Thyroxine (T4, free direct)

What Is Thyroxine?

Thyroxine (T4) is a hormone produced by the thyroid gland in the neck. Alongside triiodothyronine, thyroxine is one of the main hormones produced by the gland. Most of the thyroxine in the body is attached to a protein while less than 1% is unattached and free.[1] 

Which tests include this marker?

What Role/s Does It Play in The Body?

Thyroxine acts on several different target cells in the body. It stimulates the use of oxygen and heat production by many cells to produce energy. It also helps the metabolism of cells by using up carbohydrates to make energy and increases the break down of proteins and oxidises fats which can result in weight loss. The hormone has a wide range of functions throughout the body.[2]

How Does Thyroxine Affect My Wellbeing?

If too much of thyroxine is released into the blood this can indicate hyperthyroidism. 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
  • Cardiovascular risk

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 T4. 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 which 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 to 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 in 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.


Female Fertility

This profile analyses key biomarkers which can not only affect your fertility, but also mood, energy and weight.


Menopause Health

For women in various stages of the menopause who want to check hormone levels as well as the impact changes may be having on their overall wellbeing.



With over 40 biomarkers, this health check empowers you to understand much more about your health on the inside.



Our most advanced health check which analyses almost 50 biomarkers. For those who want a deep understanding of their health.



[1] Lab Tests Online UK. (2017). FT4. Available at:

[2] Rosol, T, J., DeLellis, R, A., Harvey, P, W and Sutcliffe, C. (2013). Chapter 58 – Endocrine System. In: Haschek and Rousseaux’s Handbook of Toxicologic Pathway. 

[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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