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What Is a T4 (thyroxine) Blood Test?

Medically Reviewed by Dan Brennan, MD on June 15, 2021

T4 is short for thyroxine, which is one of the hormones made by your hormone gland. A T4 test shows how well your thyroid is working to regulate your body’s growth and energy.

About the T4 Blood Test

Your body makes two different T4 hormones. The first is a free T4 that flows through your bloodstream independently. Bound T4 is the second type of thyroxine. It travels through your bloodstream by attaching to proteins. 

Your doctor may order a total T4 test to measure free T4 and bound T4, or they may order a free T4 test that doesn’t include results for bound T4. Many doctors order the free T4 test because of accuracy concerns surrounding the bound T4 results. ‌‌

While the primary reason for a T4 test is to check your thyroid, it can also alert your doctor to problems affecting your pituitary gland. This gland creates a thyroid-stimulating hormone (TSH) that communicates with your thyroid to release T4 into your bloodstream.

Procedures for the T4 blood test. Talk to your doctor about any medication, 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, it may cause a false-positive test result, showing that you don’t have a thyroid problem.

Some infections may also 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.

Reason for a T4 Blood Test

There are a few different reasons you may need a T4 blood test. Your doctor will request a T4 blood test if they suspect your thyroid is overactive, which is called hyperthyroidism, or underactive, which is called hypothyroidism. ‌

If your thyroid is underactive, it doesn’t produce enough thyroxine, slowing down your body and metabolism. 

Signs of an underactive thyroid may include:

  • Being constipated
  • Dry skin
  • Feeling cold
  • Feeling depressed 
  • Feeling tired
  • Gaining weight
  • Having weak muscles
  • Irregular periods‌
  • Thinning hair‌

If you have an overactive thyroid gland, your thyroid makes too much thyroxine. You may have high energy levels and fast metabolism.

Symptoms of hyperthyroidism may include:

  • Having a faster heart rate
  • Feeling hungrier than usual
  • Feeling anxious or nervous 
  • Having diarrhea
  • Feeling hot
  • Having difficulty sleeping
  • Losing weight without trying
  • Shaking hands
  • Sweating
  • Thinning hair 
  • Your eyes feeling dry or puffy

Understanding T4 Blood Test Results

Your results are usually available within a day or two of your blood test. Results are qualitative and may be affected by your:

  • Age
  • Gender‌
  • Lab testing method‌

Your doctor will look at your results with your overall health in mind. As an adult, your normal free thyroxine range is from 4.6 micrograms per deciliter (mg/dL) to 11.2 micrograms per deciliter. If your results are within this range, it typically means your thyroid is working as expected.

If your results are high, you may have an overactive thyroid. If your results are low, you may have an underactive thyroid. Because your hypothalamus and pituitary glands contribute to your thyroid function, results may show you have an issue with one of these glands. 

Your hypothalamus helps control some of your body operations. It does this by sending messages to your automatic nervous system, which controls things like body temperature, blood pressure, digestion, heart rate, and breathing.‌

Additional testing. While a T4 blood test shows that there is a problem with your thyroid, it doesn’t diagnose 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.

Additional tests may include:

  • T3 blood test. Your doctor may want to check the levels of this other hormone made by your thyroid gland because it works with T4 in your body to manage energy. 
  • Thyroid-stimulating hormone (TSH). If your doctor didn’t request this test before a T4 blood test, they may request for it to be checked afterward. 
  • Thyroid antibodies. If you have a health condition like Hashimoto's thyroiditis or Graves' disease, your body may make proteins designed to attack your thyroid. ‌

T4 blood test risks. Drawing a sample of blood for the T4 blood test is similar to other blood labs you may have done. You get a small needle prick to extract blood, and then the technician covers the area with gauze and a bandage.

Side effects of the test may include:

  • Pain following the needle stick
  • Excessive bleeding
  • Infection if the area isn’t kept clean
  • Bruising if your blood vessel bursts
  • Dizziness or fainting 
  • Feeling a lump under your skin
WebMD Medical Reference

Sources

SOURCES:

‌Cleveland Clinic: "Pituitary Gland."

KidsHealth from Nemours: “Blood Test: T4 (Thyroxine).”

Mayo Clinic: "Cirrhosis."

‌MedicineNet: “What Is a T4 Test?”

‌Medline Plus: “Thyroxine (T4) Test.”

‌UCLA Health: “T4 (thyroxine) Test.”

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