There are several blood tests that measure how well the thyroid is functioning, including the TSH blood test, the T3 blood test, and the T4 blood test. Abnormal thyroid blood test levels may indicate hypothyroidism, hyperthyroidism or other related conditions.
TSH Blood Test
The TSH test measures the amount of thyroid-stimulating hormone in the blood. It is responsible for how much thyroxine and triiodothyronine are released by the thyroid gland. If the proper amount of these hormones aren't released, certain symptoms may arise. When thyroxine and triiodothyronine levels are high, rapid heart rate, weight loss and difficulty sleeping occur. When the thyroid hormone levels are low, weight gain, dry skin and constipation are some of the symptoms.
The TSH blood test is usually ordered to diagnose a thyroid disorder. It can also be used to monitor the effects of thyroid replacement therapy. Newborns are screened for an underactive thyroid with the TSH blood test.
The TSH blood test involves taking a blood sample from the arm with a needle. For newborn screening, the sample is taken from the heel with a needle prick. No preparation is required for the test, but several things can affect the TSH level. Aspirin and thyroid-replacement therapy drugs will affect the test.
T4 Blood Test
When the TSH test reveals abnormal results, the doctor may order the T4 test. It measures the amount of thyroxine in the blood. There are two types of T4 tests. One that measures the free thyroxine and one that measures the total thyroxine available. The total thyroxine test is affected by proteins that are bound to thyroxine. The free thyroxine test is considered more accurate since it isn't affected by protein levels.
Both types of T4 tests are performed on a blood sample drawn from a vein. The total thyroxine test can be affected by several types of medications including estrogen, birth control pills and aspirin. The free T4 test isn't affected by these drugs.
T3 Blood Test
A third blood test that is used to determine how well the thyroid gland is functioning is the T3 test. It measures the amount of triiodothyronine in the blood. It is ordered when the TSH and T4 test results are abnormal and symptoms indicate an overactive thyroid. The T3 test can measure protein-bound triiodothyronine or free triiodothyronine. The protein that normally binds this hormone is thyroxine-binding globulin.
Like the total T4 test, the total T3 test can be affected by protein levels and certain medications including aspirin, estrogen and birth control pills. The free T3 test is unaffected by these medications.
Results of Thyroid Blood Tests
The TSH, T4, and T3 blood tests will reveal the concentration of thyroid stimulating hormone, thyroxine and triiodothyronine, respectively, in the blood. The normal range for TSH is 0.4 - 4.0 mIU/L (milli-international units per liter). The normal range for T4 is 4.5 to 11.2 mcg/dL (micrograms per deciliter), and the normal range for T3 is 100 to 200 ng/dL (nanograms per deciliter).
An abnormally high TSH result along with normal T4 and T3 results indicate mild hypothyroidism. An abnormally high TSH result along with low T4 and T3 results indicate hypothyroidism.
An abnormally low TSH result along with normal T4 and T3 results indicate mild hyperthyroidism. An abnormally low TSH result along with high T4 and T3 results indicate hyperthyroidism.
If all three blood tests are abnormally low, then the diagnosis is a nonthyroidal illness called pituitary or secondary hypothyroidism.
1. "TSH: The Test." Lab Tests Online. http://www.labtestsonline.org/understanding/analytes/tsh/test.html
2. "T4: The Test." Lab Tests Online. http://www.labtestsonline.org/understanding/analytes/t4/test.html
3. "T3: The Test." Lab Tests Online. http://www.labtestsonline.org/understanding/analytes/t3/test.html
4. "TSH Test." Medline Plus http://www.nlm.nih.gov/medlineplus/ency/article/003684.htm
5. "T4 Test." Medline Plus http://www.nlm.nih.gov/medlineplus/ency/article/003517.htm
6. "T3 Test." Medline Plus http://www.nlm.nih.gov/medlineplus/ency/article/003687.htm