What Do Your Thyroid Test Results Really Mean?

Understanding TSH, Free T4, Free T3, Reverse T3, and Thyroid Antibodies

Understanding Thyroid Blood Tests
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As a thyroid patient, it's can be a challenge to get a doctor to run more than the thyroid stimulating hormone (TSH) test. It can also be a challenge to understand and interpret thyroid test results once you get them.

Let's take a look at the different thyroid tests, what they're measuring, and what your test results may mean, starting with the TSH test.

The TSH (Thyroid Stimulating Hormone) Thyroid Test

TSH stands for thyroid stimulating hormone, as is a key test used to diagnose and manage thyroid disease.


What does it measure?

TSH measures a pituitary hormone that is a messenger to your thyroid gland. If your pituitary detects that your thyroid is producing too little thyroid hormone, the pituitary produces more TSH, which then prompts your gland to produce more thyroid hormone. When your pituitary detects too much thyroid hormone, it lowers TSH, as a message to the gland to slow down or stop production of thyroid hormone.

TSH is often the only test some doctors use to diagnose or manage thyroid conditions.

What do the results mean?

Generally, levels above the reference range suggest that your thyroid is underactive (hypothyroid) and not producing enough thyroid hormone. Levels below the reference range suggest that your thyroid is overactive (hyperthyroid) and producing too much thyroid hormone.

This can be confusing, so let's repeat this in a simple way: HIGH TSH is associated with an UNDERactive thyroid/hypothyroidism, and LOW TSH is associated with an OVERactive thyroid/hyperthyroidism.

If your TSH falls within the reference range and you don't have thyroid symptoms, then there is no reason for concern.

However, if your TSH falls within the reference range and you have thyroid signs and symptoms, a family history of thyroid disease, low levels of the key thyroid hormones (T4/T3), and/or antibodies, some experts believe that your thyroid may not be "normal."

This is why it is important to ask for the exact number, and the reference range at the lab. Do not accept a verbal assurance, phone call or card in the mail that says "your TSH was 'normal' or "your thyroid tests were 'normal."

This is particularly important if your TSH test results are normal, but you have symptoms.

There are several controversies about the TSH test that you will want to keep in mind: 

  • Integrative physicians believe that the true measurement of thyroid function is not TSH, but the actual, available thyroid hormones circulating in the bloodstream—measured by the Free T4 and Free T3 tests.

What is the reference range?*

The reference range is 0.5 - 4.70 µIU/mL. (Some laboratories are .3 to 4.5, or other similar ranges.)

T4/FT4 (Thyroxine / Free Thyroxine) Thyroid Tests

Thyroxine, also known as T4, is one of the key thyroid hormones. The majority of hormone produced by the thyroid gland is thyroxine. Thyroxine is considered a "storage" hormone - in that alone it is not usable by the body to produce energy and deliver oxygen to cells.

It must lose an atom of iodine, a process called monodeiodination (or T4 to T3 conversion), and become triiodothyronine (T3) in order to be used by cells.

What does it measure?

Total thyroxine measures the total amount. Free thyroxine measures the amount of thyroxine that is not bound, and is actually available to be converted into T3.

What do the results mean?

Generally, levels above the top of the reference range suggest that your thyroid is overproducing thyroid hormone - and that your thyroid is overactive or hyperthyroid. Levels below the bottom of the reference range suggest that your thyroid is underproducing thyroid hormone, and it is underactive or hypothyroid. Levels within the reference range are considered "normal" by conventional physicians.

Again, there are controversies over measuring T4/Free T4. Some integrative physicians and hormone experts believe that if your Free T4 is not in the top half of the reference range, it is not optimal.

It is important to find out where your practitioner stands on this issue.

What is the reference range?*

Total T4: 4.5 - 12.5 µg/dL
Free T4: 0.8 - 1.8 ng/dL

T3/FT3 (Triiodothyronie / Free Triiodothyronine) Thyroid Tests

Triiodothyronine (T3) is the active thyroid hormone, and delivers oxygen and energy to cells.

What does it measure?

Free T3 measures the free, unbound and available levels of the T3 hormone. Because the free levels of T3 represent immediately available hormone, free T3 is thought by some practitioners to better reflect the patient's hormonal status than total T3.

What do the results mean?

Elevated levels or levels above the reference range may be indicative of hyperthyroidism. Lower levels, or levels below the reference range may be indicative of hypothyroidism.

Testing for T3 and Free T3 is even more controversial than T4 testing. This is primarily because many conventional practitioners do not believe that the T3 level has an effect on symptoms, and that there is no place for treatment with T3 hormone.

Many integrative physicians and hormone experts believe that if free T3 is not in the top half—or even the top quarter - of the reference range, it is not optimal, and thyroid patients will not feel well.

It is important to find out where your practitioner stands on this issue.

What is the reference age?*

The following are reference ranges for adults.

Free T3: (Triiodothyronine): 2.3- 4.2 pg/mL
Total T3: 80 -200 ng/dL

Reverse Triiodothyronine - Reverse T3 (RT3)

Reverse T3 (RT3) is a form of T3 that is inactive and has no function in the body, except to in some cases block cells' use of actual T3. Reverse is produced in higher amounts during times of stress.

What does it measure?

The Reverse T3 test measures the amount of Reverse T3 in the bloodstream.

What do the results mean?

Reverse T3 is a controversial test. Conventional endocrinology for the most part dismisses the value of RT3 measurement in diagnosing, treating and managing hypothyroidism - again, because the role of T3 levels, and T3 treatment, is largely ignored.

