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If you are new to understanding thyroid labs, or even if you've had a thyroid condition for some time, it can be challenging to know what your doctor is testing and what these markers mean. There are several labs your thyroid doctor will likely use to assess your thyroid function. Still, this article will look specifically at thyroglobulin antibodies, known by its abbreviation TgAB or anti-Tg.
One of the primary ways the immune system protects the body is by producing antibodies. When a foreign invader is in the body, like bacteria, toxins, or viruses, the immune system sends out antibodies to fight it. This response is a sign of a healthy immune system.
Sometimes, the immune system goes rogue and creates antibodies that attack the healthy tissues in your body. Antibodies made to fight your own organs are a sign of autoimmune disease.
Thyroglobulin antibodies are cells made by the immune system to attack thyroglobulin in the thyroid gland. Thyroglobulin is a protein that is created and stored in the thyroid gland to help make thyroid hormones.
Your doctor may check your TgAB levels if they suspect you have an underlying autoimmune condition causing thyroid dysfunction. A doctor may want to see the results of a TGAb test if they see specific clinical findings on your lab work and physical exam, including:
- A goiter
- Inflammation of the thyroid
- Abnormalities in other thyroid function tests like T4 and T3
While this may be a test occasionally used to detect autoimmune thyroiditis, it is more common to use this test to determine if treatment for thyroid cancer has been effective or not. Indeed, providers often use TgAB as a tumor marker because your body will only continue to produce TgAB's if you still have thyroid tissue in your body. Thus, after thyroid cancer treatment, the goal is to have no TgAB's in your lab work after your thyroid has been removed.
The most common way to check for autoimmune thyroiditis is to test for the presence of thyroperoxidase antibodies (TPO antibodies, sometimes known as anti-TPO). Like TgAB, your immune system also creates TPO antibodies to attack your thyroid gland.
Thyroid peroxidase is a key enzyme involved in the production of thyroid hormone. Specifically, the TPO enzyme facilitates the chemical reaction where iodine bonds to the thyroglobulin protein. This critical step helps to create thyroxine (T4).
When your immune system sends out antibodies to disrupt this chemical reaction, it prevents thyroid hormones from being made. Over time, it can lead to chronic inflammation and eventual hypothyroidism and organ failure.
The American Thyroid Association (ATA) recommends your doctor test for either TPO antibodies or TgAB in patients with hypothyroidism because autoimmune disease is most often the culprit.
However, the ATA specifies that the TPO antibody test is more sensitive and specific than the TgAB test for autoimmune thyroid disease like Hashimoto's thyroiditis.
Thyroid-stimulating hormone (TSH)
Thyroid-stimulating hormone (TSH) is the primary marker your doctor will look at to assess your thyroid function. TSH is a hormone released by the pituitary gland that tells your thyroid how much thyroid hormone to make. When thyroid hormone levels are low, the pituitary releases more TSH to signal the thyroid to produce more hormones. Alternatively, when thyroid hormone levels are high, TSH levels fall.
- Low TSH = Hyperthyroidism (overactive thyroid)
- High TSH = Hypothyroidism (underactive thyroid)
Thyroxine (T4) is the primary hormone produced by the thyroid gland and circulated throughout the body. Most lab tests measure your free T4 level, which is the amount of T4 not attached to a protein in your blood.
Triiodothyronine (T3) is another form of thyroid hormone that circulates in your blood. Testing for free T3 in addition to TSH and free T4 can help inform how well your body is converting thyroid hormone to the active form to be used by the body
You can assess your thyroid function in one of two ways. The first way is by seeing a doctor first and having them order bloodwork based on your physical exam and medical history. The second way is to check your thyroid levels before you meet with your doctor so you can talk about your lab results in your consultation.
Most doctors require that you see them first before they order bloodwork unless you have an established relationship. However, it may make more sense to have your labs in hand so you can talk directly about your results and what actions are necessary to help you feel your best.
To have your thyroid labs assessed ahead of time, you can order an at-home thyroid test kit to measure your TSH, free T4, free T3, and looks for TPO antibodies. You also have the option to add on vitamin D and/or reverse T3, which can add value to your discussion with your doctor as well. An at-home thyroid test requires only a finger-prick (instead of a venipuncture blood draw). It delivers comprehensive results to you within five days of the lab receiving your sample.