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Free T4 + Free T3

What are free T3 and free T4?

The thyroid is a small gland located just below the Adam’s apple that produces hormones to manage your body temperature, energy use, weight, muscle strength, and mood. These hormones are: T3 (triiodothyronine), which contributes to the body’s muscle control, digestion, heart function, and bone health; and T4 (thyroxine), which helps maintain metabolism, mood, and body temperature. Your body can convert T4 to T3 if necessary.

Healthcare professionals use free T3 and T4 levels to assess the functioning of your thyroid and pituitary glands. The pituitary gland is in the brain and manages the production and release of thyroid hormones.

When should I test for free T3 and free T4?

You should test your levels of free T3 and T4 if you begin to feel symptoms associated with hormone imbalance.

If your free T3 and T4 levels are elevated, you might experience:

  • Anxiety
  • Weight loss
  • Difficulty sleeping 

If your free T3 and T4 levels are low, you might experience:

  • Hair loss
  • Weight gain
  • Tiredness 

This test may also be used to help evaluate the effectiveness of treatments taken to address a thyroid disorder.

What are normal T3 and T4  levels?

The normal range for adults for each hormone is¹:

  • Free T3: 0.26–0.48 ng/dL¹
  • Free T4: 0.7–2.0 ng/dL

What does a high level of T3 and T4 mean?

Elevated levels of free T3 and free T4 are associated with an overactive thyroid, which is also called hyperthyroidism. Hyperthyroidism is most commonly caused by Graves’ disease, an autoimmune disorder that causes the immune system to signal the thyroid to produce excess T4 and T3.

What does a low level of T3 and T4 mean?

Low levels of free T3 and T4 are associated with an underactive thyroid, also known as hypothyroidism. Hypothyroidism is most commonly caused by Hashimoto’s thyroiditis, an autoimmune disorder that leads to chronic inflammation of the thyroid gland, causing it to produce lower quantities of hormones.

How can I improve my T3 and T4 levels?

If your T3 and T4 levels are low, studies have shown that aerobic exercise increases total blood levels of T3 and T4²,³. Some foods interfere with thyroid function, particularly brassicas, peanuts, and soybeans and should be avoided by those with hypothyroidism. Always consult with your healthcare professional prior to making these lifestyle changes. Medication may be needed to improve thyroid hormones.


  1. Fischbach FT, Dunning MB III, eds. 2009. Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.  
  2. Ciloglu F, Peker I, Rehlivan A, et al. Exercise intensity and its effects on thyroid hormones. Neuro Endocrinol Lett 2005;6:830–834.
  3. Klubo-Gwiezdzinska J, Bernet VJ, Wartofsky L. Exercise and thyroid function. J Sport Exerc Psychol 2013:85–119.
  4. Walk-In-Lab. Best ways to improve thyroid function and health. Available from: https://www.walkinlab.com/blog/improve-thyroid-function-health/
Last updated:
October 13, 2021

Test for Free T4 + Free T3

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