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Thyroid Function Tests - Determining Thyroid Function Through Thyroid Hormones

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Blood work for thyroid is known by the name thyroid function test. Read the article and get an insight into thyroid function tests.

Medically reviewed by

Dr. Kaushal Bhavsar

Published At October 21, 2022
Reviewed AtOctober 21, 2022

What Is the Thyroid Gland?

An endocrine gland (glands that secrete hormones) located in the front of the neck, on the lower part, is called the thyroid gland. It is a butterfly-shaped gland that secretes thyroid hormones. The secreted thyroid hormones enter the bloodstream, from where it reaches every body tissue. These thyroid hormones are needed to maintain average body temperature and the normal functioning of the heart, muscles, brain, and other organs.

What Are the Hormones Secreted by the Thyroid Gland?

Utilizing the Iodine that is taken through food, the thyroid gland secretes two hormones, namely,

  • Triiodothyronine (T3).

  • Thyroxine (T4).

Thyroxine contains four Iodine atoms, of which one is removed in the liver and brain to become Triiodothyronine.

What Is the Mechanism of the Release of Thyroid Hormones?

The thyroid gland secretes not only these hormones but also stores and releases them when the need arises. The need to release thyroid hormone is determined by the hypothalamus (a small but significant portion of the brain) and pituitary gland (an endocrine gland located in the skull base and is the size of a pea). The hypothalamus releases Thyrotropin-releasing hormone (TRH), and the pituitary gland releases Thyroid-stimulating hormone (TSH).

When the thyroid hormone level is low, the hypothalamus senses it. It releases TRH, which stimulates the pituitary gland to release TSH, which, in turn, stimulates the thyroid gland to release more thyroid hormone. Similarly, when the levels of thyroid hormones are low, the TRH and TSH production is reduced, thus decreasing the release of the thyroid hormone.

What Is the Function of Thyroid Hormones?

The following are the functions of the thyroid gland:

  • It controls muscle contraction.

  • It regulates the basal metabolism rate (the rate at which calories are burnt), thus having an impact on weight gain or loss.

  • It regulates the rate at which the dead cells are replaced.

  • It controls the heart rate.

  • It regulates the rate of food movement through the digestive tract.

What Are Thyroid Function Tests?

The thyroid function tests are carried out by taking blood from the arm veins. This blood is evaluated for the amount of thyroid hormones present. With the help of thyroid hormone levels, the functioning of the thyroid gland is determined.

What Conditions Are Diagnosed With Thyroid Function Tests?

In general, thyroid function tests help diagnose hyperthyroidism (increased level of thyroid hormone in the blood) and hypothyroidism (decreased level of thyroid hormone in the blood). In addition, it also helps in diagnosing other thyroid dysfunctions that are associated with hyperthyroidism and hypothyroidism.

These include:

What Is the Effect of Hyperthyroidism?

Hyperthyroidism is characterized by increased metabolism, weight loss, insomnia, increased heart rate, puffiness around the eyes, anxiety, etc. When there is an increased production of thyroid hormones than the needed amount, hyperthyroidism occurs. An important cause of hyperthyroidism is Grave's disease.

What Is the Effect of Hypothyroidism?

If the thyroid hormone production is reduced than the required amount, it is called hypothyroidism. Hashimoto's disease is the most common cause of hypothyroidism. Fatigue, weight gain, skin dryness, puffiness, menstrual abnormality, etc., are the associated symptoms.

What Tests Are Included in Thyroid Function Tests?

1. T4-

The level of T4 or Thyroxine present in the blood helps diagnose hypothyroidism and hyperthyroidism. When the level of thyroxine is decreased in the blood, it denotes hypothyroidism, and an increased level indicates hyperthyroidism. For an adult, the average level of thyroxine in the blood is 5.0 – 11.0 ug/dL.

2. T3-

Decreased levels of T3 are manifested in people with hypothyroidism however, Triiodothyronine level is mainly used in the diagnosis of hyperthyroidism. Also, it helps in determining the severity of the condition. The average level of T3 in the blood is 100 - 200 ng/dL.

