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Understanding Blood Tests for Hypothyroidism

Learn about the key thyroid tests and what they mean for your thyroid health!
Understanding Blood Tests for Hypothyroidism
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Thyroid tests are a common diagnostic tool used to evaluate your thyroid function and diagnose thyroid disorders, including hypothyroidism. Understanding what these tests are measuring, and how to interpret your thyroid hormone levels can be confusing and challenging for both patients and healthcare providers. Ahead, a look at the main thyroid tests involved in diagnosing and treating hypothyroidism, and how to interpret the results.

An overview of how the thyroid works

Your thyroid and pituitary glands work together, much like a heater and a thermostat. Your pituitary gland acts as the thermostat, sensing your levels of thyroid hormones – specifically thyroxine (T4) and triiodothyronine (T3) -- in the bloodstream. Your thyroid gland acts as the heater, producing thyroid hormones.

When your circulating levels of thyroid hormone drop, the pituitary gland releases thyroid stimulating hormone – known as TSH. The job of TSH is to prompt the thyroid gland to produce more thyroid hormones. As your thyroid hormone levels rise and return to normal, your pituitary gland senses this and reduces the production of TSH. The reduced production of TSH in turn reduces the production of thyroid hormones. When it’s functioning properly, the feedback loop helps your body maintain the appropriate levels of thyroid hormones in the body.

Of the two major hormones produced by the thyroid gland, T4 is the most abundant, accounting for about 80% of the total hormone produced. T3 accounts for about 20%.

In order to be usable by the body, T4 – considered a storage hormone – needs to lose an iodine atom and be converted into T3 in various tissues throughout the body, including the liver, kidneys, and brain. T3 is the active thyroid hormone and is responsible for most of the biological effects of thyroid hormones.

Under normal conditions, the body converts T4 to T3 in certain ratios. Conditions such as genetic mutations, stress, illness, nutrient deficiencies, or extreme dieting and calorie restriction can affect the T4-to-T3 conversion process in two significant ways:

  1. The ability to convert T4 into T3 can be impaired
  2. T4 may preferentially convert into an inactive hormone called Reverse T3 (RT3).

RT3 is a metabolically and biologically inactive form of T3. It contains the same number of iodine molecules as T3, but they’re attached to different locations, making it inactive. Reverse T3 is created by the body as a way to manage and control how much thyroid hormone the body can use at any given time.

When you’re hypothyroid, your thyroid is unable to produce enough T4 and/or T3 thyroid hormones.

When you have impaired T4-to-T3 conversion – or you’re producing higher than normal amounts of RT3 – you can also end up hypothyroid due to a deficiency in the active thyroid hormone.

The reference range vs. the optimal range

Every laboratory thyroid blood test used for the diagnosis of hypothyroidism – as well as hypothyroidism treatment – has what’s known as a reference range. It’s sometimes colloquially called the “normal range.” A laboratory reference range is a set of values that includes upper and lower “normal” limits of a lab test, based on a group of otherwise healthy people. Reference ranges are used to interpret lab results. It is important to note that reference ranges can vary between different laboratories

Many thyroid practitioners, including many of Paloma’s thyroid experts, consider the reference ranges for thyroid tests as broad working guidelines. When treating patients with Hashimoto’s and hypothyroidism, they frequently target a narrower therapeutic reference range, in order to achieve the best restoration of thyroid function and resolution of symptoms. This narrower range is referred to as the “optimal range.” You can read more in this Paloma article about optimal levels.

Thyroid labs and what they mean

Thyroid Stimulating Hormone (TSH) Test

The thyroid stimulating hormone (TSH) test is a blood test used to evaluate how well the thyroid gland is working. TSH is not a thyroid hormone. Instead, it’s a messenger hormone produced by the pituitary gland that prompts the thyroid gland to produce T4 and T3. The TSH test measures the amount of TSH in the blood and is used to diagnose thyroid disorders, such as hyperthyroidism (too much thyroid hormone) or hypothyroidism (too little thyroid hormone). The test is often used in conjunction with other thyroid function tests, such as T4 and T3 tests, to evaluate thyroid health

The reference range for the TSH test ranges from around 0.45 to .5 milliunits per liter (mIU/L) on the low end, to 4.5 to 5.5 mIU/L on the high end, for healthy adults who are not pregnant. Levels above the reference range are considered indicative of hypothyroidism.

