9 mins read

Triiodothyronine (T3, Free)

Triiodothyronine or T3 is one of two thyroid hormones responsible for regulating your metabolism and other life-enhancing processes.

Author: Leanne Edermaniger

July 21, 2020

Reviewed by: Dr Thom Phillips

In this article:

What is Triiodothyronine (T3)?

Triiodothyronine also known as T3 is a thyroid hormone that has several key roles in the body, such as managing metabolism, growth, and heart rate. It is the biologically active form of the thyroid hormone, thyroxine.

Triiodothyronine is produced as part of a feedback loop involving the hypothalamus, pituitary gland and the thyroid gland. Certain body tissues including the brain, liver and kidneys, control the activation of thyroid hormones through enzymes called deiodinases. These enzymes convert thyroxine (T4) into triiodothyronine (T3), accounting for approximately 80% of the circulating T3, the rest is released into the bloodstream by the thyroid gland[1].

When T3 is released into the circulation, it mostly binds to proteins, such as thyroxine-binding globulin (TBG). When attached like this, T3 is called bound T3, the rest is free T3 (FT3), a more biologically active form of triiodothyronine. When added together, both bound and free T3 make up your total T3 levels.

What is a Normal Triiodothyronine Level?

Generally, normal triiodothyronine levels differ with age, but the average for healthy adults is between 3.1 and 6.8 pmol/L[2].

Our data shows that 98% of Forth customers are within a healthy range for T3 with an overall average of 4.7 pmol/L:

  • Men: 5.0 pmol/L
  • Women: 4.7 pmol/L

What Causes High Triiodothyronine?

Some of the common causes of high T3 levels are:

Hyperthyroidism or an overactive thyroid: The thyroid gland produces too much of the thyroid hormones, causing an array of symptoms associated with the metabolic processes being sped up in the body, such as weight loss.

Pregnancy: Pregnancy causes a multitude of hormonal changes and thyroid hormone differences are no exception. During pregnancy, the thyroid is overstimulated, which can cause changes in thyroid hormone concentrations[3].

Thyroid function must be monitored during pregnancy.

Liver disease: Higher levels of circulating T3 in the blood have been found in patients with non-alcoholic fatty liver disease (NAFLD). These high levels often present alongside low thyroxine (T4) levels and an increased T3/T4 ratio[4].


Hyperthyroidism is caused by your thyroid producing too much triiodothyronine and thyroxine. If you have high levels of T3 it can cause symptoms such as weight loss, increased appetite, and an increased heartbeat because the excess thyroid hormone causes bodily processes to speed up.

Graves’ disease is the most common cause of an overactive thyroid, affecting approximately 4 out of 5 people with hyperthyroidism[5]. Graves’ disease is an autoimmune condition where the body builds antibodies against the thyroid stimulating hormone (TSH) receptor[6]. The condition affects more women than men.

What symptoms does high T3 cause?

  • Heart: Irregular or fast heartbeat, pounding heart
  • Mood: Anxiety, mood swings, irritability, trouble sleeping
  • Hair and skin: Hair loss, fine and brittle hair, thinning skin, excessive sweating, hives, red palms, warm skin, itchy skin, loose nails
  • Eyes: Vision issues, dry or red eyes

Gastrointestinal: Diarrhoea

  • Other symptoms include:
  • Weight loss
  • Low libido
  • Needing to pee more often than usual
  • Feeling tired all the time
  • Finding it difficult to sit still
  • A swelling in the neck caused by an enlarged thyroid gland called a goitre

How do you treat high T3 levels?

If the results of a thyroid blood test indicate that your T3 and T4 levels are raised, this could be a sign that you have an overactive thyroid.

You should discuss any abnormal results with your GP, you can download your test results from your health dashboard to share.

An overactive thyroid is often treatable, and you may be referred to a hormone specialist, called an endocrinologist, who will oversee any treatment.

There are three main treatments for hyperthyroidism:

Medicines called thioamides to stop your thyroid from overproducing hormones.

Radioactive iodine treatment which is a type of radiotherapy that destroys the thyroid gland cells, limiting the amount of hormones it can produce.

Surgery to remove all or part of the thyroid gland may be recommended if it is swollen because of a large goitre, you have eye problems associated with hyperthyroidism, other treatments haven’t worked, or your symptoms have returned.

What Causes Low Triiodothyronine?

Several things can cause low T3 levels:

  • Hypothyroidism
  • Medicines
  • Chronic health conditions
  • Low energy availability caused by reduced calorie intake and overtraining


An underactive thyroid or hypothyroidism is where your thyroid gland doesn’t produce enough thyroid hormones, including triiodothyronine. Without enough of these hormones being produced and released, the body’s functions slow down, causing some of the common symptoms[8].

The most common cause of an underactive thyroid is an autoimmune condition called Hashimoto’s thyroiditis. The condition causes the body to destroy the thyroid cells, leading to the reduction of thyroid hormone production, and is more common in women than men[9].

What are the symptoms of low T3?

  • Tiredness
  • Sensitivity to cold temperatures
  • Muscle aches and weakness
  • Muscle cramps
  • Weight gain
  • Constipation
  • Depression
  • Dry and scaly skin
  • Brittle hair and nails
  • Low sex drive
  • Pain, numbing and tingling in hands and fingers
  • Slow thoughts and movements
  • Irregular or heavy periods[10]

Can low T3 cause weight gain?

