What is T3?

What is T3?

Written by

Lauren Dobischok
19 December, 2022

Medically checked article All HOMED-IQ content is reviewed by medical specialists

Triiodothyronine (T3) is one of the two major thyroid hormones and affects nearly every biological process in your body. T3 works with thyroid hormone thyroxine (T4) to control metabolism, organ function, growth, mood, and sleep. Most T3 in the blood is bound to proteins, while only a minimal part is present as a free hormone known as FT3. FT3 is the form of T3 that is usable by your body tissues. T3 is important to all body processes and should not be too high or too low for optimal health.

What is T3?

T3 is a hormone that is partly produced by the thyroid gland. The majority of T3 is made by converting T4 to T3 through a process called deiodination. Both T3 and T4 regulate various important body processes, such as body temperature, metabolism, body weight, and the quality and quantity of hair. T4 is converted into T3 according to the body’s needs.

T3 influences the following body processes:

  • Metabolism
  • Mood flow
  • Body temperature
  • Influence on growth hormone
  • Effect on heart rate
  • Influence on digestion
  • Influence on mental well-being

Source: Cleveland Clinic, 2022

The difference between T3 and FT3

Most T3 is bound to proteins in the blood, and is not usable by the body tissues. Free T3 (FT3) is the T3 in the blood that is not bound to proteins and is therefore usable by the body.

Where is T3 hormone made?

T3 and T4 are sometimes referred to as one thyroid hormone because they work together in the body. T3 is the active hormone that impacts body processes, and is created from the inactive hormone T4. T4 is converted to T3  by the enzyme deiodinase via the liver and kidneys. This process is known as deiodination. During deiodination, an iodine molecule is removed from T4, creating T3. Most of T3 arises from deiodination, with only about a fifth being produced directly by the thyroid gland. While T3 is the active thyroid hormone, T4 is important for transporting T3 to where it is needed.

Why is T3 important?

T3 is part of the body’s complex endocrine system and affects all kinds of bodily processes, including the proper functioning of the thyroid gland. Low T3 can indicate hypothyroidism, or an underactive thyroid. Conversely, high T3 levels can indicate hyperthyroidism, or an overactive thyroid. Both too much or too little T3 can cause unpleasant symptoms and may require treatment. 

What are normal T3 values?

A healthy T3 value is related to how well the thyroid gland is functioning. FT3 and T3 levels can be checked using a blood test. This can be performed at your GP or using a home test. The normal range for T3 is 0.9 to 2.8 nanomoles per liter (nmol/L) (Mount Sinai). The normal range for FT3 is 3.08 – 6.78 picomoles per liter (pmol/L). If your T3 or FT3 levels are outside this range, see your doctor. 

Symptoms of low T3

Low T3 can be caused by an underactive thyroid, also known as hypothyroidism. Symptoms of hypothyroidism include:

  • Weight gain
  • Constipation
  • Tiredness
  • Depression
  • Sensitivity to cold
  • Brittle hair and nails
  • Muscle cramps and weakness
  • Irregular periods

Source: NHS

Symptoms of high T3 

High T3 can be caused by an overactive thyroid, or hyperthyroidism. Symptoms of hyperthyroidism include:

  • Difficulty sleeping
  • Nervousness, anxiety, or irritability
  • Hyperactivity
  • Mood swings
  • Muscle weakness
  • Sensitivity to heat
  • Itchiness
  • Diarrhea
  • Persistent thirst

Source: NHS

What do abnormal T3 levels mean?

An T3 value outside the normal range can indicate a shortage or excess of thyroid hormone. Both hypothyroidism and hyperthyroidism require follow-up with your doctor and treatment to return thyroid hormones to healthy levels and reduce symptoms. 

What should I do if my T3 level is abnormal?

If you have performed a T3 test at the doctor, they will interpret the test results and provide follow-up advice. If you have performed a home thyroid test, take the laboratory report to your GP for further evaluation. 


NHS website. (2021, November 18). Symptoms. nhs.uk. https://www.nhs.uk/conditions/overactive-thyroid-hyperthyroidism/symptoms/

NHS website. (2021, November 18). Symptoms. nhs.uk. https://www.nhs.uk/conditions/underactive-thyroid-hypothyroidism/symptoms/

T3 test. (n.d.). Mount Sinai Health System. https://www.mountsinai.org/health-library/tests/t3-test

T3 (Triiodothyronine) Test: What It Is, Function & Levels. (n.d.). Cleveland Clinic. https://my.clevelandclinic.org/health/diagnostics/22425-triiodothyronine-t3

About the author

Lauren Dobischok

Lauren is a health scientist and science communicator currently living in the Netherlands. Originally from Canada, she completed a Research Master’s in Health Sciences at the Netherlands Institute of Health Sciences at Erasmus University Rotterdam (NIHES) with a specialisation in epidemiology. Prior to her master’s degree, she completed a Bachelor’s degree in Health Sciences at Simon Fraser University. With a background in public health, her goal is to create accurate scientific content that is easy to understand and empowers people to make informed decisions. Within Homed-IQ, Lauren works as a Product Developer and Content Lead, working closely with physicians and scientists on medical devices for Homed-IQ’s new products and written communications.