Many practitioners consider an optimal TSH level to be between .45 and 2.0 mIU/L.

Total Thyroxine (Total T4) Test

Total thyroxine (Total T4) is a test that measures the entire amount of thyroxine in the blood, including the amount attached to proteins that help transport the hormone through the bloodstream. It’s important to note that thyroxine that is attached to proteins is not available to convert into T3.

In general, the reference range for the total T4 test in adults runs from 5.0 to 12.0 micrograms per deciliter (μg/dL) of blood. Low-normal levels, or levels below the reference range, can be indicative of hypothyroidism.

Free Thyroxine (Free T4) Test

The Free Thyroxine (Free T4) test measures the amount of thyroxine that is not bound to proteins in the blood and is able to enter and affect body tissues.

The Free T4 test is considered more accurate than a total T4 test, as it measures the available thyroxine that can be used by the body and converted into T3.

The reference range for Free T4 is typically from .8 to 0.9 nanograms per deciliter (ng/dL) on the low end, to 1.77 to 2.3 ng/dL on the upper end of the range. You may also see a range of 12 to 30 picomoles per liter (pmol/L). Low-normal levels, or levels below the reference range, can be indicative of hypothyroidism.

Some practitioners consider a Free T4 level in the range of 1.3-1.77 ng/dL to be optimal.

Total Triiodothyronine (Total T3) Test

Total triiodothyronine (Total T3) measures both bound and free T3 in the blood.

The reference range for the total T3 test can vary depending on the laboratory and the method used to perform the test. In general, the normal range for total T3 in adults is about 71 to 76 ng/dL on the low end, up to 180 ng/dL on the high end.

Low-normal levels, or levels below the reference range, are often seen in hypothyroidism.

Free Triiodothyronine (Free T3) Test

Free T3, or free triiodothyronine, is a type of thyroid hormone that is not attached to proteins in the blood and is able to enter and affect body tissues. It is the active form of triiodothyronine that can be used by the body. A blood test can be done to measure the amount of free T3 in the blood, which is used to diagnose thyroid conditions,

The normal range for free T3, or free triiodothyronine, in adults is about 2.0 to 2.3 pg/mL on the low end, up to 4.2 to 4.4 pg/mL on the high end.

Some practitioners consider the optimal Free T3 range to be from 3.3 to 3.9 pg/mL.

Thyroid Peroxidase Antibodies (TPOAb) Test

The Thyroid Peroxidase Antibodies (TPO Ab) test is sometimes referred to as the “Antithyroid Antibodies” test. This test measures antibodies against thyroid peroxidase (TPO) in the blood. TPO is an enzyme made in the thyroid gland that is important in the production of thyroid hormone. The test is used to diagnose autoimmune thyroid diseases, such as Hashimoto's thyroiditis and Graves' disease. A TPO antibody test is considered “positive” – indicating the presence of antibodies that target TPO – when the level is above the reference range. Positive TPO antibodies are associated with inflammation and damage to the thyroid gland that are characteristic of Hashimoto’s thyroiditis – the primary cause of hypothyroidism in the U.S.

While the thyroid peroxidase antibody normal range? Various labs have huge variations in their reference range. You’ll find reference ranges for the TPO antibodies test that include:

  • 0 – 60 international units per millimeter (IU/ml)
  • 0 – 45 IU/ml
  • 0 – 34 IU/mL
  • 0 – 9 IU/mL

There is no agreement as to optimal TPOAb levels, but some practitioners work with patients to get these levels as low as possible close to 0.

