Understanding Your Thyroid Blood Tests and Results

TSH, T4, Free T4, T3, Free T3, Reverse T3, Antibodies, and Other Tests

Doctor administering blood test
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Blood tests for thyroid function are an important part of the process for diagnosing thyroid disease and treating thyroid conditions. Here is a summary to help you gain a better understanding of the key thyroid blood tests, what they are measuring, what the results mean, and the impact on the diagnosis and management of your thyroid condition. You can then delve into the details of each and gain a better understanding of what it all means.


Thyroid Test

Reference Range

TSH (Thyroid Stimulating Hormone)0.5-4.70 µIU/mL
Total T4 (Thyroxine)4.5-12.5 µg/dL
Free T4 (Free Thyroxine)0.8-1.8 ng/dL
Total T3 (Triiodothyronine)80 -200 ng/dL
Free T3 (Free Triiodothyronine)2.3- 4.2 pg/mL
RT3 (Reverse T3/Reverse Triiodothyronine)10-24 ng/dL
TPOAb (Thyroid Peroxidase Antibodies)0-35 IU/mL
TSI (Thyroid-stimulating Immunoglobulins)0-1.3
Tg (Thyroglobulin)

No thyroid gland: 0-0.1 ng/ml.
Still have a gland: 0-33 ng/mL

TgAb (Thyroglobulin Antibodies)0-4.0 IU/mL

TSH (Thyroid Stimulating Hormone) Test

Other Names: Serum Thyrotropin

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

Measures: The TSH test measures the amount of TSH in the bloodstream.

Reference Range: 0.5-4.70 µIU/mL. (Some laboratories are 0.3 to 4.5, or other similar ranges.)

Conventional Interpretation: Above range, and under 10 µIU/mL is "subclinical" hypothyroidism, over 10 µIU/mL is overt hypothyroidism. Under 0.1 to 0.5 µIU/mL is considered evidence of subclinical hyperthyroidism, less than 0.1 may be overt hyperthyroidism.

 "Normal" TSH levels are considered to exclude hypothyroidism or hyperthyroidism.

Integrative Interpretation: Levels above 1.5 to 2.0 µIU/mL may be indicative of thyroid dysfunction. Optimal level is from 1.0 to 1.5 µIU/mL.

Controversies: There are several controversies about the TSH test and what it means.

  • Among endocrinologists, there is a controversy as to what the TSH reference range should be. Typically, it runs from around 0.4/0.5 to 4.5 or so. But some endocrinologists believe that the top end of the reference range should be 3.0.
  • Among endocrinologists, there is a disagreement over whether levels under 10.0 should be treated. Some doctors believe that these levels warrant treatment with thyroid hormone replacement drugs—while others consider that "subclinical hypothyroidism"—and that treatment is only needed after levels exceed 10.0.
  • Some research suggests that even when the TSH falls within the reference range, if a patient has Hashimoto's antibodies, treatment is warranted.
  • Integrative physicians believe that TSH is only one in many diagnostic and management factors for thyroid patients. They refer to overreliance on the TSH as Tyranny of the TSH.
  • Integrative physicians believe that the true measurement of thyroid function is the actual, available thyroid hormones circulating in the bloodstream—Free T4 and Free T3.

    T4/Thyroxine and Free T4/Free Thyroxine

    About: 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.

    Measures: Total T4 measures the total amount of thyroxine circulating in the bloodstream. Free T4 measures the available, unbound amount of thyroxine in the bloodstream.

    A healthy thyroid gland produces mainly thyroxine, and the thyroxine needs to be converted into triiodothyronine (T3) in order to deliver oxygen and energy to cells.

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

    Conventional Interpretation: Many conventional physicians do not test Total T4 or Free T4. However, in some cases, along with elevated TSH, Total T4 or Free T4 levels that are below the reference range are considered evidence of hypothyroidism. Along with low/suppressed levels of TSH, Total T4 or Free T4 levels that are above the reference range are considered evidence of hyperthyroidism.

    Integrative Interpretation: For diagnosis and treatment of hypothyroidism, levels in the top half of the reference range are considered optimal and evidence of proper thyroid function.

    Controversies: Many conventional endocrinologists only use TSH tests in diagnosis and management of thyroid conditions, and as a result, do not test for total or free T4 levels.

    T3/Triiodothyronine and Free T3/Free Triiodothyronine

    About: Triiodothyronine (T3) is the active thyroid hormone. A healthy thyroid gland produces some triiodothyronine—the active thyroid hormone. The rest is the result of the conversion of thyroxine into triiodothyronine.

    Measures: The total T3 test measures the total amount of triiodothyronine circulating in the bloodstream. Free T3 measures the free, unbound levels of the hormone triiodothyronine available for use by the body.

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

    Conventional Interpretation: Many conventional physicians do not test Total T3 or Free T3. However, in some cases, along with elevated TSH, Total T3 or Free T3 levels below the reference range are considered evidence of hypothyroidism. Along with low/suppressed levels of TSH, Total T3 or Free T3 levels that are above the reference range are considered evidence of hyperthyroidism.

