Triiodothyronine (T3, free)

What Is T3?

Triiodothyronine (T3) is a hormone produced by the thyroid gland in the neck. Alongside thyroxine, triiodothyronine is one of the main hormones produced by the gland, although thyroine is also converted to triiodothyronine elsewhere in the body. Most of the triiodothyronine in the body is attached to a protein, while the rest is unattached and free (FT3).[1]  It is the FT3 that is measured and active.

Which tests include this marker?

What Role/s Does It Play in The Body?

Triiodothyronine makes up around 10% of ‘thyroid hormone’ while thyroxine or T4 makes up the rest. However, T3 is thought to be four times as active as T3 and is associated with most of the effects thyroid hormones produce. There is more T3 circulating in the body than T4. When T3 is in the liver it is converted to T4. T3 has similar functions to T4 in that it increases the rate of metabolic activity in the cells and tissues. During childhood, it is essential for growth and development while in adults it is required for metabolic control.[2]

How Does Triiodothyronine Affect My Wellbeing?

High levels of triiodothyronine can indicate an overactive thyroid.  Some of the symptoms of an overactive thyroid include:

  • Mood swings
  • Nervousness and anxiety
  • Fatigue
  • Trouble sleeping
  • Swelling in the neck, known as a goitre
  • Irregular or fast heartbeat
  • Weight loss
  • Trembling

An overactive thyroid is much more common in women than it is in men and usually occurs between the ages of 20 and 40.[3]

An underactive thyroid or hypothyroidism occurs when the thyroid gland doesn’t produce enough T3. Underactive thyroid symptoms can include:

  • Fatigue
  • Weight gain
  • Depression
  • Dry skin and hair
  • Muscular Aches
  • Sensitivity to the cold

When the thyroid becomes underactive, it doesn’t produce enough hormones that are responsible for maintaining the cell's metabolism. As a result, many of the body’s functions will slow down.[4]

How Can I Improve My Result?

Thyroid function can be a result of a deficiency or an autoimmune disorder – where the body’s immune system attacks its own cells. Although, it may require medical intervention to restore its function, changes in diet and lifestyle can be beneficial in helping to cope with the condition.


Normal thyroid function can be helped by eating a varied and healthy diet. It is important to include calcium-rich foods and keep your vitamin D at optimum levels. If this can’t be achieved through diet alone, it may be necessary to consider supplementation. 

Iodine is important for thyroid function, but too much or too little can have adverse effects.[5] Adults require 150 micrograms per day, with most of this being obtained through the diet. However, if you are taking medication such as levothyroxine for an underactive thyroid there is no need to supplement with iodine.[6]

Vitamin B12 deficiency can be prevalent in thyroid patients, particularly those with hypothyroidism. One possible cause may be the nutritional status of these individuals.[7] So, it is essential to have an adequate intake of B vitamins. Vitamin B12 is found in animal products. Although breakfast cereals and bread may be fortified with vitamin B12, vegetarians and vegans may require supplementation to ensure the correct intake. Good sources of vitamin B12 are:

  • Liver
  • Beef
  • Haddock
  • Tuna
  • Salmon
  • Trout
  • Eggs
  • Milk
  • Cheese
  • Fortified breakfast cereals[8]



Participation in physical exercise has a great effect on energy metabolism by increasing energy expenditure and the resting metabolic rate for hours after exercise. Exercise has been shown in some studies to increase the amount of circulating thyroid hormones.[9]

Although some studies have found that exercise doesn’t have any great effect on circulating levels of thyroid hormones, it can have other beneficial effects. For example, it can help with weight loss, psychological symptoms such as anxiety and depression as well as improving cardiopulmonary fitness.

Tests that include this marker

Thyroid Check

A look at the main thyroid hormones which indicate if the thyroid is functioning correctly.



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

[2] John, R., Christofides, N., Spencer, C, A and Wild, D. (2013). Chapter 9.2 – Thyroid. In: The Immunoassay Handbook (Fourth Edition).

[3] National Health Service. (2016). Overactive Thyroid (Hyperthyroidism). Available at:

[4] National Health Service. (2018). Underactive Thyroid (Hypothyroidism). Available at:

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

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

[7] 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.

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

[9] 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. 

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