The thyroid gland is a small gland located on the front of the throat. It makes two hormones, known as T3 (triiodothyronine) and T4 (thyroxine). These hormones control metabolism and energy use throughout the body. Because of this, imbalances in thyroid hormones can affect every organ system. Different problems result from levels of the two thyroid hormones being too high or too low. Thyroid issues are more common in women, but can definitely occur in men as well.
In order to know if your thyroid is functioning properly, you’ll need to get thyroid tests. How do doctors check the thyroid?
How is the Thyroid Tested?
There are a few different tests that can be used for detecting thyroid problems and diagnosing thyroid issues. Thyroid hormones are tested by a blood test. Thyroid disorders will produce characteristic patterns of changes on these tests, allowing doctors to diagnose thyroid issues.
Thyroid Stimulating Hormone (TSH) Test
TSH is a hormone that’s released by the pituitary gland, which is located at the base of the brain. It travels through the bloodstream, and stimulates the thyroid gland to produce T3 and T4. When there’s too much T3 and T4 in the body, this will usually suppress TSH, so TSH will become low. When there’s not enough T3 and T4, this will usually stimulate TSH, so TSH will become high. However, there are some situations where the secretion of TSH is actually the cause of the thyroid problem, and then the pattern will be different.
When you get regular screening bloodwork at your annual physical, the TSH test is the most common method for how to check thyroid levels. If the test comes back normal, then it’s assumed that your thyroid is functioning normally. This test is the easiest way to measure thyroid function.
TSH Reflex Test
In some cases, a TSH reflex test is used to check the thyroid. First, the TSH test is performed. If this comes back normal, the test is over. If it’s abnormal, then levels of free T4 (FT4) will then be checked as well. Free T4 is T4 that is not bound to a carrier protein in the blood, and is therefore available to affect your body’s cells.
Thyroid Stimulating Immunoglobulin (TSI)
Thyroid stimulating immunoglobulin (TSI) testing may be used when Graves disease is suspected. This is an autoimmune condition, in which the immune system produces antibodies that mimic TSH. The antibodies mistakenly tell the thyroid to make a lot of T3 and T4, leading to hyperthyroidism. If you have TSI in your blood, then this is an indication that you have Graves disease.
Antithyroid Antibodies Test
There is another type of autoimmune thyroid condition called Hashimoto’s disease. In this condition, the immune system produces antibodies that attack the thyroid gland. Rather than stimulating it, these antibodies interfere with the function of the thyroid, leading to hypothyroidism, or not enough thyroid hormones. The antithyroid antibody test may be used as a diagnostic test for hypothyroidism.
Thyroid Function Test
A full thyroid function test, also known as a full thyroid panel test, is used for testing the thyroid in situations where a doctor suspects, based on your symptoms, that you have a thyroid problem. This will test TSH, free T3, and free T4. Sometimes TSI and antithyroid antibodies are tested for as well. This is how to test the thyroid most thoroughly, checking for all types of thyroid problems.
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Do I need to fast for the TSH blood test?
Traditionally, doctors have said that it’s not necessary to fast for the TSH test. However, there’s some recent research indicating that eating before the test may affect the results. If you’re testing with a home kit, it may be better to perform your test early in the morning while you’re fasting, so that you don’t risk affecting the results.
Does alcohol affect the thyroid blood test?
Alcohol can affect your thyroid hormone levels. When you drink, alcohol blunts your brain’s release of TRH. This is the hormone that the hypothalamus releases in order to tell the pituitary to release TSH. So alcohol will generally lower your TSH level. In addition, because of the low TSH, you will also have low T3 and T4.
What does an abnormal thyroid blood test mean?
In general, in hyperthyroidism, there will be high T4 and T3, and low TSH (because thyroid hormones suppress TSH). There may also be TSI, indicating that Graves disease is the cause.
In hypothyroidism, there will be low T4 and T3. If there’s high TSH, then the thyroid gland itself is the cause. This is known as primary hypothyroidism. There may be antithyroid antibodies, indicating Hashimoto’s disease.
If there’s low TSH with low T3 and T4, then it’s likely that the problem is with the secretion of TSH; the thyroid itself may be normal, but the pituitary gland is not making enough TSH. This is known as secondary hypothyroidism.
If you have an abnormal thyroid blood test, you should discuss it with a doctor. Further testing may be needed to determine the cause of your symptoms, and your doctor will also talk with you about treatment. Treating thyroid disorders is important, both to reduce your symptoms and improve your quality of life, and to protect your long-term health.
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Nair R, Mahadevan S, et al. Does fasting or postprandial state affect thyroid function testing? Indian J Endocrinol Metab. 2014 Sep-Oct;18(5):705–707. doi: 10.4103/2230-8210.139237
Thyroid Tests. National Institute of Diabetes and Digestive and Kidney Diseases. https://www.niddk.nih.gov/health-information/diagnostic-tests/thyroid. Accessed 11 May 2022.
Thyroid Function Tests. American Thyroid Association. https://www.thyroid.org/thyroid-function-tests/. Accessed 11 May 2022.