Some practitioners also consider thyroid treatment for patients who have normal thyroid levels, but elevated/positive TPOAb.  

Reverse T3 (RT3) Test

A Reverse T3 (RT3) test is a blood test that measures the inactive form of the hormone T3.

The reference range for the RT3 test can vary depending on the laboratory and the method used to perform the test. In general, the normal range for RT3 is 8 to 10 ng/dL on the lower end of the range, up to 25 ng/dL on the higher end of the range.

Many practitioners consider RT3 levels under 15 to be optimal.

Thyroglobulin Antibodies (TgAb) Test

Thyroglobulin Antibodies (TgAb) are a type of autoantibody that targets the thyroglobulin protein in the thyroid gland. Thyroglobulin is a large protein that is synthesized in the thyroid gland and is the site of synthesis of T3 and T4. TgAb is a marker for thyroid autoimmunity. TgAb levels can be elevated in people with autoimmune thyroid diseases, such as Hashimoto's thyroiditis.

The reference range for the Thyroglobulin Antibodies (TgAb) test can vary depending on the laboratory and the method used to perform the test. In general, TgAb levels below 20 IU/mL are typically considered normal for the general population (those without any thyroid issues).

There is no agreement as to optimal TgAb levels, but some practitioners work with patients to get these levels as low as possible close to 0.

A note from Paloma

Getting diagnostic testing and following blood tests can be challenging for patients with hypothyroidism. Some doctors may resist patient requests for a thyroid test or only offer a TSH test instead of a full thyroid panel. Additionally, some doctors may have a one-size-fits-all policy of only testing annually, which may not be sufficient for some patients. Some HMOs and health insurers may also limit their coverage and approvals for thyroid lab testing and only approve semi-annual or annual thyroid testing.

Paloma Health believes that patients should get a complete panel of thyroid tests as often as needed for optimal hypothyroidism diagnosis and care. That's why Paloma has made its thyroid testing affordable and convenient. Paloma offers a Complete Thyroid Home Test kit that allows you to test your thyroid function at home, on our own schedule, with painless fingerstick testing.

The panel includes TSH, Free T4, Free T3, and TPOAb. You also have the option to add RT3 and Vitamin D tests to your panel at checkout. The test kit is affordable, easy to use, and convenient. You simply order your kit online, follow the easy instructions to take your samples, and send your test kit back to the certified lab in the prepaid mailer. Your results come back quickly to your secure online portal. This is a great option for people who find getting a blood draw at a lab or doctor's office inconvenient, expensive, or painful

If you have hypothyroidism, it's important to get the best care possible to manage your condition effectively. Paloma Health offers an unsurpassed experience with a knowledgeable team of thyroid doctors who provide quality, proven thyroid care. They offer an integrative approach to diagnosis and treatment, and Paloma’s experts work diligently to find the right treatment for you. By taking a brief quiz, you’ll be matched with a Paloma doctor who can provide the personalized care you deserve. If you're looking for a convenient and affordable way to manage your hypothyroidism, schedule an appointment with a Paloma doctor today.

Dealing with Hypothyroidism?  Video chat with a thyroid doctor

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American Thyroid Association. Thyroid Function Tests | American Thyroid Association. American Thyroid Association. Published 2016.

Thyroid Testing Example Results. Published November 11, 2022. Accessed April 16, 2023.

National Institute of Diabetes and Digestive and Kidney Diseases. Thyroid Tests | NIDDK. National Institute of Diabetes and Digestive and Kidney Diseases. Published October 12, 2019.

Thyroid Function Tests. Published November 19, 2019.

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Mary Shomon

Patient Advocate

Mary Shomon is an internationally-recognized writer, award-winning patient advocate, health coach, and activist, and the New York Times bestselling author of 15 books on health and wellness, including the Thyroid Diet Revolution and Living Well With Hypothyroidism. On social media, Mary empowers and informs a community of more than a quarter million patients who have thyroid and hormonal health challenges.

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