Diagnosing Thyroid Disease With Blood Tests

Blood tests for thyroid disease, including TSH, Free T4, Free T3, antibodies
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Diagnosing thyroid disease is a process that can incorporate numerous factors, including a clinical evaluation, various blood tests, imaging tests, biopsies, and other tests. In this article, you’ll learn more about the various blood tests that are used as part of thyroid disease diagnosis, treatment, and ongoing follow-up and management.

TSH Test

The most common thyroid test is the blood test that measures the amount of thyroid-stimulating hormone (TSH) in your bloodstream. The test is sometimes called the thyrotropin-stimulating hormone test.

TSH that is elevated, or above normal, is considered indicative of hypothyroidism. TSH that is “suppressed” or below normal, is considered evidence of hyperthyroidism.

Currently, the reference range runs from around 0.5 to 5.0. Levels above 3.0 are evidence of possible hypothyroidism, and levels below 0.5 are evidence of possible hyperthyroidism. Note that the reference range is different for women who are pregnant.

Free T4 / Free Thyroxine

Thyroxine, a hormone produced by the thyroid, is also known as T4. Free T4 measures the free, unbound thyroxine levels in your bloodstream. Free T4 is typically elevated in hyperthyroidism, and lowered in hypothyroidism.

Free or unbound T4 levels represent the level of hormone available for uptake and use by cells. Bound levels represent hormone that may not all be immediately available, because it is affected by other drugs, illness, and physical changes such as pregnancy. Because the free levels of T4 represent that actual amount of available hormone, free T4 is thought to better reflect the patient's hormonal status than total T4 (below).

Total T4/Total Thyroxine/Serum Thyroxine

This test measures the total amount of circulating thyroxine in your blood. A high value can indicate hyperthyroidism, a low value can indicate hypothyroidism. Total T4 levels can be elevated due to pregnancy, and other high estrogen states, including use of estrogen replacement or birth control pills.

Free T3 / Free Triiodothyronine

Triiodothyronine is the active thyroid hormone, and is also known as T3. Free T3 measures the free, unbound levels of triiodothyronine in your bloodstream. Free T3 is considered more accurate than Total T3. Free T3 is typically elevated in hyperthyroidism, and lowered in hypothyroidism.

Total T3/Total Triiodothyronine

Total T3 is typically elevated in hyperthyroidism, and lowered in hypothyroidism.

T3 Resin Uptake (T3RU) / T7 

When a blood test is done with a T3 and T4, the T3 resin uptake (T3RU) test is sometimes referred to as the T7 test. This test measures the amount of unsaturated binding sites on the transport (binding) hormones. Elevated T3RU is more commonly seen with hyperthyroidism.


Thyroglobulin (Tg) is a protein produced by the thyroid. Tg levels are low or undetectable with normal thyroid function but can by elevated in thyroiditis, Graves’ disease, or thyroid cancer. Monitoring of Tg levels is frequently used to evaluate the effectiveness of treatment for thyroid cancer and to monitor for thyroid cancer recurrence.

Reverse T3

When the body is under stress, instead of converting T4 into T3 - the active form of thyroid hormone - the body conserves energy by making what is known as Reverse T3 (RT3), an inactive form of the T3 hormone. The value of RT3 tests in diagnosis is controversial, as some practitioners believe that the body continues to manufacture RT3 instead of active T3, resulting in clinically significant deficiencies in the active T3 thyroid hormone.

Thyroid Peroxidase (TPO) Antibodies (TPOAb) / 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).

These antibodies work against thyroid peroxidase, an enzyme that plays a part in the T4-to-T3 conversion and synthesis process. TPO antibodies can be evidence of tissue destruction, such as Hashimoto's disease, and less commonly, in other forms of thyroiditis such as post-partum thyroiditis.

It is 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.

Thyroglobulin Antibodies / Antithyroglobulin Antibodies

Testing for thyroglobulin antibodies (also called antithyroglobulin antibodies) is used to explore autoimmune causes of thyroid conditions. If you have already been diagnosed with Graves' disease, having high levels of thyroglobulin antibodies means that you are more likely to eventually become hypothyroid. Thyroglobulin antibodies are positive in about 60 percent of Hashimoto's patients and 30 percent of Graves' patients.

Thyroid-Stimulating Immunoglobulins (TSI) / TSH Stimulating Antibodies (TSAb)

TSH receptor antibodies (TRAb) are seen in most patients with a history of, or who currently have, Graves' disease. Testing is usually done for a specific type of stimulating TRAb that goes by several different names, including:

  • Thyroid-Stimulating Immunoglobulins (TSI)
  • TSH stimulating antibodies (TSAb)

Thyroid-stimulating immunoglobulins (TSI) can be detected in the majority of people with Graves' disease, and estimates find them 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.)

Less commonly, some people with Hashimoto's disease also have these antibodies, and this can cause periodic short-term episodes of hyperthyroidism.

When monitoring TSI, elevated levels may help predict relapse of Graves' disease, and lowered TSI levels may indicate that Graves' disease treatment is working.

TSI monitoring is especially important during pregnancy, because elevated levels, particularly in early pregnancy and during the third trimester, are a risk factor for fetal or neonatal thyroid dysfunction. The mother's TSI antibodies can transfer to the unborn baby via the placenta, making a baby hyperthyroid in utero, or at birth. Research has shown that as many as 10 percent of pregnant women with elevated TSI deliver babies with transient hyperthyroidism.


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