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Thyroxine (T4, Free Direct)

Thyroxine is an important thyroid hormone that regulates metabolism, and body temperature and supports the normal function of the heart and skeletal muscles.

Author: Leanne Edermaniger

April 24, 2024

Reviewed by: Dr Thom Phillips

In this article:

What is Thyroxine (T4)?

The thyroid gland is a small, butterfly-shaped gland situated In the neck which has essential roles in the regulation of your metabolism, muscle function, growth and development. The thyroid gland is responsible for producing two thyroid hormones:

  1. Triiodothyronine (T3)
  2. Thyroxine (T4)

Thyroxine is the main hormone produced and secreted by the thyroid gland. Compared to triiodothyronine, thyroxine is the less active form of thyroid hormone. However, most of the thyroxine circulating in the blood is converted to the more active form, T3, by the liver, kidneys, heart muscles, skeletal muscle, and central nervous system[1].

Thyroxine is responsible for controlling how much energy the body uses, also known as the metabolic rate, and other organ systems. For example, too much or too little thyroxine can affect heart function.
An overactive thyroid, hyperthyroidism, occurs when too much thyroxine (and triiodothyronine) is produced and can cause the heart to beat faster, triggering abnormal heart rhythms. One example is atrial fibrillation, but high thyroxine levels may also increase the risk of cardiovascular disease and heart failure.

An underactive thyroid (hypothyroidism), on the other hand, can cause your heart rate to slow down because of a lack of circulating thyroid hormones. Low thyroxine can lessen the elasticity of the blood vessels, making it more difficult to pump blood back to the heart, increasing blood pressure. Hypothyroidism is also associated with high blood cholesterol levels and an increased risk of heart disease[2].

Around 0.2% of free T4 is unbound and active in the blood, the rest is attached to carrier or transporter proteins. Thyroxine-binding globulin (TBG) binds to two-thirds of circulating T4. When T3 and T4 reach their target cells, they can dissociate from their bound protein and enter the cells via diffusion or cell-mediated transport. Thyroid receptors can bind to T3 and T4, but they have a higher affinity for T3, the activated form of thyroid hormones. T4 can be converted into T3 through a process known as deiodination, which removes the iodine molecule from T4 transforming, it into T3[3].

What is a Normal Thyroxine Level?

A normal thyroxine level will vary depending on the testing laboratory. However, a rough guide is that a normal thyroxine range should be between 9 and 24 pmol/L[4].

Data collected in 2024 shows that 95% of our customers have healthy thyroxine levels, with an average level of 16.3 pmol/L.

  • The average thyroxine level in men is 17 pmol/L
  • The average thyroxine level in women is 15.8 pmol/L

What Causes High Thyroxine?

There are several reasons why thyroxine levels increase. They can range from underlying conditions to lifestyle choices.

Generally, high levels of thyroid hormones are associated with an overactive thyroid or hyperthyroidism. It is a relatively common condition, which affects more women than men, where excess thyroid hormones cause the body’s metabolic process to speed up, resulting in a variety of symptoms, including weight loss, increased appetite, and anxiety[5].

Some of the causes of high thyroxine levels are:

  • Autoimmune conditions: The most common cause of an overactive thyroid is the autoimmune condition, Graves’ disease. The condition causes your immune system to attack the thyroid gland, causing it to produce more thyroid hormones, including thyroxine than it needs, resulting in many of the body’s functions speeding up[6].
  • Overactive thyroid nodules: Thyroid nodules can be solid or fluid-filled lumps that appear in the thyroid gland. The thyroid can also become enlarged resulting in a goitre. Most thyroid nodules are harmless[7], but they can cause an overproduction of thyroid hormone, and increase the risk of raised thyroxine levels.
  • Thyroiditis: If the thyroid gland becomes inflamed, this is known as thyroiditis and can cause thyroxine levels to rise or fall. Several types of thyroiditis can result in high thyroid levels, including subacute, postpartum, and silent/painless.
  • High dietary intake of iodine: Iodine is essential for the production of thyroid hormones and because the human body is unable to make iodine, we must get it from our diet. However, in some people, consuming large amounts of iodine-rich foods can cause the thyroid to make too much thyroid hormone.
  • Excessive thyroid medication: If you have an underactive thyroid, are given levothyroxine, and have high thyroxine levels, you may be taking too much. It is important to monitor your thyroid hormone levels and your doctor may need to adjust your medicine if your thyroxine levels are too high.

What happens when thyroxine is too high?

  • Anxiety, nervousness, and irritability
  • Weight loss
  • Finding it difficult to sit still
  • Sleep issues
  • Mood swings
  • Fatigue
  • Muscle weakness
  • Sensitivity to heat
  • Increased thirst
  • Needing to pee more often than normal
  • Low libido
  • A swelling in the neck called a goitre
  • Increased heart rate
  • Twitching
  • Hair loss or thinning hair
  • Loose nails
  • Excessive sweating
  • Hives
  • Red or dry eyes or other vision problems[8]

Does high thyroxine cause weight gain?

Weight gain is not a common symptom of hyperthyroidism. People with an overactive thyroid and therefore high thyroxine levels are more likely to experience weight loss.

Can too much thyroxine damage your heart?

Yes, too much thyroxine can damage your heart and increase the risk of heart issues. A common effect on the heart from too much thyroid hormone is the development of an abnormal heart rate, called atrial fibrillation. Atrial fibrillation increases the risk of heart attacks in people without coronary heart disease[9].

What Causes Low Thyroxine?

If the thyroid doesn’t produce enough triiodothyronine and thyroxine, it results in a condition called hypothyroidism or an underactive thyroid.