Integrative physicians and those who focus on optimal hormone balance, however, consider elevated RT3 to be an key sign of an underactive or dysfunctional thyroid.

Ideally, reverse T3 should fall in the lower half of the normal range. You may also want to calculate the T3/Reverse T3 ratio, to help evaluate the situation.

What is the reference range?*

The reference range is typically 10-24 ng/dL

Thyroid Peroxidase Antibodies (TPO) / Antithyroid Peroxidase Antibodies

Thyroid peroxidase (TPO) antibodies, are also known as antithyroid peroxidase antibodies. (In the past, these antibodies were also called antithyroid microsomal antibodies or antimicrosomal antibodies).

TPO work against thyroid peroxidase, an enzyme that plays a role in the conversion of T4 to T3. TPO antibodies can be evidence of inflammation of the gland, or tissue destruction such as Hashimoto's disease. Less commonly, TPO are seen in other forms of thyroiditis such as post-partum thyroiditis.

What do the results means?

Elevated levels of TPO suggest inflammation of the gland, typically due to autoimmune Hashimoto's thyroiditis.

It's estimated that TPO antibodies are detectable in approximately 95 percent of patients with Hashimoto's thyroiditis, and 50 to 85 percent of Graves' disease patients. The concentrations of antibodies found in patients with Graves' disease are usually lower than in patients with Hashimoto's disease.

Some patients have elevated TPO antibodies, but are otherwise "euthyroid"—normal T4/T3 and TSH levels. Some research has shown that preventative treatment with levothyroxine may be warranted in those patients, as it may slow down elevation of antibodies, and help prevent progression to overt hypothyroidism.

What is the reference range?*

The reference range is less than 35 IU/mL

Thyroid Stimulating Immunoglobulins (TSI)

Thyroid stimulating immunoglobulin - TSI - is a measurement of antibodies that stimulate the thyroid gland to enlarge (goiter) and release excessive thyroid hormone, resulting in hyperthyroidism. This test is sometimes also called TSH receptor stimulating antibody.

What does it measure?

Thyroid stimulating immunoglobulin - TSI - measures the amount of these antibodies in your bloodstream. 

Ths TSI test is typically done to detect Graves' disease and to evaluate toxic multinodular goiter.

It is also commonly done in a pregnant woman with Graves' disease, during the last three months of pregnancy, to assess the newborn's risk of being born with Graves' disease.

Patients with Hashimoto's disease may have an elevated thyroid-stimulating immunoglobulins (TSI) index

What do the results mean?

Elevated levels can indicate hyperthyroidism due to Graves' disease.

Elevated levels in late pregnancy way also indicate a higher risk of neonatal thyrotoxicosis/hyperthyroidism.

What is the reference range?*

Less than or equal to 1.3 TSI index

Thyroglobulin (Tg)

Thyroglobulin is a protein produced by the thyroid gland, and its presence in the blood is a sign that there is still some thyroid gland present in a person, whether the entire gland, a remnant left after surgery, or a remnant that is still intact after radioactive ablation (RAI).

What does it measure?

Thyroglobulin is tested mainly in thyroid cancer patients, to determine if the cancerous thyroid tissue is producing thyroglobulin prior to treatment, to determine if treatment is working, and to help detect recurrence after treatment. Since most of the commmon thyroid cancers—i.e., papillary and follicular—produce thyroglobulin—increased levels of thyroglobulin may be a sign of cancer recurrence.

Thyroglobulin testing may also be done less commonly, to evaluate hyperthyroidism and Graves' disease.

What do the results mean?

A low level of thyroglobulin is normal in people who don't have thyroid disease.

Elevated levels in someone with thyroid cancer means that the thyroglobulin levels can be monitored later to help detect recurrence.

Thyroglobulin levels should be 0 or very low after thyroid surgery or after radioactive iodine (RAI) treatments. If they are still detectable, additional treatment may be required.

If levels begin to rise after thyroid cancer treatment, that may be a sign that the cancer has recurred.

Thyroglobulin levels that drop may be a sign that Graves' disease and hyperthyroidism treatment is working.

Conditions that cause inflammation of the thyroid gland - i.e., goiter, thyroiditis, or hyperthyroidism—may have elevated thyroglobulin levels. The test is not typically ordered during treatment for these conditions, however.

What is the reference range?*

If you have no thyroid gland, it should be less than 0.1 ng/mL
If you still have a gland, it should be less than or equal to 33 ng/mL

Thyroglobulin Antibodies, Antithyroidglobulin Antibodies (TgAb)

Thyroglobulin antibodies—known as TgAb—are antibodies against thyroglobulin.

They are found in around 10 percent of people with normal thyroid function, and as many as 15 to 20 percent of people with thyroid cancer.

What does it measure?

TgAb can interfere with the thyroglobulin (Tg) results, and so it's important for those with thyroid cancer to have TgAb levels monitored, along with Tg, at regular intervals.

What do the results mean?

When levels are elevated, this can mean that the Tg test levels are not accurate, and that Tg levels may actually be higher than test results show.

What is the reference range?*

The reference range is less than 4.0 IU/mL

*Note: Reference Ranges can differ from lab to lab, and geographically, so refer to your blood test results for the reference range at your laboratory.


Braverman, L, Cooper D. Werner & Ingbar's The Thyroid, 10th Edition. WLL/Wolters Kluwer; 2012.

Garber, J, Cobin, R, Gharib, H, et. al. "Clinical Practice Guidelines for Hypothyroidism in Adults: Cosponsored by the American Association of Clinical Endocrinologists and the American Thyroid Association." Endocrine Practice. Vol 18 No. 6 November/December 2012.

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