Reading Your Thyroid Blood Test Results

Blood tests for thyroid function are an important part of diagnosing and treating thyroid disorders.

Learning about the various tests and numbers can be complicated. But taking the time to understand them can help you better manage your disease.

This article looks at the different thyroid tests and what numbers you might see in your results. It also explains why many healthcare providers prefer some thyroid tests while others make different choices.

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How to Work With Your Thyroid Medical Team

What Thyroid Blood Tests Measure

The purpose of thyroid testing is to measure its function. This is done by looking at substances produced by the thyroid gland and other organs that control thyroid function.

Thyroid tests measure:

  • Thyroid-stimulating hormone (TSH): A messenger hormone from the pituitary that increases thyroid activity
  • Thyroxine (T4): An inactive thyroid hormone that's converted to T3 when needed
  • Triiodothyronine (T3): The active thyroid hormone used for many bodily functions
  • Thyroglobulin (Tg): A protein made b the thyroid that's relevant to thyroid cancer
  • Thyroid antibodies (TPOAb, TRAb, TgAb): Cells in the immune system designed to target the thyroid or associated substances; indicates autoimmunity

A single test offers helpful information about your thyroid health. But it usually takes more than one to get a complete picture.

Types of Tests

By comparing the results of various thyroid tests, a healthcare provider can tell if you have:

For example, the pituitary gland in the brain makes TSH. It controls how much T3 and T4 the thyroid produces.

Taken together, levels of TSH, T3, and T4 say a lot about how well (or not) your thyroid gland is working.

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Thyroid Stimulating Hormone

TSH acts as a messenger to the thyroid gland.

If the pituitary gland finds too little thyroid hormone in your blood, it makes more TSH. This, in turn, causes the thyroid to make more hormones (T3 and T4).

When the pituitary finds too much thyroid hormone, it slows the TSH supply. That signals the thyroid gland to slow down too.

Thyroxine

T4 works as a "storage" hormone. It starts out as inactive. Then your body converts it into T3 when and where it's needed.

To be converted, it goes through a process called monodeiodination. That means it loses an atom of iodine to become T3. The T4 test measures two key values:

  • Total T4: The total amount of thyroxine found in the blood. This includes T4 that has bonded with protein, and T4 that has not. The difference affects its ability to enter tissues.
  • Free T4: The type not bonded to protein, so it's usable by your tissues.

Triiodothyronine

T3 is the active thyroid hormone. The thyroid creates some and the rest is created in your tissues through monodeiodination.

T3 tells your cells to produce more energy, along with many other functions. Three different tests can measure T3:

  • Total T3: The total amount of triiodothyronine in the blood, bonded to protein or not.
  • Free T3: Not bound to protein, usable by your tissues.
  • Reverse T3: An inactive "mirror image" of T3 that attaches to thyroid receptors but can't activate them.

Thyroglobulin (Tg)

Thyroglobulin (Tg) is a protein made by the thyroid gland. It's mostly measured as a tumor marker to help guide thyroid cancer treatment.

Treatment often involves thyroidectomy (surgical removal of the thyroid) or radioactive ablation (RAI) therapy to destroy the gland. A high Tg level is a sign cancer cells are still present after these treatments. 

By comparing baseline values with later results, the Tg test can tell healthcare providers if cancer treatment is working. It also tells them whether cancer is still in remission or if it's come back.

Thyroid Antibodies

There are some thyroid disorders caused by an autoimmune disease. These diseases are caused by the immune system targeting and attacking normal thyroid cells by mistake. Antibodies are made during this process and then "matched" to antigens, or receptors, on a targeted cell.

There are three common antibodies associated with autoimmune thyroid disease: 

  • Thyroid peroxidase antibodies (TPOAb) are detected in 95% of people with Hashimoto's disease and around 70% of those with Graves' disease, two types of autoimmune thyroid diseases. A high TPOAb is also seen in women with postpartum thyroiditis.
  • Thyroid stimulating hormone receptor antibodies (TRAb) are seen in 90% of Graves' disease cases, but only 10% of Hashimoto's cases.
  • Thyroglobulin antibodies (TgAb) are produced by your body in response to the presence of thyroglobulin. They are detected in 80% of people with Hashimoto's, and 50% to 70% of those with Graves' disease. One in four people with thyroid cancer will have elevated TgAb.

Some thyroid tests check for the presence of antibodies, including TgAb. It is often present in the most common types of thyroid-related autoimmune disorders, but also in about 25% of people with thyroid cancer.

Thyroid Binding Proteins

Testing the blood level of proteins that bind to T3 and T4 can help healthcare providers understand thyroid problems in people whose glands still work normally. Three common tests are:

  • Thyroid binding globulin (TBG): Measures levels of a protein that carries thyroid hormones in the blood
  • T3 resin uptake (T3RU): Counts the percentage of TBG in a sample of blood
  • Free thyroxine index (FTI): An older testing method; Uses total T4 multiplied by the T3RU to find either low or excessive thyroid function

Side Effects of Thyroid Blood Tests and Care

https://www.healthline.com/health/thyroid-function-tests#side-effects

Test Reference Ranges

The results of these blood tests are listed with a reference range. This range gives the expected values that healthcare providers typically see from tests among all people.

