required for a thyroid test? Most providers will suggest you do not fast before your thyroid function test. Research shows that fasting, especially early in the morning, may impact TSH levels. A fasting test typically results in higher TSH levels versus one done in the afternoon.

Can a thyroid blood test detect anything else? ›

These tests alone aren't meant to diagnose any illness but may prompt your healthcare provider to do additional testing to evaluate for a possible thyroid disorder. Additional blood tests might include: Thyroid antibodies: These tests help identify different types of autoimmune thyroid conditions.


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