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Triiodothyronine (T3, Free)

What is Triiodothyronine?

Triiodothyronine (T3) is a hormone produced by the thyroid gland in the neck. Triiodothyronine is the more active form of the thyroid hormone, thyroxine. Around 80% of the T3 is produced from the conversion of thyroxine by organs including the liver and kidneys, while the rest is secreted directly into the blood. The thyroid hormones have many roles in the human body, particularly its metabolic rate. [1]

Why take a Triiodothyronine blood test?

Most of the triiodothyronine in the body is bound to special proteins and the rest is free. A triiodothyronine test can measure both (total) or it can measure just the free triiodothyronine (FT3).

An FT3 test helps to determine if the thyroid gland is working properly. It is mainly used to diagnose an overactive thyroid or hyperthyroidism.

You can test your triiodothyronine levels along with other key thyroid hormones by purchasing a simple at-home finger prick test kit which is then analysed at an accredited lab. Forth offers a number of blood tests which include triiodothyronine such our thyroid function check and our Vitality package which includes analysis of over 30 key biomarkers for good health.

What function does Triiodothyronine have in the body?

Triiodothyronine is one of two major thyroid hormones. It makes up approximately 10% of the total thyroid hormone in the body, the rest is thyroxine or T4. T3, however, is around four times as active as T4 and is associated with most of the effects produced by thyroid hormones. When T3 enters the liver, it is converted to T4.

Both T3 and T4 have roles in the metabolic rate of the body, making all the cells work harder and increasing their energy needs, this causes the following effects:

  • A rise in body temperature
  • Increased pulse rate and stronger heartbeat
  • Brain maturation and growth in children
  • Food is used up at a faster rate because energy stored in the liver and muscles are broken down [2]

How do changes in Triiodothyronine affect health and wellbeing?

Changes in triiodothyronine levels in the body can affect health and wellbeing. The thyroid gland can become underactive and overactive causing increases or decreases in hormone secretion into the blood.

An overactive thyroid happens if the thyroid gland produces too many hormones or underactive thyroid is where the thyroid gland doesn’t produce enough hormones. The imbalance in these hormones can both cause several symptoms.

If you are worried about your thyroid function or just want to check where you fall on the range, you can test your triiodothyronine level with a simple at-home blood test.

What can cause Triiodothyronine to change?

Hyperthyroidism causes excess thyroid hormones to be produced and is also known as an overactive thyroid. The most common cause of hyperthyroidism is an autoimmune disease where the immune system acts on its own thyroid cells, called Graves’ Disease. It is believed Graves’ disease is caused by a genetic predisposition.[3]

Hyperthyroidism, thyroid inflammation or a benign tumour can cause a condition called thyrotoxicosis. Thyrotoxicosis occurs when there is too much thyroid hormone in the blood and can result in swelling in the neck called a goitre.

Hypothyroidism occurs because of an underactive thyroid. In this case, the thyroid does not produce enough thyroid hormone. A variety of factors can cause hypothyroidism.

The most common cause of hypothyroidism is autoimmune thyroid disease where the body’s immune system attacks its own thyroid cells as if they were foreign. A common form is Hashimoto’s thyroiditis. The condition has a genetic susceptibility with both immune-related and thyroid-specific genes involved in the development of the disease.[4]

Other causes of hypothyroidism include radioactive iodine treatment and antithyroid drugs which are used to correct and treat an overactive thyroid. Some cough medicines also contain high amounts of iodine which can interfere with thyroid function. This is also true of some health foods like kelp, another food high in iodine. Iodine is essential to produce thyroid hormones.[5]

What are the most common symptoms?

The symptoms of an overactive thyroid are:

  • Changes in mood
  • Anxiety
  • Nervousness
  • Difficulty sleeping
  • Irregular or fast heartbeat
  • Weight loss
  • Trembling
  • Increased risk of cardiovascular risk
  • Swelling in the neck, goitre

The symptoms of an underactive thyroid, include:

  • Fatigue
  • Weight gain
  • Dry skin and hair
  • Depression
  • Muscle aches
  • Sensitivity to the cold

How to keep Triiodothyronine in the healthy range

Thyroid dysfunction is often caused by an autoimmune condition and although it may require medical intervention, lifestyle changes can help to restore its function. Changes to diet and lifestyle can help to control the symptoms of an over or underactive thyroid.

