Understanding Your Thyroid Blood Test Results

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Blood tests for thyroid function—TSH, total T4, free T3, TSI, and others—are an important part of diagnosing and treating thyroid disorders. Figuring out what the various names and numbers mean can be complicated, but taking the time to learn them can help you better manage your disease.

A single test will give helpful information on a person's thyroid health, but more than one is usually needed to get a complete picture. By comparing the results of thyroid tests, a healthcare provider can tell if you have hypothyroidism (low thyroid function), hyperthyroidism (too much function), or an autoimmune disease such as Graves' disease.

This article looks at the different thyroid tests and what values you might see in your results. It also gives a general look at why many doctors prefer some thyroid tests, but other doctors might disagree and choose a different option.

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How to Work With Your Thyroid Medical Team

Types of Tests

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

For example, the pituitary gland in the brain makes a hormone known as thyroid stimulating hormone (TSH). It controls how much of the hormones triiodothyronine (T3) and thyroxine (T4) are produced by the thyroid gland. Taken together, these values can tell you a lot about how well or poorly your thyroid gland is working.

Thyroid function tests usually look at six of these key substances in the blood, including hormones, proteins, and immune system agents known as antibodies.

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Thyroid Stimulating Hormone (TSH)

Thyroid stimulating hormone (TSH) acts as a messenger to the thyroid gland. If the pituitary gland finds too little thyroid hormone in the blood, it will make more TSH. This, in turn, causes the thyroid to make more hormone too. When the pituitary finds too much thyroid hormone, it slows the TSH supply and signals the thyroid gland to slow down too.

Thyroxine (T4)

Thyroxine (T4) works as a "storage" hormone. Until it is converted into T3, it is inactive. It must go through a process known as monodeiodination in which it loses an atom of iodine to become triiodothyronine (T3). The T4 test measures two key values:

  • Total T4 is 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 is the type not bonded to protein, and is viewed as the active form of T4.

Triiodothyronine (T3)

Triiodothyronine (T3) is the active thyroid hormone created when T4 is converted in the chemical process described earlier. T3 tells cells to produce more energy, along with many other functions. Three different tests are used to measure T3:

  • Total T3 is the total amount of triiodothyronine in the blood, bonded to protein or not.
  • Free T3 is not bound to protein and is considered the active form of T3.
  • Reverse T3 is the inactive "mirror image" of T3 that attaches to thyroid receptors but is unable to activate them.

Thyroglobulin (Tg)

Thyroglobulin (Tg) is a protein made by the thyroid gland. It is mostly measured as a tumor marker to help guide thyroid cancer treatment. A high Tg level is a sign that cancer cells are still present after surgery to remove the thyroid (thyroidectomy) or radioactive ablation (RAI) therapy to destroy the gland.

By comparing baseline values with later results, the Tg test can tell doctors if cancer treatment is working. It also tells them if a person is still in cancer remission, or if there are signs cancer has 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 doctors 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) is an older testing method. It uses total T4 multiplied by the T3RU to see if a person has either low or excessive thyroid function.

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 doctor'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

T3 and T4 Interpretations

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.

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