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What Is Thyroid Function Testing?

A number of blood tests may be used to learn if your thyroid gland is working normally. These thyroid function tests reveal if your thyroid is producing the right amount of thyroid hormones. Thyroid blood tests may be ordered:

  • To check for thyroid disease before it causes symptoms, especially in newborns
  • To diagnose an underactive or overactive thyroid
  • To determine if an underlying disorder, such as Hashimoto’s thyroiditis, Graves’ disease, or thyroid inflammation, may be causing an underactive or overactive thyroid
  • To evaluate a lump on the thyroid gland, called a thyroid nodule
  • To evaluate an enlargement of the thyroid gland, called a goiter
  • To monitor treatment for thyroid disorders

Understanding how your thyroid is functioning enables your doctor to treat a thyroid disorder if one is present.

The Role of Thyroid Function Tests

Thyroid function testing can be used in several ways, including:

  • Screening: Screening means testing for disease in the absence of symptoms. All newborns in the United States are routinely screened for an underactive thyroid. Some adults may also be screened for thyroid disorders, although not all experts agree that this is beneficial.
  • Diagnosis: Diagnostic thyroid function testing aims to find the cause of a person’s symptoms. Thyroid function testing can reveal whether the thyroid gland is functioning normally or abnormally and what specific disorder a person may have.
  • Treatment monitoring: Thyroid blood tests provide valuable information about whether treatment for thyroid disorders is working as it should or if it needs adjustment.

Who should get testing?

Thyroid function testing is often ordered when patients have symptoms of a thyroid disorder. Testing can assist with diagnosing or ruling out thyroid problems as a cause of a patient’s symptoms.

Many of the symptoms of common thyroid problems are nonspecific, meaning that there is a wide range of diseases and disorders which may cause them. Thyroid function testing may be included with other tests to evaluate a patient who is having trouble with a nonspecific symptom like fatigue, depression, or difficulty becoming or staying pregnant.

The table below provides examples of symptoms associated with thyroid disorders:

Thyroid Disorders Possible Symptoms
Underactive Thyroid or Hypothyroidism; Hashiomoto’s Thyroiditis Fatigue, weight gain, cold intolerance, dry skin and hair, fertility problems, depression, constipation
Overactive thyroid or Hyperthyroidism; Graves’ Disease Weight loss, irritability, anxiety, insomnia, mood swings, heat intolerance, rapid heartbeat, frequent bowel movements
Thyroid Nodule Visible lump in neck, hyperthyroid symptoms, hypothyroid symptoms, hoarseness, pain
Goiter Thyroid gland enlargement, hypothyroid or hyperthroid symptoms, hoarseness, difficulty swallowing, pain
Thyroid Cancer A lump or swollen glands in the neck, hoarseness, pain, difficulty swallowing

In addition to being used for diagnosis, thyroid function tests may be performed to screen for thyroid disease in patients who have no symptoms. Newborn infants are routinely screened for hypothyroidism shortly after birth.

Screening for thyroid disease in adults is controversial. Some experts recommend screening certain groups who are at higher risk of having an underactive thyroid. These risk factors may include:

  • Age
  • Family history of thyroid disease
  • Personal history of type I diabetes
  • Personal history of autoimmune disease
  • Personal history of radiation to the head and neck

Other experts do not recommend routine screening for thyroid disorders in adults. The United States Preventive Services Task Force, which issues screening recommendations, has concluded that there is currently not enough evidence to assess the benefits and drawbacks of  screening for thyroid disorders.

Thyroid function tests are also used for treatment monitoring. Patients with known thyroid disorders will have periodic thyroid function testing to ensure their treatment is effective.

Getting test results

Typically, your doctor will interpret several thyroid function tests results along with your symptoms and health history to determine whether a thyroid disorder is present.

It often takes a few business days before the results of your thyroid function test(s) are available. You may receive the results by mail or through an online health portal, or your doctor may schedule an appointment to review the results with you and discuss next steps.

Types of Thyroid Function Tests

A number of blood tests may be used to screen for and diagnose thyroid disorders.

The table below lists the thyroid function tests that may be used for screening and diagnosis:

Test Name What it Measures
Thyroid-stimulating hormone (TSH) A pituitary hormone that stimulates the thyroid gland and reflects thyroid function
Free Thyroxine (T4) A thyroid hormone that is not bound to protein
Total T4 A thyroid hormone that is both bound to protein and unbound, or free
Free Triiodothyronine (T3) A thyroid hormone that is not bound to protein
Total T3 A thyroid hormone that is both bound to protein and unbound, or free
Thyroglobulin (Tg) A protein made by healthy thyroids and by certain thyroid cancers

Often, doctors will begin with a TSH test. If it is abnormal, more tests will be ordered to learn more about how the thyroid is functioning. Many labs offer reflexive tests, meaning that if TSH is found to be abnormal, another test such as free thyroxine (T4) will automatically be performed using the same blood sample.

Thyroid antibody tests may also be used to diagnose thyroid disorders. Antibodies are made by the immune system and target specific proteins on cells. The following antibodies may be measured to determine whether an autoimmune disease, such as Hashimoto’s thyroiditis or Graves’ disease, is affecting thyroid function:

  • Thyroid peroxidase antibody (TPOAb)
  • Thyrotropin receptor antibody test (TRAb)
  • Antithyroglobulin antibody test (TgAb)

In many cases, patients will also have imaging tests such as an ultrasound, thyroid scan, or a radioactive iodine uptake in order to establish their diagnosis.

Patients who have been previously diagnosed with a thyroid disorder will typically have thyroid function tests for treatment monitoring. The selection of tests depends on the disorder being treated. Common treatment monitoring tests include:

  • TSH testing is used to monitor patients with hypothyroidism and hyperthyroidism
  • Free T4 and sometimes total T3 tests are used to monitor patients undergoing active treatment for hyperthyroidism. Measuring TRAb may be used for patients whose hyperthyroidism is caused by Graves’ disease.
  • Tg is a test used to monitor patients after treatment for thyroid cancer.

Getting Thyroid Function Testing

Thyroid function tests are typically ordered by a primary care physician, endocrinologist, or other medical professional. Thyroid function tests are most often performed in a doctor’s office, clinic, medical lab, hospital, or other medical setting.

At-home testing

Some thyroid gland tests are available as at-home tests. Commercially available test kits enable testing of the most commonly used tests, such as thyroid-stimulating hormone (TSH) and thyroid panels with TSH, free T3, and T4. Some at-home thyroid panels also include thyroid antibody tests.

Diagnosis of thyroid disorders involves interpreting multiple thyroid function tests along with individual patient factors. It is important to discuss concerns about thyroid function and results of at-home thyroid function tests with a doctor.

Abnormal test results obtained on at-home tests, especially thyroid antibody tests, will likely need to be retested by a doctor for confirmation.

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