3. TSH-

A low level of TSH and T4 or T3 indicates a problem with the pituitary gland. As mentioned earlier, the T4 and T3 hormones level is determined by the thyroid-stimulating hormone released by the pituitary gland. A decreased amount of TSH release accompanies an increased thyroid hormone level in the blood. In the same way, a reduced level of thyroid hormone in the blood causes the pituitary gland to release more TSH, thus increasing its level in blood. Therefore, raised TSH values denote hypothyroidism, while decreased TSH values indicate hyperthyroidism. 0.40 - 4.50 mIU/mL is the average level of TSH for a healthy adult.


4. Free T3 and T4-

Some proteins usually bind with T3 and T4. As a result, the accurate measurement of the level of T3 and T4 may be difficult. In the case of free T3 or T4 measurement, the proteins are eliminated to give accurate results. Therefore, it is the most preferred test by many physicians to diagnose thyroid dysfunction. An adult's average free T3 level is 2.3 - 4.1 pg/ml, and the normal free T4 level is 0.9 - 1.7 ng/dL.

5. Calcitonin-

It is also a hormone produced by the parafollicular cells of the thyroid gland, and the normal value is any value less than 10 pg/mL. Calcitonin is used to diagnose rare thyroid disorders like medullary thyroid cancer and C-cell hyperplasia.

6. Thyroid Antibodies-

A thyroid antibody test is indicated when there is an abnormality in the level of thyroid hormones. The antibodies indicate a presence of an autoimmune thyroid disorder. Each type of antibody indicates a particular thyroid dysfunction.

There are several thyroid antibodies, which include the following:

  • Thyroglobulin antibodies (Tg Ab) Hashimoto's thyroiditis.

  • Thyroid peroxidase antibodies (TPOAb) Hashimoto's thyroiditis or rarely in Grave's disease

  • Thyroid-stimulating hormone receptor antibodies (TSHR Ab, also known as TRAb) Grave's disease.

7. Thyroglobulin-

It is a protein synthesized by the thyroid glands used in the production of T3 and T4 hormones. Thyroglobulin helps diagnose thyroiditis and monitor thyroid cancer.

What Medications Should Be Taken With Caution Before Taking Thyroid Function Tests?

  • Oral Contraceptive Pills-

When taking oral contraceptive pills, increased T3 and T4 levels are seen. This is because proteins that bind with the T3 and T4 hormones are raised due to estrogen present in the birth control pills. Therefore, it is always advisable to check free T3 or free T4 levels in such cases.

  • Biotin-

An over-the-counter supplement present in many hair products impacts thyroid hormone levels. It gives a false higher value despite having an average level in blood. Therefore, it is recommended to stop Biotin at least two days before taking thyroid function tests.

What Other Tests Help in Determining Thyroid Function?

The radioiodine uptake test helps in evaluating the function of the thyroid gland. In this test, radioactive Iodine is injected and is tracked by a physician. The amount of radioactive Iodine that has been absorbed is determined, which helps ascertain the function of the thyroid gland.


The thyroid function test denotes the group of blood tests taken to assess the function of the thyroid gland. Although it is easy to identify if there is decreased or increased level of thyroid hormone in the blood with the help of the results, treatment should not be started on your own without the advice of a healthcare professional. Because only a professional can help you identify the cause and provide you with the best treatment.

Frequently Asked Questions


Which Thyroid Test Is the Best?

The most reliable thyroid tests are as follows:
- TSH levels.
- Free T3.
- Total T4.
- Total T3.
- Anti-thyroglobulin and anti-TPO antibodies (Hashimoto’s thyroid antibodies).
- Reverse T3.
- Free T4.
- Thyroid-binding globulin (TBG levels).


Which Test Identifies Hashimoto’s?

The following test is used to identify Hashimoto’s Thyroiditis:
- Thyroglobulin Antibodies.
- C reactive protein (CRP).
- Anti-TPO Antibodies.
- Erythrocyte sedimentation rate (ESR).


What is a Complete Thyroid Panel Called?