Although iodine is essential for the function of the thyroid gland, too much or too little can have negative effects. [6] Adults need 150 micrograms of iodine per day, most of which should be able to be obtained through eating a healthy, balanced diet. Individuals who are already taking thyroid medication like levothyroxine for an underactive thyroid, then you should not need to supplement your diet with iodine. [7]

Individuals with a thyroid condition are susceptible to vitamin B12 deficiency. It is particularly prevalent in hypothyroidism possibly because of their nutritional status. Plus, autoimmune thyroid disease is associated with autoimmune disorders such as pernicious anaemia which can lead to vitamin B12 malabsorption. [8] Therefore, an adequate intake of B vitamins is essential. Vegans and vegetarians are also susceptible to deficiency because vitamin B12 is found in animal products. Although some foods are fortified with the vitamin, susceptible groups may need supplementation. Good sources of vitamin B12 are:

  • liver
  • beef
  • haddock
  • tuna
  • salmon
  • trout
  • eggs
  • milk
  • cheese
  • fortified breakfast cereals [9]

Physical exercise has great effects on energy metabolism by increasing energy expenditure as well as increasing the body’s resting metabolic rate for hours following exercise. Some research has shown that exercise performed at 70% of maximum heart rate increases the level of T3 and FT4. [10]

However, some studies contradict this and state exercise doesn’t have any major effect on circulating levels of thyroid hormones. Yet, exercise has other beneficial advantages for weight loss, psychological symptoms like depression and anxiety, all of which can affect individuals with a thyroid issue.

Triiodothyronine (T3, Free) Tests

All these tests include Triiodothyronine (T3, Free). Select the test that suits your personal needs.

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Ultimate Health Check

Advanced health test that checks heart, liver, kidney, thyroid, and muscle function, nutrition, immune health, hormones and more.

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Advanced Thyroid Blood Test

Test thyroid function and check for thyroid autoimmune conditions with our advanced thyroid blood test.

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[1] Lab Tests Online UK. (2017). FT3. Available at:

[2] [Internet]. (2010). How does the thyroid gland work Cologne, Available from:

[3] Longo, D, L. (2016). Graves’ Disease. The New England Journal of Medicine: 375, pp 1552-65.

[4] Ajjan, R, A and Weetman, A, P. (2015). The Pathogenesis of Hashimoto’s Thyroiditis: Further Developments in Our Understanding. Horm Metab Res: 47, pp 702-710.

[5] British Thyroid Function. (2018). Hypothyroidism. Available at:

[6] Chung, H, R. (2014). Iodine and Thyroid Function. Ann Pediatr Endocrinol Metab: 19, pp 8-12

[7] British Thyroid Foundation. (2018). Thyroid and Diet Factsheet. Available at:

[8] Collins, A, B and Pawlak, R. (2015). Prevalence of Vitamin-B12 Deficiency Among Patients with Thyroid Dysfunction. Asia Pac J Clin Nutr: 25(2), pp 221-226.

[9] National Institutes of Health: Office of Dietary Supplements. (2018). Vitamin B12. Available at:

[10] Ciloglu, F., Peker, I., Pehlivan, A., Karacebey, K., Ilhan, N., Saygin, O and Ozmerdivenli, R. (2005). Exercise Intensity and its Effects on Thyroid Hormones. Neuro Endocrinol Lett: 26(6), pp 830-834.

Medically reviewed

This information has been medically reviewed by Dr Nicola Keay.

Nicola has extensive clinical and research experience in the fields of endocrinology and sport and exercise medicine. Nicky is a member of the Royal College of Physicians, Honorary Fellow in the Department of Sport and Exercise Sciences at Durham University and former Research Fellow at St. Thomas’ Hospital.

Nicky Keay
Dr Nicola Keay

BA, MA (Cantab), MB, BChir, MRCP.​

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