I. Introduction

The thyroid gland impacts our overall health by regulating metabolism, the chemical processes that convert food to energy, and keep our cells working. The gland affects vital functions, such as heart rate, digestion, body temperature, and energy levels.

A T3 test measures levels of triiodothyronine, a thyroid hormone also known as T3, in the body. It’s one of several thyroid function tests that evaluates how well the thyroid gland is working.

This guide provides a basic overview of how the thyroid gland functions and the role of T3 hormones in the body. It also provides information about T3 testing, what it means if your T3 levels are too low or too high, and options for treatment.

II. Overview of T3 Testing

Why should I get tested?

The American Thyroid Association reports that up to 60% of people with thyroid disease don’t know they have the condition. Left undiagnosed, these disorders can increase the risk of heart disease, osteoporosis, and infertility. In pregnant women, it can lead to miscarriage, early delivery, or developmental problems in babies.

A T3 test, also known as a triiodothyronine test or thyroid function test, measures levels of T3 in the body to provide insight into how well the thyroid is working. Thyroid disorders occur when there’s too much or too little of any thyroid hormone in the body. In addition to T3, thyroid hormones include thyroxine (T4) and thyroid-stimulating hormone (TSH).

When should I get tested?

Your doctor may recommend T3 testing to evaluate thyroid function if:

  • You have signs of an underactive thyroid (hypothyroidism), such as fatigue, weight gain, low heart rate, or intolerance to cold temperatures
  • You have signs of an overactive thyroid (hyperthyroidism), such as irritability, anxiety, rapid heartbeat, or intolerance of heat
  • You’re taking thyroid medication

You have abnormal levels of other hormones, such as T4 or TSH

What is required for the test?

A blood sample is used to test levels of triiodothyronine in your body.

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

Let your doctor know of any medicines or supplements that you’re taking, as these may affect your test results. It’s important that you do not stop taking any medication unless specifically requested by your doctor.

Otherwise, no preparation is required for a T3 blood test.

III. The Basics of Thyroid Disorders

Importance of the Thyroid Gland

The thyroid is a butterfly-shaped gland with two lobes, located in the lower part of the neck just below the larynx.

This gland produces vital hormones that affect the body’s metabolism and how it creates energy for important body functions. Thyroid hormones affect:

  • Heart rate
  • Digestion
  • Muscle function
  • Brain function
  • Bones
  • Energy
  • Mood
  • Growth

Types of Thyroid Hormones

The thyroid gland produces two types of thyroid hormones:

  • Thyroxine (T4), which has four iodine atoms
  • Triiodothyronine (T3), which has three iodine atoms

The thyroid produces only about 20% of the T3 in the body. The rest is T4 that’s converted to T3 in the liver, kidneys, and brain. This is accomplished by enzymes called deiodinases that remove one of the iodine atoms. While both T3 and T4 affect metabolism, T3 is considered the more active hormone.

Most of the T3 in the body travels through the bloodstream attached to proteins. This is called bound T3, and it doesn’t affect body tissues. Free T3 is unbound and can enter body tissues.

Levels of T3 and T4 are controlled by the pituitary gland, which detects how much T4 is in the body. If the pituitary gland senses that increased levels of hormones are needed, it activates the thyroid gland by releasing thyroid-stimulating hormones (TSH). Thyroid disorders result when T3, T4, or TSH levels are out of normal range.

Symptoms of Hyperthyroidism (Overactive Thyroid)

When there’s too much T3 in the body, you can develop hyperthyroidism. Symptoms may include:

  • Rapid heartbeat or heart palpitations
  • Nervousness or irritability
  • Increased sweating or intolerance of heat
  • Unexplained weight loss
  • Increased appetite
  • Difficulty sleeping
  • Depression
  • More frequent bowel movements
  • Irregular menstruation
  • Tremors in the hands
  • Hair loss or thinning
  • Enlarged thyroid gland

Causes of an overactive thyroid include:

  • Graves’ disease, which is an autoimmune disease
  • Thyroid medication
  • Birth control pills or estrogen
  • Pregnancy
  • Liver disease
  • T3 thyrotoxicosis (rare)

Symptoms of Hypothyroidism (Underactive Thyroid)

When there’s too little T3 in the body, you can develop hypothyroidism, or a slowing of the body’s functions. Symptoms may include:

  • Fatigue
  • Intolerance to cold temperatures
  • Low heart rate
  • Lack of appetite
  • Poor memory or cognitive function
  • Depression
  • Stiff muscles
  • Fertility problems

Hypothyroidism is most common in women and people over the age of 50. It’s typically caused by chronic illness or thyroiditis, including Hashimoto’s disease.

IV. How a T3 Test Works

There are two types of tests for measuring levels of T3 in the bloodstream. Both are performed on blood samples.

  • A total T3 test measures both T3 that’s attached to proteins (bound), and free T3 that’s not attached to proteins (unbound).
  • A free T3 test measures only unbound T3. This test is considered less accurate than a total T3 test.

If your physician recommends a T3 test, you’re given a doctor’s order that you can take to a clinic or laboratory to have the test performed.

A clinician takes the blood sample using a small needle syringe, usually from a vein on your arm.

