I. Introduction

The hormones produced by the thyroid gland affect critical body functions. Hormone levels that are too low or too high can impact energy, body temperature, heart rate, muscles, and cognitive function.

This guide provides an overview of how the thyroid gland works, reasons you may need a free T4 test, some of the thyroid disorders that may cause abnormal T4 test results, and basic treatment options.

II. Overview of a Free T4 Thyroxine Test

Why should I get tested?

Thyroid disorders impact many parts of the body, including the heart, brain, and muscles. If left untreated, they can also increase the risk of cardiovascular disease, osteoporosis, and infertility. Thyroid disease affects an estimated 20 million Americans, with up to 60% of cases undiagnosed.

A free T4 test measures the level of unbound, or free, T4 hormones in your blood to screen for a possible thyroid disorder. Your doctor may also use a free T4 test to monitor your progress if you’re undergoing treatment for a thyroid condition, such as hyperthyroidism or hypothyroidism. Changes in T4 levels can determine whether medication dosages should be increased or decreased.

When should I get tested?

Your physician may recommend a free T4 test if you have signs of a thyroid disorder, such as:

  • Abnormal levels of other thyroid hormones, such as thyroid-stimulating hormone (TSH)
  • Symptoms of an overactive or underactive thyroid (see below for details)
  • Symptoms of an underactive pituitary gland
  • An enlarged thyroid gland or lump in the thyroid
  • Fertility problems

Thyroid problems are most prevalent in middle-aged and older women, but can also occur in babies, children, adolescents, and men and women of all ages. It’s five to eight times more likely to occur in women than men, with 12.5% of women developing a thyroid disorder during their life. In some cases, thyroid disorders are inherited.

What is required for the test?

A free T4 test requires you to provide a blood sample that is analyzed at a diagnostic laboratory.

What do I need to do to prepare for the test?

Advise your clinician of any medications, herbs, or supplements that you are taking. For example, birth control pills, heparin, or biotin may affect test results. Otherwise, no preparation is required.

III. The Basics of Thyroid Disorders

How the Thyroid Gland Works

The thyroid produces important hormones that affect cells, tissues, organs, and many different chemical systems and body functions. These hormones regulate the speed at which cells work, influencing metabolism, weight, body temperature, heart rate, energy, concentration, reflexes, muscles, organs, and fertility, to name a few. Thyroid hormones are essential in infants, children, and adolescents for proper growth and brain development.

The thyroid gland is located in the lower part of the neck, just under the larynx. It’s responsible for producing, storing, and releasing hormones, which are carried in the bloodstream to other parts of the body. Normally, this small, butterfly-shaped gland is difficult to see or feel, but it’s easy to detect during a physical examination if it becomes enlarged.

The thyroid uses iodine to produce thyroxine, a hormone known as T4 because it has four iodine atoms. In certain parts of the body, such as the liver or brain, T4 is converted into another type of thyroid hormone known as triiodothyronine, or T3.

Thyroid Hormones

There are two types of T4:

  • Total T4 accounts for more than 99% of the thyroxine in the body. It’s bound to a protein called thyroxine-binding globulin as it moves through the bloodstream. Because it’s attached, it can’t enter body tissue and doesn’t affect tissue function.
  • Free T4 comprises less than 1% of all T4 in the body. It’s unbound and enters body tissue when needed and affects tissue function.

Production of T4 is managed by the pituitary gland, which makes a hormone called thyroid-stimulating hormone (TSH). If the pituitary gland determines that levels of T4 are too low, it produces TSH, which then stimulates the thyroid to produce more T4. Thyroid disorders result when there’s too much or too little TSH, T4, or T3.

Symptoms of an Overactive Thyroid (Hyperthyroidism)

When the thyroid is overactive, it produces too much T4, resulting in a condition known as hyperthyroidism. Symptoms of hyperthyroidism include:

  • Irritability
  • Nervousness or anxiety
  • Unexplained weight loss
  • Tremors in the hands
  • Rapid or pounding heartbeat or palpitations
  • Enlarged thyroid gland or swelling in the lower neck
  • Heat intolerance
  • Increased perspiration
  • Menstrual irregularity
  • Difficulty sleeping
  • Muscle weakness

Graves’ disease is an autoimmune disorder in which your immune system attacks your thyroid gland, causing it to create excess hormones. This autoimmune disorder is genetic and affects an estimated 0.5% of the population. Along with hyperthyroidism, Graves’ disease can cause Graves’ ophthalmopathy, resulting in eye discomforts, such as inflammation, irritation, light sensitivity, blurry vision, and dryness.

Symptoms of an Underactive Thyroid (Hypothyroidism)

When the thyroid is underactive, it doesn’t produce enough T4, resulting in a condition known as hypothyroidism. Symptoms of hypothyroidism include:

  • Extreme fatigue
  • Slow pulse or heartbeat
  • Weight gain
  • Depression
  • Forgetfulness/cognitive dysfunction
  • Hair loss
  • Feeling cold
  • Menstrual irregularity
  • Constipation
  • Joint or muscle aches
  • High levels of cholesterol

Hypothyroidism may also occur in newborns. Symptoms of a thyroid disorder in infants include:

The most common cause of hypothyroidism is Hashimoto’s thyroiditis, an autoimmune disease, which attacks the thyroid.