    Integrative Interpretation: For diagnosis and treatment of hypothyroidism, levels in the top half of the reference range are considered evidence of sufficient thyroid function, and levels in the top 25th percentile of the reference range are considered optimal. In the integrative view, sub-optimal levels may warrant treatment with thyroid hormone replacement medications, or a medication that specifically includes T3.

    Controversies: 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.

    Because the free levels of T3 represent immediately available hormone, free T3 is thought by some integrative practitioners to best reflect a patient's hormonal status, compared to TSH and/or total T3.

    RT3/Reverse T3/Reverse Triiodothyronine 

    About: Reverse T3 is a form of T3 that is inactive and is produced in higher amounts during times of stress.

    Measures: An inactive, useless form of T3 that is produced when the body is under stress.

    Reference Range: Typically 10-24 ng/dL

    Conventional Interpretation: This test is rarely performed by conventional physicians, who see no value in this measurement.

    Integrative Interpretation: Integrative physicians and those who some doctors who focus on optimal hormone balance consider elevated RT3 or an RT3/T3 ratio imbalance to be a key sign of an underactive or dysfunctional thyroid. They believe that the reverse T3 should fall in the lower half of the normal range.

    Controversies: Reverse T3 is a controversial test. Conventional physicians for the most part dismiss the value of RT3 measurement in diagnosing, treating and managing hypothyroidism. Integrative physicians and those who focus on optimal hormone balance, however, consider elevated RT3 to be a key sign of an underactive or dysfunctional thyroid. 

    TPOAb/Thyroid Peroxidase Antibodies

    Other Names: Antithyroid Peroxidase Antibodies

    About: Thyroid peroxidase (TPO) antibodies, also abbreviated as TPOAb, are antibodies that develop as a result of an autoimmune attack on the thyroid gland. They target the gland, and usually result in destruction of the gland over time. TPOAb antibodies attack thyroid peroxidase, an enzyme that plays a role in the conversion of T4 to T3. Elevated TPOAb levels 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.

    Measures: This test measures the level of TPO antibodies.

    Reference Range: The reference range is from 0—35 IU/mL

    Conventional Interpretation: If TPOAb levels fall within the reference range, they are considered normal. This does not entirely rule out Hashimoto's disease but makes it far less likely. Elevated TPOAb levels suggest inflammation of the gland, typically due to autoimmune Hashimoto's thyroiditis or other forms of thyroiditis.

    It's estimated that TPOAb 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. In the conventional view, however, elevated TPOAb require no treatment unless accompanied by overt hypothyroidism or hyperthyroidism.

    Integrative Interpretation: Some patients have elevated TPOAb, but are otherwise "euthyroid,” with 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.

    Controversies: Many endocrinologists do not believe in testing for TPOAb, preferring instead to base thyroid diagnosis and treatment management on the TSH test results alone.

    TSI/Thyroid-stimulating Immunoglobulins

    About: Thyroid stimulating immunoglobulin—TSI—are antibodies that stimulate the thyroid gland to enlarge and release excess thyroid hormone, resulting in hyperthyroidism. This test is sometimes also called TSH receptor stimulating antibody.

    Measures: The TSI test measures the circulating level of these antibodies in the bloodstream.

    Reference Range: Less than or equal to 1.3

    Conventional Interpretation: TSI levels are elevated in as many as 75 to 90 percent of Graves' disease patients. The higher the levels, the more active the Graves' disease is thought to be. (The absence of these antibodies does not, however, rule out Graves' disease.) Note: some people with Hashimoto's disease also have these antibodies, and this can cause periodic short-term episodes of hyperthyroidism.

    The 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 hyperthyroidism or Graves' disease.


    About: Thyroglobulin (Tg) is a protein produced by the thyroid gland, and its presence in the blood is a sign that a patient still has some thyroid gland—whether the entire gland or a remnant left after surgery or radioactive ablation (RAI).

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

    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

    Conventional Interpretation:  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 cancer has recurred.

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

    TgAb/Thyroglobulin Antibodies

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

    Measures:  The TgAb test measures the level of these antibodies circulating in the bloodstream.

    Reference Range: The reference range is less than 4.0 IU/mL

    Conventional Interpretation:  Elevated TgAb levels 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. TgAb levels are also elevated in about 60 percent of Hashimoto's patients and 30 percent of Graves' patients. If you have already been diagnosed with Graves' disease, having elevated TgAb levels also means that you are more likely to eventually become hypothyroid.

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

    A Word From Verywell

    Reference ranges and units of measurement used can vary from lab to lab. Always determine the specific reference ranges and test values at the laboratory where your tests are conducted. 

    Some doctors or their office staff call to tell you your medical test results. You may hear "your results were fine," or "your tests were normal" as a test result. This is not enough information. Always ask for an actual copy of any medical test results, including thyroid tests. Particularly with thyroid blood tests, you need to know your actual levels, as well as the reference range, in order to advocate for the best possible care for your thyroid condition. 

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