Reading Your Thyroid Blood Test Results

Thyroid blood tests are an important part of diagnosing and treating thyroid disease. Based on how high or low the different values are—including key hormones such as TSH and T4—the lab can determine which type of thyroid disease you have (hypothyroidism vs. hyperthyroidism) and narrow the possible causes.

The results can be difficult to understand because the diagnosis can change based on the relationship of values. For instance, a high TSH and low T4 may suggest Hashimoto’s disease, while a low TSH and high T4 may suggest Graves' disease. Different combinations of values can mean different things, and there is often more than one meaning.

This article looks at the different types of thyroid blood tests, including how they are measured and how the results are interpreted,

How to Work With Your Thyroid Medical Team

Thyroid Function

Your thyroid gland is situated in the front of your throat. It produces hormones that regulate many important body functions, including your heart rate, blood pressure, digestion, and metabolism (the conversion of calories to energy):

When you have thyroid disease, your thyroid gland doesn't produce hormones at levels needed to keep your body functioning normally. This is broadly described as either:

• Hypothyroidism: Also referred to as an underactive thyroid gland, it is most commonly caused by an autoimmune disorder called Hashimoto's thyroiditis.
• Hyperthyroidism: Also referred to as an overactive thyroid gland, it is most commonly caused by an autoimmune disorder called Graves' disease.

The decrease or increase in thyroid hormones can cause adverse symptoms. With hypothyroidism, the underproduction of hormones can cause weight gain, fatigue, and a slow heart rate, With hyperthyroidism, the overproduction of hormones can cause weight loss, agitation, and a racing heart.

Having a normally function thyroid gland is referred to as euthyroid .

Types of Thyroid Function Tests

The purpose of thyroid blood tests is to measure how well your thyroid gland is functioning. This is done by measuring hormones produced by the thyroid gland as well as other organs (like the pituitary gland) that influence the thyroid gland.

Thyroid-Stimulating Hormone (TSH)

Thyroid-stimulating hormone (TSH) is produced by the pituitary gland situated at the base of the brain. It acts as a chemical messenger, telling the thyroid gland when to start producing thyroid hormones.

When thyroid hormone levels are low, more TSH is produced to stimulate production. When levels are normal, the pituitary gland stops producing TSH.

The TSH test measures how much or how little TSH there is in the bloodstream.

Thyroxine (T4)

Thyroxine (T4) is one of two hormones produced by the thyroid gland. It is an inactive thyroid hormone that freely circulates in the bloodstream, ready to be converted to the active form called T3.

There are two types of T4: bound T4 (which is attached to proteins that prevent it from entering tissues) and free T4 (which is not attached to proteins and can enter tissues freely).

When measuring T4, the lab will look at two different values:

• Total T4: The total amount of T4 (bound and unbound) in your bloodstream
• Free T4: The total amount of unbound T4 available for use in tissues

Free T4 is arguably more important to measure because it is the type that will be more active once converted to T3.

Triiodothyronine (T3)

Triiodothyronine (T3) is the active thyroid hormone converted from T4. As with T4, there is both bound and free T3.

T3 blood tests include:

• Total T3: The total amount of T3 (bound and unbound) in your bloodstream
• Free T3: The amount of unbound T3 available for use in tissues
• Reverse T3: An inactive form of T3 that has no utility but can block free T3 from entering tissues

Is T3 or T4 More Important?

T4 is generally the more important thyroid hormone to measure. This is because it describes how much thyroid hormone is available for use rather than how much has been converted.

Thyroglobulin (Tg)

Thyroglobulin (Tg) is a protein made by the thyroid gland. It's mostly used to help guide the treatment of thyroid cancer.

Thyroidectomy (surgical removal of the thyroid gland) or radioactive ablation (RAI) therapy are commonly used to treat thyroid cancer. A high Tg level is a sign that cancer cells are still present after these treatments.

By comparing baseline values with later values, the Tg test can tell your provider if cancer treatments are working. It can also tell them if the cancer is in remission.

Thyroid Antibodies

Some thyroid diseases like Hashimoto's thyroiditis and Graves' disease are autoimmune disorders. These are diseases in which the immune system targets and attacks normal thyroid cells.