One blood sample is used for the thyroid panel, which tests for several variables related to thyroid function. It specifically calculates the body's levels of thyroid hormones and thyroid-stimulating hormones. A typical thyroid panel contains the following three hormones:
- TSH (thyroid-stimulating hormone) 
- Free T4 (thyroxine). 
- Total T3 or the free T3 (triiodothyronine).


How Long Must You Fast Before a Thyroid Test?

There is no need to fast before a thyroid test, and patients can eat whatever they want. But fasting may be required if further tests need to be performed on the same sample; for instance, fasting is necessary for lipid profiles, fasting blood sugar, etc.


What Thyroid Test Is the Most Accurate?

The most accurate thyroid test are as follows:
- Free triiodothyronine (FT3).
- Thyroid-stimulating hormone (TSH).
- Free thyroxine (FT4).


How Reliable Are Thyroid Exams?

Today, the accuracy of every medical test is solely dependent on the accuracy of the reference range. The normal reference range presents a significant challenge regarding the thyroid-stimulating hormone (TSH) Test, the current gold standard in thyroid diagnostics. Most laboratories define normal test results as those that fall between 0.5 and 5.0 mIU/L. Five percent of the population was expected to have hypothyroidism when this normal reference range was created. Therefore, this upper reference range was developed based on population studies for TSH, which revealed that 5 % of individuals had a TSH greater than 5.0 mIU/L.


How Can Hypothyroidism and Hashimoto’s Be Distinguished From One Another?

While immune system issues occasionally bring on hypothyroidism, Hashimoto's illness is always brought on by a problem with the immune system. When Hashimoto's disease is present, thyroid peroxidase and anti-thyroglobulin antibodies are invariably high.


What Activities Must You Avoid Before a Thyroid Test?

Hashimoto's disease sometimes precedes hypothyroidism, but it is possible to have Hashimoto's disease independently of hypothyroidism. Hypothyroidism can be identified even if a test for Hashimoto's thyroiditis is negative. Hashimoto's illness can induce hypothyroidism, although it is not the only cause.


What Could Influence Thyroid Test Outcomes?

Factors like stress, dieting, lack of sleep, and the timing of the menstrual cycle may affect the results of a thyroid test. The easiest method to prevent erroneous lab test results variations is to monitor thyroid levels consistently.


When Does TSH (Thyroid-Stimulating Hormone) Drop the Most?

TSH levels vary and are found to be highest in the middle of the night. According to recent studies, TSH serum levels reach their highest point between two and four in the morning and their lowest point between four and eight in the evening.


What Drugs Have an Impact on TSH Levels?

In the absence of clinical signs of thyroid malfunction, commonly administered medications may cause incorrect thyroid function tests:
- Anti-convulsants.
- Heparin.
- Beta-adrenoceptor antagonists.
- Non-steroidal anti-inflammatory medicines.
- Steroid hormones.


What Foods Cause TSH (Thyroid-Stimulating Hormone) Levels to Rise?

The following foods can either raise the thyroid activity; as a result, TSH may fluctuate:
- Tyrosine.
- Iodine-containing vitamins.
- Kelp.
- Bladderwrack supplements.
- Tofu.
- Soy milk.
- Peaches.
- Pears and strawberries.
- Coffee.
- Green tea.
- Alcohol.


How Fast May TSH(Thyroid-Stimulating Hormone) Levels Fluctuate?

Blood tests are used to measure thyroid levels like TSH. A second test is typically performed two or three months later because a single test has the potential to be deceiving. As TSH levels might vary daily, blood is drawn simultaneously for both tests.


Is Hypothyroidism Reversible?

Hypothyroidism very seldom goes away" on its own. While under normal conditions, it typically either burns out or is given to stop the thyroid from pumping out all the hormones, it changes for some people during pregnancy and then adjusts again.


Can Weight Gain Result From Low TSH(Thyroid-Stimulating Hormone)?

The active thyroid hormones the doctor might prescribe can enhance the body's metabolism and lower TSH levels with a gain in body weight. In addition, TSH levels can be decreased without medication by reducing body fat. Exercise and a healthy diet are effective methods for losing weight and fat.
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Dr. Kaushal Bhavsar
Dr. Kaushal Bhavsar

Pulmonology (Asthma Doctors)


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