Blood tests for thyroid function (TSH, total T4, free T3, TSI, and others) are an important part of the diagnosis and treatment of thyroid diseases. Figuring out what the different names and numbers mean can be difficult, but taking the time to study them can help you better manage your illness.
A test will provide useful information about a person's thyroid health, but multiple tests are usually required to get a complete picture. By comparing thyroid test results, your doctor can determine if you have hypothyroidism (low thyroid function), hyperthyroidism (excessive function), or an autoimmune disorder such as Graves' disease .
This article looks at the various thyroid tests and the values you may see in their results. It also provides a general idea of why many health care providers prefer some thyroid tests, but other doctors may disagree and choose another option.
How to work with the thyroid team
The purpose of thyroid tests is to determine its function. It does this by studying substances produced by the thyroid gland and other organs that control the function of the thyroid gland.
For example, the pituitary gland in the brain produces a hormone known as thyroid stimulating hormone (TSH). It controls the amount of the hormones triiodothyronine (T3) and thyroxine (T4) produced by the thyroid gland. Taken together, these values can tell you a lot about how well or poorly your thyroid is working.
Thyroid function tests generally detect six of these key substances in the blood, including hormones, proteins, and immune system agents known as antibodies .
Thyroid stimulating hormone (TSH)
Thyroid stimulating hormone (TSH) acts as a mediator in the thyroid gland. If the pituitary gland detects too little thyroid hormone in the blood, it will make more TSH. This, in turn, causes the thyroid to produce more hormones. When the pituitary gland detects too much thyroid hormone, it slows down the production of TSH and signals a slowdown in the thyroid gland.
Thyroxine (T4) acts as a storage hormone. Until it becomes T3, it is idle. It must go through a process known as monodeiodination , in which it loses an iodine atom to become triiodothyronine (T3). The T4 test measures two key values:
- Total T4 is the total amount of thyroxine in the blood. This includes T4, which is protein bound, and T4, which is not. The difference affects its ability to penetrate tissues.
- Free T4 is a non-protein bound type and is considered the active form of T4.
Triiodothyronine (T3) is an active thyroid hormone produced by the conversion of T4 in the chemical process described above. T3 tells cells to produce more energy and has many other functions. Three different tests are used to measure T3:
- Total T3 is the total amount of triiodothyronine in the blood, whether it is bound to protein or not.
- Free T3 is not bound to proteins and is considered the active form of T3.
- Reverse T3 is an inactive "mirror image" of T3 that attaches to thyroid receptors but cannot activate them.
Thyroglobulin (TG) is a protein produced by the thyroid gland. It is primarily measured as a tumor marker in the treatment of thyroid cancer . A high Tg level is a sign that cancer cells are still present after surgery to remove the thyroid gland ( thyroidectomy ) or radioactive ablation therapy (RAI) to destroy the gland.
By comparing baseline values with later results, the Tg test can tell healthcare providers if cancer treatments are working. It also tells them if the person is still in remission from the cancer or if there are signs of cancer recurrence.
There are some thyroid disorders caused by autoimmune diseases . These diseases are caused by the immune system mistakenly attacking normal thyroid cells. During this process, antibodies are produced, which then "pair" with antigens or receptors on the target cell.
There are three common antibodies associated with autoimmune thyroid disease:
- Thyroid peroxidase antibodies (TPOAb) are detected in 95% of people with Hashimoto's disease and in about 70% of people with Graves' disease, two types of autoimmune thyroid diseases. High levels of TPOAb are also seen in women with postpartum thyroiditis .
- Antibodies against the thyroid stimulating hormone receptor (TRAb) are found in 90% of Graves disease cases, but only in 10% of Hashimoto cases.
- Thyroglobulin antibodies (TgAb) are produced by your body in response to the presence of thyroglobulin . They are found in 80% of people with Hashimoto's disease and between 50% and 70% of people with Graves' disease. One in four people with thyroid cancer has elevated TgAb levels.
Some thyroid tests look for antibodies, including TgAb. It is often present in the most common types of thyroid-related autoimmune diseases, but it also occurs in about 25% of people with thyroid cancer.