There are three common antibodies associated with autoimmune thyroid disease:

• Thyroid peroxidase antibodies (TPOAb): The type of antibody is 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 people with postpartum thyroiditis.
• Thyroid-stimulating hormone receptor antibodies (TRAb): These are found in 90% of Graves' disease cases, but only 10% of Hashimoto's cases.
• Thyroglobulin antibodies (TgAb): These are produced by your body in response to the presence of Tg. One in four people with thyroid cancer will have elevated TgAb. It is also detected in 80% of people with Hashimoto's and between 50% to 70% of those with Graves' disease.

Thyroid Binding Proteins

These blood tests measure the amount of proteins that can bind to T3 and T4. They can help determine the cause of a thyroid problem if your thyroid gland is functioning normally.

The tests include:

• Thyroid binding globulin (TBG): This is the specific protein that binds to T4 and T3.
• T3 resin uptake (T3RU): This calculates the percentage of TBG in the blood.

Interpreting Results

The results of thyroid blood tests are described in a reference range of values. The reference range has a high and low value between which results are considered normal. Values near the upper or lower limit are borderline, while anything outside of these limits is considered abnormal.

Abnormal values may indicate thyroid disease, either primary thyroid disease (caused by damage to the thyroid gland) or central thyroid disease (caused by damage to organs like the pituitary gland that regulates thyroid function).

Thyroid disease may also be subclinical, meaning that there is only mild thyroid failure, usually with few notable symptoms.

Reference ranges and units can vary from lab to lab. To ensure consistency in your test results, use the same lab for every test.

Thyroid Disease Healthcare Provider Discussion Guide

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

TSH Interpretations

On its own, the TSH test can diagnose thyroid disease, albeit with limitations.

According to the American Association of Clinical Endocrinologists (AACE) and the American Thyroid Association (ATA), TSH values can be interpreted as follows:

• Subclinical hypothyroidism: 4.7 to 10 milliunits per liter (mU/L)
• Primary Hypothyroidism: 10 mU/L or higher
• Subclinical hyperthyroidism: 0.1 to 0.5 mU/L
• Primary Hyperthyroidism: 0.1 mU/L and lower

Comparing High/Low TSH and T3/T4 Levels

While the TSH test is valuable, it is more useful when referenced to T3/T4 results. By comparing TSH with T3/T4 values, your healthcare provider may have a clearer picture of the cause of thyroid disease.

Other Interpretations

Other thyroid tests may be included in the workup. Some have specific aims, while others are used for screening purposes or to evidence possible causes.

• RT3 tests can detect euthyroid sick syndrome (ESS), a condition in which abnormal thyroid hormone levels are due to an illness unrelated to the thyroid gland.
• Tgtests can be used to predict long-term treatment outcomes. Research has shown that 4% of people with a Tg level under 1 will experience cancer recurrence within five years.
• TPOAb tests can help confirm Hashimoto's disease if TSH is high and T4 is low.
• TRAb tests can help diagnose Graves' disease or confirm a diagnosis of toxic multinodular goiter.

Summary

Many thyroid blood tests are available, including tests for the thyroid hormones T3 and T4, thyroid stimulating hormone (TSH), and thyroid antibodies. The combined results can paint a clearer picture of the cause of thyroid disease. The tests can also monitor how effective thyroid treatments are, including the treatment of thyroid cancer.

Frequently Asked Questions

• TSH: 0.5 to 5.0 mIU/L (milli-international units per liter)
• Total T4: 5.0 and 12.0μg/dL (micrograms per deciliter)
• Total T3: 80 and 220 ng/dL (nanograms per deciliter)

It can. That's because caffeine makes the thyroid drug levothyroxine (a synthetic form of T4) move too quickly through your digestive tract so you can't absorb it properly. This can make your T4 levels fall.

• Mild bleeding at the puncture site
• Mild redness or swelling
• Bruising or hematoma (a small pocket of blood under the skin)

While rare, a skin infection may occur. See a healthcare provider immediately if you develop a fever with chills, a pus-like discharge, and increased swelling, redness, pain, or heat at the puncture site.

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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By Mary Shomon
Mary Shomon is a writer and hormonal health and thyroid advocate. She is the author of "The Thyroid Diet Revolution."

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