IV. How a Free T4 Thyroxine Test Works

There are two types of blood tests that detect levels of the thyroxine hormone in the bloodstream:

  • A total T4 test measures both unbound and bound T4 hormones.
  • A free T4 test measures only free or unbound T4 hormones. It’s considered more accurate than a total T4 test for testing thyroid function.

Thyroxine tests are performed on blood samples drawn from a vein on the inside of your forearm, near the elbow.

  • An area of your arm is cleaned and disinfected with an alcohol swab.
  • A tourniquet or rubber band is applied on the upper arm to increase blood flow to the vein and make it easier to draw blood.
  • A small needle is inserted into the arm to collect a blood sample in a tube.
  • The tourniquet and needle are removed. Gentle pressure is applied to the puncture site to stop bleeding.
  • The blood sample is sent to a laboratory for testing.

Blood samples for infants with suspected thyroid disorders are usually obtained using a heel prick.

If your physician orders a free T4 test, you can visit a clinic or laboratory that offers testing to provide a blood sample. You may also be able to order a thyroid home test kit on some websites and collect a blood sample for testing in the privacy of your home.

V. Treatment for Thyroid Disorders

Most thyroid diseases are lifelong conditions but they can be managed. Treatment depends on the type of disorder and your symptoms. Some common treatment options are outlined below, but your doctor will recommend a plan that is appropriate for your specific condition.


Hypothyroidism can be managed with synthetic thyroid hormones that replicate the ones that your thyroid needs to function normally. An example of a standard thyroid hormone pill is levothyroxine, which is taken orally to keep your hormones at an appropriate level.

Large doses of thyroid medicines can cause side effects, so it’s likely your doctor will start treatment with a low dosage that’s increased over time. A free T4 test may be used to regularly monitor the effects of the medication on your hormone levels, and to determine if dosages need to be adjusted.

As most thyroid conditions are lifelong, you may need to continue taking the medication even if you feel better to ensure your hormone levels are maintained and symptoms don’t return.


Hyperthyroidism can be managed with anti-thyroid medicines, such as methimazole and propylthiouracil. These drugs stop your thyroid from producing more hormones. Beta-blockers can also be prescribed to reduce the effects of thyroid hormones on your body and help slow your heart rate or ease other symptoms.

Other conditions, such as Graves’ disease, may require treatment with radioactive iodine, a one-time medication that eliminates the cells that produce thyroid hormones. The thyroid can also be removed through surgery, but this often causes permanent hypothyroidism.

VI. Frequently Asked Questions

What is considered a normal result for a free T4 test?

A normal level of free T4 hormones typically falls between 0.9 to 2.3 nanograms per deciliter. However, this range varies by laboratory. Your test results also depend on factors, such as age, gender, medical history, testing methods, and levels of thyroid-stimulating hormones in your body, so it’s important to discuss your results with your physician.

What does it mean if my T4 levels are too high or too low?

If your T4 levels are higher than normal, your thyroid is overactive and is producing too much of the hormone. It may mean that you have hyperthyroidism.

If your T4 levels are lower than normal, your thyroid is not producing enough of the hormone. It may mean that you have hypothyroidism.

What happens if my results are abnormal?

If your doctor is concerned about your T4 test results, additional blood tests may be requested to check levels of other hormones, such as thyroid-stimulating hormone (TSH), free T3, and total T3.

Depending on your symptoms, your doctor may screen for specific conditions, such as Graves’ disease or Hashimoto’s thyroiditis, or order imaging tests, such as an ultrasound or nuclear scan.

What might affect my test results?

Test results can be affected by pregnancy, kidney and liver problems, estrogen levels, some medications, and other illnesses. It’s important to review your test results with a physician.

How does a home test kit for thyroid disorders work?

A home test kit for thyroid disorders can be purchased from some websites. This kit is sent to you directly so that you may collect your blood sample at home using a finger-prick test. The kit contains all of the supplies required, including detailed instructions, alcohol swabs, disposable lancets, gauze and bandages, and a sample collection card. Once you’ve collected a sample, return it to the laboratory in a prepaid shipping envelope. Your test results are provided online when available.

Why is it important to diagnose thyroid conditions in children?

Hypothyroidism that’s untreated in babies, children, and teens can affect physical growth, brain development, development of permanent teeth, and puberty.

What if I'm pregnant?

Thyroid changes can occur during pregnancy. About 2.5% of pregnant women experience hypothyroidism, and less than 0.4% develop hyperthyroidism. These conditions pose a significant risk if untreated. In most cases, treatment is successful, and women give birth to healthy babies. Sometimes, the condition can develop postpartum.

Why is it difficult to diagnose thyroid conditions in older adults?

An underactive thyroid can be difficult to diagnose in older patients as symptoms may be more subtle. For example, signs of slowing cognitive function, such as confusion or forgetfulness, may be attributed to Alzheimer’s disease or dementia. About 10% of older women and 6% of older men experience hypothyroidism.

VII. Additional Resources

To learn more about thyroid disorders and testing for these conditions, use the following resources.

Name Web Summary
American Thyroid Association www.thyroid.org General information for thyroid patients
American Thyroid Association www.thyroid.org Information about thyroid disease in older patients
Hormone Health Network www.hormone.org Information about thyroxine disorders
John Hopkins Medicine www.hopkinsmedicine.org Information about thyroid disorders
Merck Manual www.merckmanuals.com Overview of thyroid gland disorders

VIII. Sources Used in This Article