Yes, decreased T3 levels can cause weight gain, a symptom of an underactive thyroid. Thyroid hormones are responsible for regulating metabolism, heat production, the metabolism of glucose and fats, and food intake. Therefore, low thyroid hormone levels, including T3, can cause the metabolism to slow down and lower the number of calories burnt, resulting in weight gain[11].

How do you fix low T3?

If an underactive thyroid is diagnosed, it is often treated with a medication called levothyroxine.

Levothyroxine is a type of hormone replacement therapy which replaces the natural thyroxine your thyroid is not making enough of.

How Overtraining And Underfueling Can Affect T3 Levels?

There is a link between low T3 levels and a low body mass index (BMI) which is deeply rooted in the body’s response to energy availability and metabolism. For example, suppose the body detects an energy deficit, like not consuming enough calories (underfueling) to sustain physical performance or overtraining. In that case, there can be a reduced conversion of T4 into T3 as the body desperately tries to conserve its energy. This state is often associated with people who have a chronic low energy intake and may correspond with a low BMI.

At Forth, our data shows a correlation between individuals who have a low BMI and low T3 levels.

Because T3 is a sensitive biomarker for energy balance, a combined low T3 level and BMI could be indicative of low energy syndromes, such as Relative Energy Deficiency in Sport (RED-S).

RED-S expands on the diagnosis formerly known as the Female Athlete Triad, to incorporate a state of low energy availability that can have serious health consequences for any athlete regardless of their sex13.

RED-S causes several physical and psychological signs which, at first, can go unnoticed. For example, it is not uncommon for the athlete to have several recurring common infections, such as coughs or colds, or feeling generally fatigued. But these can be an underlying sign of the dysfunction going on within the body because of the inconsistency between nutrition and energy expenditure.

Some of the consequences of RED-S are:


  • increased risk of injury
  • reduced coordination skills
  • reduced muscle strength
  • fatigue
  • reduced glycogen storage
  • low libido


  • irritability
  • depression
  • lack of concentration
  • reduced judgment


  • reduced response to training
  • reduced endurance performance[12]

One of the key features of RED-S in female athletes is an absence of periods because of altered sex hormone levels, and at its most severe can cause reproductive dysfunction.

In men, RED-S can also affect sexual function, causing secondary hypogonadism because of low testosterone levels.

Low T3 levels are indicative of RED-S in both men and women. The reductions in this thyroid hormone negatively affects the conversion of energy from glycogen stores, resulting in the manifestation of physical and psychological symptoms.

Treatment for RED-S must focus on correcting the energy deficit alongside multi-disciplinary support.

Can I Control My Triiodothyronine Level?

It is possible to control your triiodothyronine levels. Depending on whether they are high or low will determine the type of action you may need to take.

How can I increase my T3 naturally?

If you have low T3 levels, you may be looking to increase them naturally. Although your triiodothyronine levels may be influenced by your lifestyle, raising them naturally will depend on what’s causing them to be low.

Underfueling and overtraining could be playing havoc with your hormone levels, particularly T3, influencing your overall energy availability. You are also at risk of REDs which can have some serious negative health consequences. This syndrome can lead to increased risks of injury, performance loss and, if not addressed, ultimately affect your long-term health. Treatment for this syndrome should centre around improving energy status with the overall goal of eliminating energy deficiency.

If you have an underactive thyroid, following a healthy, balanced diet will help ensure you are getting all the nutrients your body needs, including iodine, selenium, and zinc. Some recent research has found that some people with an underactive thyroid also have low vitamin B12 levels[13]. Therefore, it is important to ensure you are getting an adequate intake of B12 as well as other B complex vitamins. If you follow a vegan diet, you are at a greater risk of B12 deficiency as it is mostly found in animal-based products, so you may require supplementation.

Getting regular exercise can also benefit your metabolism and thyroid function, but if you have an underactive thyroid, the symptoms can make it more challenging to be physically active.

Here are some tips to help you get started:

  • Always speak to your doctor before starting a new exercise regime.
  • Stretch before you exercise. This is particularly important if you experience muscle or joint pain.
  • Try to mix up your routine with some gentle aerobic exercises and strength training.
  • Start slowly and build up to fitness goals.
  • Make sure you allow your body enough time to rest and recover.

How can I lower my thyroid T3?

If you have an overactive thyroid, you may need to take medication to stop your thyroid gland from producing excess hormones. Thionamides, such as carbimazole and propylthiouracil, are typically prescribed if your thyroid gland is overactive.

Following a healthy diet may also help to keep your thyroid levels within a normal range. For example, you may need to follow a low-iodine diet for a while to help naturally lower your T3 levels.

Iodine is essential to produce thyroid hormones, but too much or too little can have side effects. A high iodine intake in some people may induce hyperthyroidism, so a low iodine diet is often recommended for anyone who will be treated with radioactive iodine for thyroid cancer. Therefore, you may need to avoid foods such as:

  • Seaweed
  • Seafood and fish
  • Eggs including egg yolks
  • Soy products like soy sauce, tofu, soy milk
  • Fresh pasta
  • Cows and goat’s milk
  • Milk and white chocolate[14]

- Health scores calculated


Article references

  1. Ehlers, M. (2019) ‘Graves’ disease in clinical perspective’, Frontiers in Bioscience, 24(1), pp. 35–47. doi:10.2741/4708.

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