Reading Your Thyroid Blood Test Results

Thyroid blood tests are an important part of diagnosing and treating thyroid disorders. Understanding results, however, takes some knowledge about what these tests check for, how they affect your thyroid, and how they do or don't interact with one another.

Without this, some of the test results can be confusing or seem contradictory, though. For instance, why does a high TSH level mean low thyroid activity? Why can you have high TSH and normal T4?

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

Thyroid Function

Your thyroid gland sits in the front of your throat. It's responsible for many important bodily functions, such as:

• Metabolism (breaking down food for energy)
• Heart function
• Muscle function
• Digestion
• Bone maintenance

When you have low thyroid function, those processes slow down. You may gain weight, feel sluggish and fatigued, and be slow to heal.

Excessive thyroid function speeds up these processes. That can mean unintended weight loss, agitation, and a racing heart.

Thyroid Disease

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

• Hypothyroidism (low thyroid function)
• Hashimoto's thyroiditis (autoimmune hypothyroidism)
• Hyperthyroidism (excessive thyroid function)
  
• Graves' disease (autoimmune hyperthyroidism)

In autoimmune thyroid disease, your immune system mistakenly attacks healthy tissues as if they were a virus or bacteria.

Types of Thyroid Function Tests

The purpose of thyroid blood tests is to measure the thyroid gland's function. This is done by looking at hormones and other substances produced by the thyroid gland and organs that control thyroid function.

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

Thyroid-Stimulating Hormone

Thyroid-stimulating hormone (TSH) acts as a messenger to the thyroid gland. It comes from the pituitary gland in the brain, not the thyroid itself.

What happens is:

• Your body constantly monitors thyroid hormone levels.
• If levels are low, your pituitary releases TSH.
• TSH tells the thyroid to step up hormone production.

Or:

• If levels are high, the pituitary releases less TSH.
• This signals the thyroid to reduce hormone production.

The thyroid hormones are called triiodothyronine (T3) and thyroxine (T4).

Triiodothyronine

T3 is the active thyroid hormone. It tells your cells to produce more energy, along with many other functions. The thyroid produces some T3 and the rest is converted from T4 in other tissues.

Much of the T3 in your system at any given time is bound to proteins, which makes it not immediately usable by your body.

Blood tests can measure T3 in three different ways:

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

When reverse T3 occupies thyroid receptors, it prevents free T3 from doing so. That can lead to hypothyroidism symptoms. This generally happens only when the body is trying to conserve energy due to severe illness or starvation.

Thyroxine

T4 works as a "storage" hormone. It starts out inactive, then your body converts it to 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 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, so it's usable by your tissues.

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 that cancer cells are still present after these treatments. 

By comparing baseline values with later results, the Tg test can tell healthcare providers if cancer treatments are working. It also tells them whether or not the cancer is in remission.

Thyroid Antibodies

Some thyroid disorders are caused by autoimmune diseases. In these diseases, the immune system targets and attacks normal thyroid cells by mistake. Three common antibodies are associated with autoimmune thyroid disease: 

• Thyroid peroxidase antibodies (TPOAb) are detected in 95% of people with Hashimoto's and around 70% of those with Graves' disease. A high TPOAb is also seen after childbirth in postpartum thyroiditis.
• Thyroid-stimulating hormone receptor antibodies (TRAb) are found 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. One in four people with thyroid cancer will have elevated TgAb. It's also detected in 80% of people with Hashimoto's and between 50% to 70% of those with Graves' disease.

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 using total T4 multiplied by the T3RU to check for either low or excessive thyroid function

Side Effects of Thyroid Blood Tests and Care

Thyroid blood tests are performed using a routine blood draw. That's when blood is drawn from you with a syringe and sent to a lab. This is a safe procedure with few potential side effects.

It's rare, but some people get nauseous or feel faint when they have blood drawn. Let the medical personnel know immediately if you experience these side effects.

Later, you may notice a small bruise or have some tenderness at the needle insertion site. An over-the-counter pain reliever or an ice pack can help with this.

You should get medical attention if the insertion site is:

• Red
• Swollen
• Highly painful

These are signs of an infection, which needs to be treated with antibiotics.

Test Reference Ranges

The results of thyroid blood tests are listed with what's called a reference range. This range gives the "normal" or expected levels healthcare providers typically see from the tests.

Generally speaking, anything in this range is 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. 

Interpreting Results

Test results have to be interpreted to give them meaning. The one test that arguably gives the most insight into your thyroid function is the TSH. Alongside results of free T3 and free T4 tests, the TSH may even suggest the cause of a thyroid problem.

Thyroid Disease Healthcare Provider Discussion Guide

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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.

Here's what TSH values mean, according to guidelines from the American Association of Clinical Endocrinologists (AACE) and the American Thyroid Association (ATA)

• 4.7 to 10 milliunits per liter (mU/L) = subclinical low thyroid function
• 10 mU/L or higher = symptomatic low thyroid function
• 0.1 to 0.5 mU/L = subclinical overactive thyroid function
• 0.1 mU/L and lower = symptomatic overactive thyroid function

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:

• Normal TSH + normal T4 = normal thyroid function
• High TSH + normal T4 = you may have a higher risk of developing an underactive thyroid
• Low TSH + high T4 = overactive thyroid
• High TSH + low T4 = underactive thyroid
• Low TSH + low T4 = low thyroid function due to another problem, such as pituitary gland dysfunction
  

Looking at TSH alongside T3 test results can also help with diagnosis:

• Low T3 + high TSH = low thyroid function
• High T3 + low TSH = overactive thyroid

Other Interpretations

Other thyroid tests may be a part of a 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 predict long-term treatment outcomes. Research has suggested only 4% of people with a Tg level under 1 will see the cancer recur after five years.
• TPOAb tests can help confirm Hashimoto's disease if TSH is high and T4 is low.
• TRAb tests help diagnose Graves' disease, confirm a diagnosis of toxic multinodular goiter, or, in late pregnancy, to check a baby's risk of Graves' disease or an overactive thyroid.
• TgAb tests help diagnose autoimmune disease or help clarify post-cancer treatment test results. This is because TgAb sometimes interferes with Tg readings.

Summary

Many thyroid blood tests are available, including tests of thyroid hormones, thyroid stimulating hormone, antibodies to the thyroid, and markers of thyroid cancer. Their combined results can paint a complete picture of your thyroid function.

The test results are interpreted based on a common standard for each test, all of which look at thyroid function in different ways. They can be used for diagnosis or to monitor how effective your treatments are.

A Word From Verywell

Thyroid blood test results are complex and can be confusing. The important thing is to pay attention to how your healthcare provider interprets them.

If you have abnormal results, don't panic. Thyroid disorders are common and highly treatable. Your healthcare provider can go over potential treatments with you so together you can decide together how to manage your condition.