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Thyroxine (T4) Blood Test: What You Need to Know

A thyroxine test is used to help diagnose disorders of the thyroid.
Thyroxine (T4) Blood Test: What You Need to Know
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In this article:

  • What is a thyroxine (T4) test? 
  • When is a thyroxine test needed? 
  • How to prepare for a thyroxine test? 
  • What happens during a thyroxine test?
  • What do the results mean? 


What is a thyroxine (T4) test?


A thyroxine test is used to help diagnose disorders of the thyroid. The thyroid is the butterfly-shaped gland at the base of your neck. It produces hormones that regulate your body's energy use, along with many other essential functions. As part of the endocrine system, the thyroid gland makes and stores hormones that help regulate the body's metabolism in the form of blood pressure, blood temperature, and heart rate.

Thyroxine (T4) is the primary hormone that is produced and secreted by the thyroid gland. It is responsible for digestion, heart rate, muscle formation, brain development, and healthy bones. T4 is biologically inactive (called a storage hormone); its primary function is to transport the hormone triiodothyronine (T3) to the proper organs in your body. Without enough T4 (and its T3 counterpart), the body's metabolism slows down.

Too much or too little T4 may indicate thyroid disease.

Thyroxine exists in two forms in the body: Free T4 and Bound T4. While most of these hormones are bound to protein and inactive, the "free" active component helps influence metabolism. Normal levels of Free T4 are essential for normal metabolism and temperature regulation. Free T4 does not bond to the protein in your blood, which allows it to enter the body tissues that need to use it. Most of the T4 in the bloodstream, however, is bonded to protein, which prevents it from entering these tissues; this is Bound T4.

While you can test both forms of T4 with a Total T4 test, a Free T4 test is considered to be more accurate and helpful because T4 converts into Triiodothyronine (T3), another thyroid hormone. Any changes show up in T4 first. 

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When is a thyroxine test needed? 

A thyroxine test can help you, and your doctor understand how your thyroid is working or if there may be a need for further evaluation. 

A T4 test may be required if:

  • You have symptoms that indicate a problem with your thyroid
  • A thyroid-stimulating hormone (TSH) test comes back with abnormal results
  • To assess the progress of a known thyroid issue

Two common disorders that affect thyroid function include: 

  • Hyperthyroidism, or overactive thyroid
  • Hypothyroidism, or underactive thyroid

Hypothyroidism may be present if you have symptoms like:

  • Slowed thinking and mental activity ("brain fog")
  • Thinning hair or eyebrows
  • Peripheral neuropathy
  • Mental health issues
  • Goiter (enlarged thyroid gland)
  • High blood pressure
  • Intolerance to cold
  • Weight gain
  • Stomach bloating
  • Heartburn
  • Constipations
  • Irregular or heavy menstrual periods
  • Weak or achy muscles
  • Dry or rough skin

Hyperthyroidism may be present if you have symptoms like:

  • Nervousness or tremor
  • Racing heartbeat or palpitations
  • Agitation or irritability
  • Poor concentration
  • Irregular or light menstrual periods
  • Heat intolerance and/or unusual sweating
  • More frequent bowel movements
  • Enlargement of the thyroid gland
  • Increase in appetite, feeling hungry
  • Skin thinning
  • Brittle hair

How to prepare for a thyroxine test? 

You should stop taking biotin, a vitamin commonly found in many hair, nail, and skin supplements, 12 hours before your test. Otherwise, continue your medication and diet as usual. If you have any questions about a specific drug, you should consult with your physician before the test.

What happens during a thyroxine test?

Conventionally, a health care professional will collect a blood sample from a vein in your arm, using a small needle, and sends it to a lab for testing.

With Paloma's at-home blood test kit, you collect a blood sample yourself: 

  1. Using one of the lancets included in your test kit, twist and pull the safety plastic off the end of the lancet.
  2. Place the lancet between your index and middle finger, similar to holding a syringe.
  3. Place and hold onto the finger you wish to draw blood from and squeeze the lancet.
  4. Massage your forearm to the base of your finger, avoiding the pierced area.
  5. Allow your blood to drop freely onto the collection card, filling all the circles.
  6. Clean your finger and apply a bandaid.
  7. Let the card dry for 30 minutes. Avoid touching or smearing.

Collecting your sample should take no more than 5 minutes. You will then send it to a CLIA-certified lab for analysis.

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What do the results mean?

Test results can vary depending on your age, gender, history, laboratory methods, and other variables.

T4 isn't the only hormone participating in thyroid function, so an abnormal result on this test alone may not provide enough information to understand or diagnose your condition. Looking at TSH, Free T4, Free T3, and TPO antibodies together can give a complete picture. 

High levels of T4 may indicate hyperthyroidism (overactive thyroid) or Grave's disease (autoimmune thyroiditis).

Other reasons for abnormally high results may include: 

  • Too much thyroid replacement medication
  • High levels of protein in the blood
  • Iodine surplus

Low levels of T4 may indicate hypothyroidism (underactive thyroid) or Hashimoto's Disease (autoimmune thyroiditis).

Other reasons for abnormally low results may include: 

  • Medications that affect protein levels
  • A pituitary problems
  • Iodine deficiency

To note: if you're pregnant or planning to be, you should discuss your values with a doctor as normal ranges may be different. Both Total T4 and Free T4 vary in children; you should talk to your doctor about expected ranges for your child. 

Talk with a doctor you trust about what the test results may mean about your thyroid function.


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Katie Wilkinson

Katie Wilkinson, previously serving as the Head of Content and Community at Paloma Health, fervently explores the nexus between healthcare and technology. Living with an autoimmune condition, she's experienced firsthand the limitations of conventional healthcare. This fuels both her personal and professional commitment to enhancing patient accessibility to superior care.

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