The most common causes of hypothyroidism are:

  • Autoimmune disease: The most common cause of hypothyroidism is Hashimoto’s disease. The condition causes your body’s immune system to attack the thyroid cells, resulting in the thyroid becoming inflamed, and not making enough thyroid hormones
  • Thyroiditis: Thyroiditis is a condition which causes the thyroid to become inflamed. It can initially cause a rise in blood hormone levels but will eventually lead to the thyroid becoming underactive
  • Congenital hypothyroidism: Some babies are born with an underactive thyroid because the gland doesn’t develop properly in the womb.
  • Surgical removal of the thyroid: Some treatments for an overactive thyroid, such as radioactive iodine therapy and surgery, mean that the thyroid either doesn’t produce enough thyroid hormones or if it has been completely removed, produces none

What symptoms does low T4 cause

Because low levels of T4 are indicative of hypothyroidism, you may experience the following symptoms:

  • Weight gain
  • Fatigue
  • Constipation
  • Sensitivity to cold temperatures
  • Depression
  • Muscle aches and weakness
  • Dry, scaly skin
  • Brittle hair and nails
  • Slow movements
  • Delayed thoughts
  • Pain, numbness and tingling in the hands and fingers
  • Irregular or heavy periods[10]

Does low T4 make you tired?

Yes, low thyroxine or an underactive thyroid can cause tiredness and fatigue. There are many reasons why thyroid disorders can make you feel like this, but a major reason is that with an underactive thyroid, the body’s metabolism slows down[11]. For most people with hypothyroidism, levothyroxine treatment improves the symptoms of fatigue[12].

Should low T4 be treated?

If you have low thyroxine, you may need hormone replacement therapy. Some people with sub-clinical hypothyroidism, that is high thyroid stimulating hormone (TSH) levels but regular thyroid hormones, may not require treatment. Many people with a persistently low thyroxine level will be prescribed a medication called Levothyroxine, which is a synthetic form of Thyroxine hormone.

Thyroxine is the most important of the thyroid hormones because it is responsible for regulating many of the functions related to metabolism.

Can I Control My Thyroxine Level?

Treatment for an underactive thyroid usually consists of a daily hormone replacement called levothyroxine. It can also be prescribed following treatment with radioactive iodine or surgery for an overactive thyroid.

Levothyroxine replaces the natural hormone that the thyroid gland isn’t producing enough of. Usually, you will be given a low dose to begin with, and it may be increased gradually depending on how your body responds to the treatment.

What to expect when you start taking thyroxine

If you have an underactive thyroid that requires medical treatment, your doctor will prescribe levothyroxine, a synthetic version of thyroxine. Although it begins to work straight away, you may not notice any effects or benefits for a few weeks.

Because levothyroxine is replacing a natural hormone produced by the body, it is rare for side effects to occur. However, if you do experience side effects, then it may be because your dose is too high. If you experience new side effects while taking levothyroxine, you should inform your doctor so they can look to reduce your dose, if required.

How can I boost my underactive thyroid naturally?

It’s natural to look for ways to improve T4 levels if your thyroid blood test results show you have low levels. A good place to start is often with your diet. Although there are no specific diets recommended for thyroid disorders, there are some things you can do to help improve its function:

  • Make sure you eat five portions of fruit and vegetables per day, that’s equivalent to 400g
  • Use high-fibre starchy foods as a base for each meal, like potatoes, brown rice, whole grain pasta
  • Incorporate some dairy products or dairy alternatives into your diet
  • Choose beans, pulses, fish, meat, and eggs, as protein sources
  • Replace saturated fats with unsaturated oils and spreads, but limit your intake of these
  • Cut down your sugar intake
  • Drink 6 to 8 glasses of water or fluid per day[14]

Research shows that a deficiency in the micronutrient, selenium could be implicated in people with Hashimoto’s thyroiditis. Therefore, supplementation could be beneficial[15]. Low thyroid hormones may also affect the body’s vitamin B12 level[16], so it may be useful to consider taking a supplement or increasing your consumption of B12-rich foods, such as:

  • Fish and shellfish
  • Liver
  • Red meat
  • Eggs
  • Dairy products
  • Poultry
  • Fortified cereals

Although exercise may not directly increase the natural production of thyroid hormones[17], physical activity may help to manage some of the symptoms associated with an underactive thyroid. However, some of the symptoms, such as muscle aches, low mood, and fatigue can make exercise challenging. Here are some of our top tips to help you get started:

Speak to your GP before starting any new exercise regime.

Build up to more vigorous exercise. You should begin by setting yourself manageable targets.

Exercise doesn’t just mean joining a gym, it can mean building in everyday activities, such as walking the dog or gardening.

Find an activity you enjoy doing. If there is a particular type of activity you like, such as dancing or swimming, have a look to see what classes are available in your local area.

Find an exercise buddy. Try taking up a hobby with a friend or find a local exercise class. Working in a group can be a real motivator and help you stick to your activity plans.

Written by Leanne Edermaniger

Based in the UK, Leanne specialises in writing about health, medicine, nutrition, and fitness.

She has over 5 years of experience in writing about health and lifestyle and has a BSc (hons) Biomedical Science and an MSc Science, Communication and Society.

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Article references

This article was written by Leanne Edermaniger

This information has been medically reviewed by Dr Thom Phillips

Thom works in NHS general practice and has a decade of experience working in both male and female elite sport. He has a background in exercise physiology and has published research into fatigue biomarkers.

Dr Thom Phillips

Dr Thom Phillips

Head of Clinical Services