What is T3?

T3 (triiodothyronine) is a thyroid hormone that plays a crucial role in regulating metabolism, growth, and development. It is mainly produced by the thyroid gland in its inactive form, T4, and then converted into the active form, T3. While most T3 in the blood is bound to proteins, free T3 (FT3) is the unbound, active form of T3 that circulates in the bloodstream and is immediately available for use by the body.

Table of Contents

• What is T3?
• The difference between T3 and FT3
• Where is the hormone T3 produced?
• Why is T3 important?
• What are normal T3 and FT3 levels?
• Symptoms of low T3 levels
• Symptoms of high T3
• What do abnormal T3 levels mean?
• What should I do if my T3 level is abnormal?

What is T3?

T3 is a hormone that is partly produced by the thyroid gland. Most T3 is made by converting T4 into 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 based on the body’s needs.

T3 affects the following bodily processes:

• Metabolism

• Mood regulation

• Body temperature

• Influence on growth hormone

• Impact on heart rate

• Influence on digestion

• Influence on mental well-being

Source: Cleveland Clinic, 2022

The difference between T3 and FT3

Most T3 in the blood is bound to proteins and cannot be used by body tissues. Free T3 (FT3) is the T3 in the blood that is not bound to proteins and can therefore be used by the body.

Where is the hormone T3 produced?

T3 and T4 are sometimes referred to as a single thyroid hormone because they work together in the body. T3 is the active hormone that affects body processes and is formed from the inactive hormone T4. T4 is converted into T3 by the enzyme deiodinase in the liver and kidneys. This process is called deiodination. During deiodination, one iodine molecule is removed from T4, forming T3. Most of the T3 is produced through deiodination, and only about one-fifth is directly produced 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 a wide range of bodily processes, including the proper functioning of the thyroid gland. A low T3 level may indicate hypothyroidism. Conversely, high T3 levels may indicate hyperthyroidism or an overactive thyroid. Both too much and too little T3 can cause unpleasant symptoms and may require treatment.

What are normal T3 and FT3 levels?

The normal range for free T3 (FT3) is between 3.5 and 7.8 pmol/L, while total T3 levels range between 1.3 and 3.1 nmol/L. Abnormal FT3 and T3 values can indicate thyroid problems such as hyperthyroidism (too high) or hypothyroidism (too low). This can be checked by your general practitioner or with a home test. FT3 and T3 levels can be measured with a blood test. This can be done by your GP or with 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 considered too high if it is above 7.8 pmol/L (450 pg/dL). Elevated levels of T3 or FT3 may indicate hyperthyroidism, which can lead 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 is below 3.5 pmol/L (230 pg/dL). Low T3 and FT3 levels may indicate hypothyroidism, in which the thyroid does not produce enough hormones, leading to fatigue, weight gain, and mood changes.

Symptoms of low T3 levels

A low T3 level may be caused by hypothyroidism. Symptoms of hypothyroidism include:

• Weight gain

• Constipation

• Fatigue

• Depression

• Sensitivity to cold

• Brittle hair and nails

• Muscle cramps and weakness

• Irregular periods

Source: NHS

Symptoms of high T3

A high T3 level may be caused by hyperthyroidism. Symptoms of hyperthyroidism include:

• Sleep problems

• Nervousness, anxiety, or irritability

• Hyperactivity

• Mood swings

• Muscle weakness

• Sensitivity to heat

• Itching

• Diarrhoea

• Persistent thirst

Source: NHS

What do abnormal T3 levels mean?

A T3 level outside the normal range may indicate a deficiency or excess of thyroid hormones. Both hypothyroidism and hyperthyroidism require medical follow-up and treatment to restore thyroid hormone levels to a healthy range and reduce symptoms.

What should I do if my T3 level is abnormal?

If you had a T3 test done by your doctor, they will interpret the test results and advise you on the next steps. If you performed a home thyroid test, bring the lab report to your GP for further evaluation.