Thyroid Function Test

A thyroid function test is the most accurate method of diagnosing thyroid disorders and keeping track of their management. Your doctor will take the time to interpret the results of this blood test and then explain them to you. He will work with you to determine your treatment based on the thyroid test results.

What Are Thyroid Function Tests?

The thyroid function test includes multiple blood tests that take a look at the degree to which your thyroid gland is functioning. Typically, these tests will look at your T3, T4, and TSH levels. The portion of the T4 and T3 that is typically measured is the active or “free” part, known as FT4 and FT3.

Why Do We Test These Hormones?

Your thyroid is butterfly-shaped gland that is fairly small and in the lower-front area of the neck. Your doctor will feel the thyroid by pressing gently against the throat with his fingers. It is your thyroid that regulates many crucial body processes, such as mood, energy generation and metabolism.

The hormones tested by the standard thyroid function test include two major hormones that this gland produces: T3 (triiodothyronine) and T4 (thyroxine). The hypothalamus and the pituitary, two parts of the brain, control the activity of the thyroid. Input concerning various bodily functions is sent to the hypothalamus and if this organ notices that T3 and/or T4 levels are low or your body’s metabolic rate has dropped, it will release TRH (thyrotropin-releasing hormone). This hormone goes to the pituitary gland via connecting blood vessels and then stimulates the gland to secrete TSH (thyroid-stimulating hormone). And then TSH goes to your thyroid and makes the cells produce more T4 and T3. These hormones then enter the bloodstream to increase cells' metabolic activity. Due to their key roles, T3, T4, and TSH are the most important hormones to measure in a thyroid test.

In cases when the thyroid gland produces insufficient quantities of the hormones, you may experience depression, lack of energy and weight gain due to hypothyroidism. In other cases, the thyroid may produce too many hormones, known as hyperthyroidism. This is accompanies by weight loss, tremors, a feeling of a “high” and increased anxiety.

When Should You Have Thyroid Function Tests?

There are specific situations in which you should schedule a thyroid function test or ask your doctor for advice. These include when you experience:

  • Symptoms associated with under- or over-active thyroid
  • Early pregnancy or planning history with personal or family history of thyroid disorders, type 1 diabetes or postpartum thyroiditis
  • Feelings of being unwell following giving birth
  • A family history with autoimmune disorders
  • Low libido, recurrent miscarriage, abnormal menstrual cycles or fertility issues
  • Osteoporosis
  • High cholesterol
  • Fast or irregular heartbeat
  • Thickening or swelling of the neck

You will need to have at least annual blood tests to check TSH levels and other thyroid-related hormones if you:

  • Are treated with lithium or amiodarone
  • Have irradiation of the neck or head following surgery for cancer
  • Previously treated an overactive thyroid
  • Have a thyroid disorder

Regular tests are also important for those with Addison’s disease, Turner syndrome and Down’s syndrome as well as anyone over 65.

How Is the Thyroid Function Test Performed?

Before you take a thyroid function test, your doctor will ask whether you are pregnant or taking any medications as both factors can influence the results.

Your doctor’s office or lab will draw your blood as an outpatient procedure. They will either have you lie or sit down comfortably and expose one arm. The technician will tightly tie a rubber band around the upper arm so your veins swell up with blood. He will then insert a needle underneath your skin, entering the vein. The technician will then collect blood into test tubes which are sent to be analyzed by a laboratory. He will remove the needle, applying pressure to your puncture wound to stop the bleeding. At last, he will put a bandage on your wound and you can go back to normal activities.

Sometimes your doctor may order a follow-up test, such as an ultrasound or thyroid uptake test if the blood test indicates unusual thyroid gland activity.

Interpretation of Thyroid Function Test

Laboratories rely on reference ranges to help compare the results of your blood test with those of the standard healthy population. This range will vary slightly based on the testing kit used. There will also be varying ranges for those who are pregnant based on how far along they are. Testing young children or babies also requires different ranges. The following reference ranges for the thyroid function test apply to healthy adults who are not pregnant:


Normal Range


0.4 to 4.5 milliunites per liter (mU/L)


9.0 to 25.0 picomoles per liter (pmo/L)


3.5 to 7.8 picomoles per liter (pmo/L)

In cases where your results fall outside of this reference, your doctor will consider the following information to help diagnose the potential problem causing these unusual levels.

  • High TSH levels combined with low FT4 levels typically indicate an underactive thyroid, known as hypothyroidism.
  • Low TSH levels combined with high FT4 levels and high FT3 levels typically indicate an overactive thyroid, known as hyperthyroidism.
  • Low TSH levels combined with low FT4 levels may indicate a disorder that affects the pituitary gland.
  • TSH levels that are abnormal and combined with normal levels of FT4 show a risk of developing some sort of thyroid disorder.

No matter your blood test indicates the thyroid gland is either underactive or overactive, your doctor may then recommend an ultrasound or thyroid uptake test. These tests help double check out whether the issue is caused by a tumor or structural problems with your thyroid gland.

In cases where these scans are normal, but your thyroid works unwell, your doctor will most likely prescribe medicine that regulates your thyroid activity. In this case, you will take another thyroid function test shortly to ensure the medication is producing the desired result.