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Old 03-29-2010, 11:11 AM   #1
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Thyroid Tests

I found this last week while searching for thyroid answers for my sister. It kind of helped me understand the different tests Pam, Mims, Ilparata and others say to get tested for.

There are 2 types of thyroid hormones easily measurable in the blood, thyroxine (T4) and triiodothyronine (T3). For technical reasons, it is easier and less expensive to measure the T4 level, so T3 is usually not measured on screening tests. Additionally, most diseases impact both T4 and T3 similarly, so T4 is typically measured first.

Please be clear on which test you are looking at. We continue to see a tremendous amount of confusion among doctors, nurses, lab techs, and patients on which test is which. In particular, the "Total T3", "Free T3" and "T3 Uptake tests" are very confusing, and are not the same test.

Thyroxine (Total T4, or TT4) . This shows the total amount of the T4. The total T4 consists of two portions; T4 which is bound to carrier proteins and is inactive, and 'free' or unbound T4 that is available to cells and therefore active. High levels may be due to hyperthyroidism, however technical artifact occurs when the bound/inactive T4 is increased. This can occur when estrogen levels are higher from pregnancy, birth control pills or estrogen replacement therapy. A Free T4 (see below) can avoid this interference.

Free T4 : (FT4) This test directly measures the free T4 in the blood rather than estimating it like the FTI. Because it is a more reliable test that the Total T4, many labs such as ours do the Free T4 routinely rather than the Total T4. High levels suggest hyperthyroidism, and low levels are found in hypothyroidism and chronic illness.

Total T3: (TT3) This is usually not ordered as a screening test, but rather when thyroid disease is being evaluated. T3 is the more potent and shorter lived version of thyroid hormone. Some people with high thyroid levels secrete more T3 than T4. In these (overactive) hyperthyroid cases the T4 can be normal, the T3 high, and the TSH low. The Total T3 reports the total amount of T3 in the bloodstream, including T3 bound to carrier proteins plus freely circulating T3.

Free T3: (FT3) This test measures only the portion of thyroid hormone T3 that is "free", that is, not bound to carrier proteins.

T3 Resin Uptake or Thyroid Uptake. (T3RU) This is a test that confuses doctors, nurses, and patients. First, this is not a thyroid test, but a test on the proteins that carry thyroid around in your blood stream. Not only that, a high test number may indicate a low level of the protein! The method of reporting varies from lab to lab. The proper use of the test is to compute the free thyroxine index.

Free Thyroxine Index (FTI or T7) : A mathematical computation allows the lab to estimate the free thyroxine index from the T4 and T3 Uptake tests. The results tell us how much thyroid hormone is free in the blood stream to work on the body. In contrast to the Total T4 alone, it is less affected by estrogen levels. While this test is less commonly ordered, it is still of use in special situations such as pregnancy.

Thyroid Stimulating Hormone (TSH) : This protein hormone is secreted by the pituitary gland and regulates the thyroid gland. A high level suggests your thyroid is underactive, and a low level suggests your thyroid is overactive. This test can vary by time of day, so a single abnormal measurement does not always mean there is a problem. Also, levels tend to be higher in older people, so it is not uncommon to see mild elevations in people in their 70's or 80's that do not necessarily indicate a medical problem.

Thyroid Antibodies
Antimicrosomal Antibody is also known as Anti-Tissue Peroxidase (or Anti-TPO). It becomes elevated in autoimmune thyroid disease such as Hashimoto's Thyroiditis, or Graves' Disease.
Anti-Thyroglobulin Antibody also becomes elevated in some cases of autoimmune thyroiditis. It tends to be positive more frequently in Graves' Disease than Hashimotos. This test is also commonly used when following patients with thyroid cancer. In thyroid cancer patients, the Thyroglobulin test is used a marker for residual thyroid tissue. However if antibodies such as Antithyroglobulin Antibody are present, this makes the Thyroglobulin test uninterpretable.


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