Generally speaking, anything between the high and low ends of this range can be considered normal. Test numbers near the upper or lower limit are borderline, while anything outside of these limits is considered not normal.

In the middle of the reference range is a "sweet spot," called the optimal reference range, in which thyroid function is considered ideal. 

It's important to know that reference ranges and the units of measurement used can vary from lab to lab. To ensure consistency in your test results, try to use the same lab for every test.

Interpretation of Results

Test results are numbers until they are interpreted to give them meaning. The one test that arguably gives the most insight is the TSH. When used with the free T3 and free T4 tests, the TSH may even suggest the cause of any problem.

Thyroid Disease Healthcare Provider Discussion Guide

Get our printable guide for your next healthcare provider's appointment to help you ask the right questions.

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TSH Interpretations

TSH values outside of a "normal" range suggest a thyroid disorder is at work. Values at or near the upper or lower range may suggest a subclinical disorder without any symptoms.

According to guidelines issued by the American Association of Clinical Endocrinologists (AACE) and the American Thyroid Association (ATA), a TSH value:

  • Between 4.7 and 10 milliunits per liter (mU/L) is a subclinical low thyroid problem
  • Over 10 mU/L is a low thyroid problem that's symptomatic
  • Between 0.1 and 0.5 mU/L is a subclinical overactive thyroid problem
  • Less than 0.1 mU/L is an overactive thyroid problem that's symptomatic

Comparing High/Low TSH and T3/T4 Levels

By comparing TSH with T4 values, your healthcare provider may be able to see a more clear picture of the thyroid issue. For example:

  • A normal TSH and normal T4 show a normal thyroid
  • A low TSH and high T4 generally means an overactive thyroid
  • A high TSH and low T4 means an underactive thyroid
  • A low TSH and low T4 mean low thyroid function because of another problem, such as with the pituitary gland or hypothalamus of the brain

A low T3 value with a high TSH value is diagnostic evidence of low thyroid function. By contrast, a low TSH value with a high T3 value is the opposite, and means an overactive thyroid.

Other Interpretations

Other thyroid tests may be a part of standard thyroid workup or used when needed. Some have specific aims. The others are used for screening purposes or to evaluate possible causes.

  • RT3 tests can help find problems with thyroid control. People with euthyroid sick syndrome (ESS), for example, have abnormal thyroid hormone levels due to an illness not related to the thyroid.
  • Tg tests can also predict long-term treatment outcomes. Research published in the journal Thyroid finds only 4% of people with a Tg level under 1 will see it recur after five years.
  • TPOAb tests can help confirm Hashimoto's disease if your TSH is up but your T4 is low.
  • TRAb tests help diagnose Graves' disease, but may confirm a diagnosis of toxic multinodular goiter. The test is also used during the last three months of pregnancy to check a baby's risk of being born with Graves' disease or an overactive thyroid.
  • TgAb tests help diagnose autoimmune disease, but also can help clarify any post-cancer treatment test results. This is because TgAb can interfere with Tg readings in some people.

There are many thyroid function tests, and their names and lab value ranges may seem confusing. They all are important, though often for different reasons. What's more important are the results, a shared understanding of what those results mean, and how they will help to guide care for a thyroid-related disorder.

Summary

Your test results, symptoms, medical history, and current health are all factors when a healthcare provider looks at how well your thyroid is working. The test results are based on a common standard for each test, all of which look at thyroid function in different ways.

It's not just the results, though. Many people find it helpful to know what the tests are, and which ones they had or will have. It's one way to ensure that both healthcare provider and patient are on the same page, and know how to talk about test results or tests they think may be needed.

A Word From Verywell

Frequently Asked Questions

  • What are normal thyroid levels?

    Normal thyroid levels, specifically for TSH (thyroid stimulating hormone), fall within the range of 0.5 to 5.0 mIU/L (milli-international units per liter). In adults, normal total T4 levels are within 5.0 to 12.0μg/dL (micrograms per deciliter). Also in adults, the normal range for T3 levels is between 80 to 220 ng/dL (nanograms per deciliter).

  • How long does a thyroid blood test take?

    An average thyroid blood test only takes a few minutes. The test itself is short like many other blood tests. However, receiving the results may take some time. Be sure to ask your healthcare provider when they expect to receive the test results.

  • What is the most accurate thyroid blood test?

    The most accurate thyroid blood test measures your thyroid stimulating hormone (TSH). The results of this test show how many thyroid hormones the thyroid gland currently produces which help determine its range of function.

  • How do you know if you have thyroid problems?

    The only way to know for certain if you have thyroid problems is to receive a diagnosis from a healthcare provider. However, there are certain symptoms associated with hypothyroidism and hyperthyroidism that may clue you in that something isn't right. A few symptoms associated with both of these conditions includes fatigue, hair loss, puffy face or eyes, and insomnia.

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7 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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