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T3 (triiodothyronine) is a thyroid hormone that plays a key role in regulating metabolism, growth, and development. It is primarily produced by the thyroid gland as inactive T4, which is then converted into the active form, T3. While most T3 in the blood is bound to proteins, free T3 (FT3) comprises the unbound, active form of T3 circulating in the bloodstream, available for immediate use by the body.
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 and FT3 values?
The normal range for free T3 (FT3) is between 3.5 and 7.8 pmol/L, while total T3 levels fall between 1.3 and 3.1 nmol/L. Abnormal FT3 and T3 values may indicate thyroid problems, such as hyperthyroidism (too high) or hypothyroidism (too low). FT3 and T3 levels can be checked using a blood test. This can be performed at your GP or using a home test (Mount Sinai).
When is T3 or FT3 too high?
T3 is considered too high when levels exceed 3.1 nmol/L (200 ng/dL), while FT3 is too high when it surpasses 7.8 pmol/L (450 pg/dL). Elevated levels of either T3 or FT3 can indicate hyperthyroidism, leading to symptoms such as rapid heartbeat, weight loss, tremors, and nervousness.
When is T3 or FT3 too low?
T3 is considered too low when levels fall below 1.3 nmol/L (80 ng/dL), and FT3 is too low when it drops below 3.5 pmol/L (230 pg/dL). Low T3 and FT3 levels can indicate hypothyroidism, where the thyroid produces insufficient hormones, leading to fatigue, weight gain, and mood changes.
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.
References
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
