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Thyroxine (T4, Free Direct)

What is Thyroxine (T4)?

Thyroxine (T4) is the major hormone secreted by the thyroid gland, located in the neck. Thyroxine is converted to its more active form, triiodothyronine by the liver and kidneys. Most thyroxine is bound to specific proteins, while less than 1% travels freely around the body. It is this free thyroxine or FT4 which affects the functions of the body.[1]

Why take a Thyroxine blood test?

A thyroxine test measures the amount of free thyroxine in the blood, that is the amount of hormone which is not bound to proteins. Most of what we refer to as thyroid hormone is thyroxine  (T4) because triiodothyronine (T3) constitutes a smaller percentage.

Most laboratory tests measure free thyroxine or FT4 because it is this which is thought to be responsible for the biological effects of the hormone in the body.Thyroid hormones have important roles in metabolism, heart function, muscle control, brain development and bone maintenance.[2]

You can test your thyroxine 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 thyroxine such our thyroid function checkFemale FertilityMenopause Health and our Vitality package which includes analysis of over 30 key biomarkers for good health.

What function does Thyroxine have in the body?

Thyroxine (T4) is the major hormone secreted by the thyroid gland, the other is triiodothyronine (T3). Both T3 and T4 have roles in the metabolic rate of the body, making al 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 store in the liver and muscles are broken down[3]

Therefore, thyroxine stimulates the production of energy by encouraging the use of oxygen and the production of heat by the cells in the body. It also helps the metabolism of cells through the using up of carbohydrates to produce energy and increasing the break down of proteins and the oxidation of fats causing weight loss.

​How do changes in Thyroxine affect health and wellbeing?

Changes in thyroxine 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 thyroxine level with a simple at-home blood test.

What can cause Thyroxine 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.[4]

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.[5]

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.[6]

What are the most common symptoms of an underactive or overactive thyroid?

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 Thyroxine 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. [7] 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, should not need to supplement their diet with iodine. [8]

Individuals with 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. [9] 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 [10]

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. [11]

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.

Thyroxine (T4, Free Direct) Tests

All these tests include Thyroxine (T4, Free Direct). 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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Female Hormone Imbalance Test

This single-day test gives a simple snapshot of your key female hormones along with thyroid function.

hormone test, estrogen, prolactin, fsh, Follicle-Stimulating Hormone, Follicle Stimulating Hormone, estradiol,
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[1] Lab Tests Online UK. (2017). FT4. Available at:

[2] You and Your Hormones. (2018). Thyroxine. Available at:

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

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

[5] 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.

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

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

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

[9] 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.

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

[11] 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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