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Blog / Thyroid Health

What is a Thyroid Function Test?

Nov. 16 2019 by Sheherzad Raza Preisler Blog Editor, PR, & Social Media Coordinator
What is a Thyroid Function Test?

The thyroid is a butterfly-shaped organ with three major regions: a left lobe, right lobe, with an isthmus connecting the two; the lobes are shaped like a butterfly’s wings, while the isthmus is shaped like its body. Functioning as a gland, the thyroid produces and secretes the hormones T3 and T4, which are instrumental in determining our basal metabolic rate (or how much energy we consume at rest), aid in tissue development from the fetal stage through early childhood, and are essential in the early development of the nervous system and its continued maintenance in adulthood. T3 and T4 are also inextricably linked to our reproductive hormones, and can influence our fertility, sex drive, and other reproductive behaviors.

When T3 and T4 are downregulated (aka produced too little), you could develop a condition known as hypothyroidism. This could lead to a number of symptoms, such as joint pain. Another side effect could be hair loss; it may also be a symptom if T3 and T4 are upregulated in your body (hyperthyroidism). 

Physicians will use a collection of assessments called thyroid function tests to see how well your thyroid is functioning, as well as to find the root cause of issues you may be having, such as hypothyroidism or hyperthyroidism. These tests help your doctor diagnose a host of other thyroid-related issues as well, such as Graves’ disease, Hashimoto’s disease, thyroid cancer, and thyroid nodules

Thyroid function tests generally check for levels of T3, T4, as well as various thyroid-specific antibodies circulating in the blood. There are four major types of thyroid function tests:

  1. TSH test: physicians often perform this test first. TSH is a hormone made by the pituitary gland (located in the brain). It tells your thyroid how much T3 and T4 it should produce. If you have low TSH, you likely have hyperthyroidism; if you have high TSH, you likely have hypothyroidism. If your TSH test comes back abnormal, you’ll need at least one more thyroid function test to hone in on the root cause of your thyroid issue.
  2. T4 tests: Low levels of T4 mean you could have hypothyroidism, while high levels of it could mean you have hyperthyroidism. However, abnormal results could also not be due to thyroid issues, as some severe illnesses or other medications (such as oral contraceptives and asthma medications) could skew your results.
  3. T3 test: Your physician may order this test if they think you could have hyperthyroidism even if your T4 level is normal. This is because sometimes T4 could be normal while T3 is high.
  4. Thyroid antibody tests: These tests could help diagnose assorted autoimmune thyroid disorders, including Hashimoto’s disease (the most common culprit behind hypothyroidism) and Graves’ disease (the most common origin of hyperthyroidism). This is because thyroid antibodies are made in inordinately high amounts when your immune system is attacking the thyroid gland by mistake. Often times, your physician may order these tests to follow-up other abnormal thyroid function tests.

Your doctor may follow up thyroid function tests with imaging tests, such as ultrasounds, thyroid scans, or radioactive iodine uptake tests.