Proteins that bind to the thyroid gland.
Checking the blood levels of proteins that bind to T3 and T4 can help healthcare providers understand thyroid problems in people whose glands are still functioning normally. Three common tests:
- Thyroid transport globulin (GTG) measures the levels of a protein that carries thyroid hormones in the blood.
- T3 resin uptake (T3RU) determines the percentage of TBG in a blood sample.
- The Free Thyroxine Index (FTI) is an older test method. It uses the total T4 multiplied by the T3RU to see if a person has an overactive or low thyroid function.
Control test ranges
The results of these blood tests are indicated with a reference range . This range provides the expected values that healthcare professionals typically see when testing everyone.
Generally speaking, anything between the upper and lower edges of this range can be considered normal. The number of tests near the upper or lower limit is at the limit, and anything outside these limits is considered abnormal.
In the middle of the reference range is the "sweet spot", called the optimal reference range , in which thyroid function is considered ideal.
It is important to note that the control ranges and units used may differ from laboratory to laboratory. To ensure consistent test results, try to use the same lab for each test.
interpretation of results
Test results are numbers until they are interpreted to be meaningful. The only test that is likely to provide more information is TSH. When used with free T3 and T4 tests, TSH can even indicate the cause of any problems.
Thyroid Discussion Guide for Healthcare Professionals
Get our printed guide to your next doctor's appointment to help you ask the right questions.
TSH values outside the "normal" range suggest that thyroid disease is working. Values at or near the upper or lower range may indicate a subclinical disorder without any symptoms.
According to the recommendations of the American Association of Clinical Endocrinologists (AACE) and the American Thyroid Association (ATA), the TSH value:
- 4.7 to 10 milliunits per liter (mU / L) is a subclinical problem with low thyroid levels.
- More than 10 mU / L is a symptomatic problem with low thyroid levels.
- 0.1 to 0.5 mU / L is a subclinical overactive thyroid problem.
- Less than 0.1 mU / L is a symptomatic overactive thyroid problem.
Interpretations T3 and T4
By comparing TSH values to T4, your healthcare provider can see a clearer picture of your thyroid problem. For instance:
- Normal TSH and normal T4 indicate normal thyroid
- Low TSH and high T4 levels generally signify an overactive thyroid gland.
- High TSH and low T4 mean an underactive thyroid gland.
- Low levels of TSH and T4 mean low thyroid function due to another problem, such as the pituitary gland or the hypothalamus of the brain.
A low T3 value with a high TSH value is a diagnostic test for low thyroid function. In contrast, a low TSH value with a high T3 value is the opposite and means an overactive thyroid gland.
Other thyroid tests can be part of a routine thyroid exam or used as needed. Some have specific goals. The rest is used to check or evaluate possible causes.
- RT3 tests can help find problems with thyroid control. For example, people with euthyroid disease syndrome (ESS) have abnormal thyroid hormone levels due to a non-thyroid disorder.
- TG tests can also predict long-term treatment results. A study published in the journal Thyroid shows that only 4% of people with a TG level below 1 will recur after five years.
- TPOAb tests can help confirm Hashimoto's disease if your TSH is high but your T4 levels are low.
- TRAb test help diagnose Graves disease, but can confirm the diagnosis of toxic multinodular goiter . The test is also used during the last three months of pregnancy to check the risk of having a baby with Graves' disease or an overactive thyroid gland.
- TgAb tests help diagnose an autoimmune disease, but it can also help clarify the results of any tests for cancer treatment. This is because TgAb can interfere with Tg readings in some people.
There are many thyroid function tests available, and their names and lab value ranges can be confusing. They are all important, although often for different reasons. More importantly, these are the results, a general understanding of what these results mean and how they will help guide the treatment of thyroid-related conditions.
Your test results, symptoms, medical history, and current health are factors when a doctor checks how well your thyroid is working. Test results are based on a common standard for each test, each of which assesses thyroid function differently.
However, it's not just about results. Many people find it helpful to know what these tests are and what has happened or will happen. This is a way to ensure that both the healthcare provider and the patient are on the same page and know how to discuss test results or tests that they think might be necessary.