What does low function of thyroid blood work mean?

Updated November 22, 2016

Many people go for routine blood tests and hear back that the results show low function of the thyroid, but many others have strange symptoms and go get tested specifically for low thyroid function. Low function of the thyroid is called hypothyroidism. Many doctors who are not specialists in endocrine system disorders do not pay attention to all of the results of thyroid blood work, looking only at one basic test. Anyone who believes that he may have hypothyroidism needs to find a doctor who runs at least five blood tests to check on the health and functioning of the thyroid.

Levels of TSH In the Blood

The most common blood test for thyroid disorders is a check on the Thyroid Stimulating Hormone (TSH). This hormone is made by the pituitary gland and is sent to the thyroid to tell it to do its job. If the thyroid is low functioning, the pituitary gland will make too much TSH in an effort to get the thyroid working as it should. Currently, the standard is that a range of .3 to 3 is normal for a thyroid. Anything higher than a 3 on the TSH test means you have hypothyroidism. Some doctors still hold to the idea that anything under 5 is normal for TSH and will try to tell you that you are fine, even if your TSH levels are 4.9. Make sure your doctor gives you the actual number rather than saying "it was normal."

Free Thyroxine (fT4)

Many doctors will also test the body for Free Thyroxine (Free T4). T4 is a hormone made by the thyroid when the thyroid is stimulated by TSH. Free T4 is a form of this hormone that is not bound to proteins in the blood and is able to enter the tissue to control metabolic rates in the tissue. Free T4 should be in a range of 0.7 to 2.0. If your tests results are less than the normal range, you are likely suffering from hypothyroidism.

Free Triiodothyronine (T3)

Triiodothyronine (T3) works with T4 to control metabolic rates in the body. The difference between T3 and T4 is one iodine molecule. Free T3 is T3 that has not bonded with proteins in the blood and is free to work in muscle tissue. One of the tests a doctor will do to check for low thyroid function is the amount of T3 in the blood. The accepted range of normal results is 2.3 to 4.2. If your tests results come back with less than 2.3 T3, you probably have hypothyroidism. Your thyroid is not creating enough T3; therefore, it is a low functioning thyroid.

Total Triiodothyronine (T3)

Triiodothyronine exists in the body in as free T3, regular T3 and reverse T3. A blood test that checks the total of all three types of T3 is called Total T3 test. The normal range on this test is 80 to 220. If your total T3 is less than 80, it is likely that you have hypothyroidism. However, in many people, the levels of total T3 in the body are normal even when the amounts of TSH and T4 indicate a thyroid problem. Sometimes the body still makes enough T3 even if the thyroid does not function properly. The results of the TSH and T4 blood work are much more important in diagnosing a low-functioning thyroid.

Anti-thyroglobulin Antibodies (anti-TG)

The doctor may also check the blood for Anti-thyroglobulin antibody. This test is looking for thryoglobulin that is sometimes found in the blood. This is a protein that attacks the thyroid and can destroy its function. The test for Anti-thyroglobulin antibody is a positive or negative test. Most people will test negative. A positive result means that this antibody is in the blood and it is attacking the thyroid.

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About the Author

R.J. Bowman has a Bachelor's degree in accounting with a minor in English from Pensacola Christian College. After college, she taught English to seventh graders until becoming a mom. At that time, she found freelance writing to be a great way to keep her writing skills sharp.