  • The puncture site is cleaned and disinfected with an alcohol swab.
  • A rubber tourniquet is tied around your upper arm to help blood flow to the vein.
  • A small needle is inserted into the puncture site and a sample of blood is drawn into a tube.
  • The needle and tourniquet are removed.
  • Gentle pressure and a small bandage are applied to the puncture site.
  • The blood sample is sent for testing.

You can purchase T3 tests directly on some websites without consulting a doctor. This includes a test requisition that you can take to a laboratory or a test kit to use at home.

V. Treatment for Thyroid Disorders

Thyroid disorders have many causes. Your physician can recommend a treatment plan based on your diagnosis, age, and medical history.

Most thyroid disorders are lifelong conditions but can be managed with appropriate treatment.


If you have an overactive thyroid, your doctor may recommend treatment that involves:

  • Reducing the gland’s ability to produce thyroid hormones through antithyroid medication, such as methimazole or propylthiouracil. These treatments result in prolonged remission of Graves’ disease in 20% to 30% of cases.
  • Destroying the cells that produce thyroid hormones through radioactive iodine, which is typically taken once in the form of a capsule or a liquid. Thyroid cells will take up any type of iodine. Radioactive iodine kills overactive cells, returning the body’s level of thyroid hormones to normal. However, patients with Graves’ disease most often develop hypothyroidism as a result, requiring daily treatment with thyroid hormones.
  • Surgery to remove part or all of the thyroid gland. This usually results in hypothyroidism, which can be treated with daily thyroid hormones.


When your body isn’t making enough thyroid hormones, your doctor may recommend thyroid hormone replacement therapy to make up for the one that you’re lacking.

A commonly used synthetic hormone is levothyroxine, which is available under a number of different brand names. Thyroid hormones come in pill form and are best taken at the same time each day to ensure consistent levels in the bloodstream. Your doctor may need to adjust dosages based on regular testing of your hormone levels to ensure proper thyroid function.

Levothyroxine is a T4 replacement but is also used to treat low levels of T3, as most of the T3 in the body is converted from T4. This conversion process functions normally in patients with hypothyroidism, so maintaining levels of T4 helps ensure adequate levels of T3.

If you’re taking thyroid hormones, your physician will monitor your improvement with regular blood tests to check your hormone levels and adjust dosages if necessary.

A synthetic form of T3 called liothyronine is available, but at this time, clinical studies don’t support its use for treating hypothyroidism. Liothyronine can cause fluctuations in T3 levels and numerous side effects.

Your doctor can recommend the appropriate treatment.


Can I test my triiodothyronine levels without seeing a doctor?

Yes. You can find T3 tests or thyroid panels that include screening for T3 available for purchase on some websites.

  • A direct to consumer lab test provides you with an order that you can take to a local laboratory. Be sure to confirm that there is a lab near you that accepts the order before purchasing it. A doctor’s order, which is required for blood testing, is included with your purchase of the lab test. A clinician will draw the sample for testing when you visit the lab. You can access the results online when they’re available.
  • A home test kit is mailed directly to the address that you provide, so you can collect a blood sample in the privacy of your own home. This is typically done using a finger prick collection method. Home test kits include the supplies required to collect the sample, such as instructions, alcohol swabs, single-use lancets, collection cards, and a return shipping envelope. Be sure to follow directions carefully to ensure more accurate results. You can access the results online when available.

What are normal results for a T3 test?

Results considered normal vary depending on the laboratory as well as factors, such as your age and health. It’s best to speak to your physician to interpret your results, but the following are considered typical normal T3 ranges for adults:

  • Total T3: between 60 and 80 ng/dL or 0.92 to 2.76 nmol/L
  • Free T3: between 130 and 450 pg/dL or 2.0 to 7.0 pmol/L

There are different ranges for pediatric patients under the age of 20.

What happens if my results are abnormal?

There are many reasons T3 levels may be outside of the normal range, including pregnancy, liver disease, certain medications, and chronic illness. Your physician can discuss your results with you and recommend additional tests if there are concerns.

You may require blood tests to check the levels of other hormones in your body. Reviewing levels of T4 or TSH, along with T3, gives a more accurate picture of thyroid function. In some cases, ultrasound and other imaging tests may be required.

How do thyroid disorders affect women?

Women are more likely than men to develop thyroid disease, especially after pregnancy or menopause. About 12.5% of women develop a thyroid condition in their life. This may result in difficulties with menstruation, ovulation, and pregnancy. If thyroid disorders are untreated in pregnant women, they can cause low birth weight, miscarriage, or problems with the baby’s growth and development.

Can children have thyroid problems?

Yes. The thyroid is vital to growth and development and can have a significant impact on babies, children, and adolescents if left untreated. Some effects include delayed growth resulting in short stature, delayed development of permanent teeth, delayed puberty, and poor mental development.

VII. Additional Resources

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

Name Web Summary
American Association of Clinical Endocrinologists www.empoweryourhealth.org Overview of how the thyroid works
U.S. National Library of Medicine www.medlineplus.gov General information about thyroid diseases
Texas Department of State Health Services www.dshs.texas.gov FAQs on hypothyroidism in infants
U.S. Office on Women’s Health www.womenshealth.gov Fact sheet about thyroid disease in women
Merck Manual www.merckmanuals.com Overview of thyroid gland disorders

VIII. Sources Used in This Article