What Is a Thyroxine (T4) Blood Test?

Medically Reviewed by Zilpah Sheikh, MD on March 16, 2024
6 min read

Thyroxine (T4) is one of the hormones made by your thyroid gland. Your thyroid gland is the small, butterfly-shaped organ in your throat that wraps around your windpipe (trachea). It makes and releases a few different hormones, including T4, that help control the following processes:

  • How your body turns the food you eat into energy (metabolism)
  • How fast food moves through your digestive system
  • Your heart rate and body temperature
  • Brain development in children
  • How your muscles contract
  • The rate at which your body replaces dying cells

T4 is the inactive form of your thyroid hormone; it must be changed into the active form, triiodothyronine (T3), for your organs and cells to use. The cells in a few of your organs can convert T4 into T3, including your:

  • Brain and spinal cord
  • Pituitary gland
  • Thyroid
  • Muscles
  • Brown fat tissue (the kind of fat that stores and burns energy to keep your body temperature stable)
  • Liver
  • Kidneys

Your doctor may have you take a T4 blood test for a few different reasons, such as:

  • To help diagnose and monitor you for thyroid disease, like an underactive or overactive thyroid.
  • To help diagnose for disorders in your pituitary gland, which is a walnut-sized organ in your brain that, among other hormones, makes thyroid-stimulating hormone (TSH), which tells your thyroid how much T4 and T3 to make.
  • To check how well your thyroid is working if you have had an abnormal finding on other thyroid functions tests (such as a TSH or T3 test), a lump or nodule on your thyroid, an enlarged or irregular thyroid gland, or trouble getting pregnant.

Low T4 symptoms

When your thyroid is underactive, it doesn't make enough thyroid hormone for your body to function well, and your metabolism will slow down. This is called hypothyroidism.

Symptoms of hypothyroidism include:

  • Depression
  • Inability to concentrate and remember things
  • Swelling around your eyes
  • Hair loss
  • Swelling in your thyroid (also called goiter)
  • Severe tiredness
  • Low tolerance for cold
  • Weight gain
  • Not feeling hungry
  • Cold, dry skin
  • Slow heart rate
  • Shortness of breath
  • Constipation
  • Irregular periods

High T4 symptoms

When your thyroid is overactive, it makes too much thyroid hormone, and your metabolism speeds up. Doctors may call this hyperthyroidism.

Symptoms of hyperthyroidism may include:

  • Anxiety and depression
  • Restlessness
  • Irritability
  • Inability to sleep
  • Swelling around your eyes that makes them bulge out (Graves' eye disease)
  • Weight loss
  • Feeling extra hungry
  • Low tolerance for heat
  • Raised sweating
  • Fine tremors in your fingers, head, face, or tongue
  • Fast heart rate
  • Feeling like your heart is fluttering, pounding, or skipping beats
  • High blood pressure
  • Diarrhea

Your body makes two different forms of T4 hormone:

  1. Free T4, which flows through your bloodstream without being attached to anything. This form is ready to be taken up into any cells that need thyroid hormone right away.
  2. Bound T4, which travels through your bloodstream by attaching to transport proteins. These transport proteins make sure your T4 gets to the places in your body it needs to go and also keeps it from getting taken up into other organs and cells along the way. Almost all the T4 your body makes is bound.

Procedures for the T4 blood test

Tell your doctor about medications, vitamins, or supplements you're taking. Medications for seizures, heart conditions, and birth control may affect the results of the test. If you take a biotin supplement, the test may show that you have a higher T4 level than you actually do. This may cause your doctor to miss that you have a thyroid problem.

Tell your doctor if you are pregnant or have any medical conditions or infections because they may affect your test results. This includes chronic diseases like kidney failure or cirrhosis. Cirrhosis is scarring on your liver which can be caused by different diseases and conditions, including hepatitis and alcoholism. If you have a short-term infection, your doctor may want to wait to do the test until you feel better.

T4 blood test risks

The procedure for the T4 blood test is similar to other blood tests you may have had done. A technician will draw your blood with a small needle, and then cover the area with a bandage.

The risks from having your blood drawn are slight, but they can include:

  • Pain following the needle stick (for most people this is mild)
  • Bleeding
  • Bruising, if blood leaks under your skin
  • Infection, if the area isn't kept clean
  • Dizziness or fainting
  • Feeling a lump under your skin

Normal T4 range

Normal range on the free T4 test vary by age, sex assigned at birth, and lab testing method, but in general, they are as follows:

  • Children up to 5 years old: 0.8-2.8 nanograms per deciliter (ng/dL)
  • Children 6-15 years old: 0.8-2.1 ng/dL
  • Teens 16-17 years old assigned male at birth: 0.8-2.8 ng/dL
  • Teens 16-17 years old assigned female at birth: 0.8-1.5 ng/dL
  • Adults over 18 years old: 0.9-1.7 ng/dL

For adults who are pregnant, the normal range on the free T4 test varies by trimester:

  • First trimester: 0.8-1.53 ng/dL
  • Second and third trimester: 0.7-1.20 ng/dL

Normal ranges (also called reference ranges) may vary between labs. Reference ranges are also set from studying large populations of people, so your result could be outside of the reference range but still normal for you. Ask your doctor about what your test results mean for you and your medical situation.

Low T4 levels

A few different conditions can cause low levels on a free T4 test, including:

  • Hypothyroidism, which can be caused by not getting enough iodine in your diet. However, the main cause in most places where iodized salt is available is Hashimoto's thyroiditis, which is an autoimmune condition. An autoimmune condition is when your immune system mistakenly attacks your organs as if they were an invader, like a virus or bacteria.
  • Untreated congenital (before you are born) hypothyroidism
  • Severe short-term illness
  • Malnutrition
  • Some medicines, including such as amiodarone, dexamethasone, lithium, propranolol, and thionamide

High T4 levels

A few different conditions can cause high levels on a free T4 test, including:

  • Too much iodine in your diet
  • Benign (not cancer) tumor (thyroid adenoma) or nodules on your thyroid
  • Graves' disease, an autoimmune condition
  • Swelling of your thyroid (goiter)
  • Taking too much thyroid replacement hormone
  • Some tumors on the testes or ovaries (rare)
  • Having medical imaging with a contrast dye that contains iodine (also rare)

Additional testing. While a free T4 test may show that there is a problem with your thyroid, it won't tell your doctor what the problem is. The next step is to figure out why your thyroid isn't working correctly. If your T4 levels aren't in a normal range, your doctor may order additional tests, such as:

TSH blood test. This is usually the first thyroid function test that your doctor will order, but if not, they may check it after your free T4 test comes back abnormal. Your TSH and T4 levels directly affect each other because TSH tells your thyroid when to make T4 and how much to make.

Free T3 blood test. Your doctor may want to check the levels of this other main hormone made by your thyroid gland.

Thyroid antibodies blood test. If you have a health condition like Hashimoto's thyroiditis or Graves' disease, your body may make proteins that attack your thyroid. If you're positive for thyroid antibodies, you may have one of these autoimmune conditions.

Radioactive iodine uptake imaging. Your thyroid needs iodine from your diet to make thyroid hormones. This test requires you to swallow a small amount of radioactive iodine. The technician will be able to see this iodine as it is being taken up into your thyroid using a special camera.

A T4 blood test measures your thyroxine hormone levels, which can show how well your thyroid is working. Low T4 levels are called hypothyroidism, and high T4 levels are called hyperthyroidism. Abnormal levels can be a short-term or long-term problem. Your doctor may order other thyroid function tests to help figure this out and decide on a treatment plan if you need one.