T3 and T4 are hormones secreted by the thyroid gland (thyroid hormones) and they have a great impact on your health. T4 is the thyroid hormone thyroxine and T3 is the hormone triiodothyronine. These affect almost every organ in your body. The levels of T4 and T3 in your body are regulated by thyroid stimulating hormone, or TSH for short. Thyroid tests to monitor thyroid function usually look for abnormal levels of the hormones TSH and T4.

Symptoms of hypothyroidism (underactive thyroid) are generally associated with low levels of T4 and T3. This can cause you to feel fatigued, have a hard time losing weight, feel tired, and suffer from hair loss. An underactive thyroid is also associated with high TSH levels and low T4 levels.

An overactive thyroid or hyperthyroidism is caused by high levels of T4 and T3 in the bloodstream. When your thyroid secretes too many thyroid hormones, you can become restless, have trouble sleeping, have frequent digestive disorders, and sweating out of the ordinary. If you have hyperthyroidism, your thyroid test results will generally show low levels of the hormone TSH.

This article contains a comprehensive guide to thyroid hormones and thyroid function. You will learn what the thyroid hormones T4 and T3 do in your body and what the recommended range of T3 and T4 is. It will also explore why testing for free T4 levels is sometimes better than a blood TSH test.

The thyroid gland and what it does

Your thyroid is an important hormonal gland in the body that has a direct impact on your metabolism. The thyroid is in the front of your neck just below your voice box and is shaped like a butterfly.

The thyroid releases hormones into the bloodstream. Your own thyroid releases triiodothyronine (T3), thyroxine (T4), and calcitonin (produced by the parathyroid glands). T4 is the main hormone that affects brain growth, metabolism, and development.

What is TSH?

Your thyroid function depends on the hormones released by the glands at the base of your brain. The hypothalamus produces thyrotropin-releasing hormone (TRH) which then stimulates the pituitary gland. The pituitary gland then secretes TSH which stimulates the thyroid to produce the hormones T3 and T4 which are released into the bloodstream.

TSH levels can fluctuate depending on the levels of T3 and T4 in the blood serum. For example, if your thyroid does not produce enough thyroid hormones (underactive thyroid), messages are sent to your pituitary gland to increase TSH levels. On the other hand, the high levels of T4 and T3 hormones caused by an overactive thyroid can signal your brain to decrease TSH.

This connection between T3, T4, and TSH is described as a feedback loop. Doctors say that TSH testing may not be the most accurate way to monitor your thyroid health if you are receiving T4 thyroid replacement therapy for hypothyroidism.

What are T3 and T4?

T3 is the thyroid hormone triiodothyronine and T4 is the thyroid hormone thyroxine.

T3 and T4 are the two main hormones your thyroid gland secretes. Together they affect and influence almost every cell in your body. Generally, your thyroid produces more T4 hormone than T3, but T4 turns into T3 when it reaches the cells and tissues of your body. Therefore, T4 is the most important hormone to measure when testing for thyroid dysfunction.

Let’s take a closer look at what T3 does and why T4 is the most powerful hormone.

What is T3?

Triiodothyronine (T3) has 3 iodine molecules in its structure. This is secreted directly by the thyroid gland and is classified as the most powerful thyroid hormone. This is because it is most metabolically active when it controls your body’s metabolism, temperature, and digestive system.

What is free T3 (FT3)?

Thyroid hormones bind to proteins in your body, but there are certain hormones that do not and these are classified as free T3. Free T3 is the active part of triiodothyronine. The FT3 level can help diagnose hyperthyroidism and monitor T3 replacement therapy.

What is T3 uptake?

On occasion, you may hear doctors mention T3 uptake when discussing thyroid tests. What is T3 uptake (T3RU)? The T3RU (T3 uptake) test measures the levels of proteins in the blood that carry thyroid hormone. T3 uptake can help your doctor interpret the T3 and T4 results of blood tests. The T3RU test is rarely used these days because tests to measure TBG (thyroxine-binding globulin) and free T4 in the blood are already available.

What is T4?

The thyroid gland also secretes T4, but in greater amounts than T3. Thyroxine (T4) has 4 iodine molecules attached to its molecular structure. When T4 enters the cells of the body, it loses a molecule and becomes T3. Thyroid function tests that check TSH and T4 levels are the most accurate ways to diagnose thyroid conditions.

What is free T4 (FT4)?

Similar to free T3, free thyroxine (FT4) levels are the unbound levels of the hormone T4 (the active part of thyroxine). You have freer T4 (FT4) in your blood than FT3. High T4 levels or low T4 levels can help accurately diagnose hyperthyroidism or hypothyroidism. Blood tests for TSH and T4 help accurately assess thyroid health.

The amount of free T4 helps doctors determine the amount of thyroid hormones available in your blood to provide you with the energy you need.

Normal levels of T3 and T4

If you show signs of abnormal thyroid function, doctors may need to take a blood test to check your FT4 levels and possible free T3. This is usually done at the same time as a TSH blood test, or they may also check T4 levels only if the TSH test readings are abnormal.

Normal levels of T3 (Normal levels of free T3 and total T3)

The normal ranges for total T3 and free T3 are as follows:

Total T3 (Triiodothyronine) Range:

  • Children: 125 – 250 ng / dL
  • Adults: 80 – 220 ng / dL

Free T3 (FT3) normal range:

Adults: 260 – 460 pg / dL (4 – 7.4 pmol / L)

Normal levels of T4 (Normal levels of free T4 and total T4)

The T4 thyroid hormone normal levels test is a common lab test along with TSH to diagnose thyroid problems. The range of total T4 (TT4) and the range of free T4 (FT4) are as follows:

Total T4 (TT4) range:

  • Newborns up to 14 days of age: 11.8 – 22.6 mcg / dL (152 – 292 nmol / L)
  • Infants and older children: 6.4 – 13.3 mcg / dL (83 – 172 nmol / L)
  • Adults: 5.4 – 11.5 mcg / dL (57 – 148 nmol / L)

Free T4 (FT4) normal range:

  • Children and adolescents: 0.8 – 2 ng / dL (10 – 26 pmol / L)
  • Adults: 0.7 – 1.8 ng / dL (9 – 23 pmol / L)
  • Pregnant women: 0.5 – 1.0 ng / dL (6.5 – 13 pmol / L)

How TSH Affects T3 and T4 Levels

TSH, T4, and T3 are related to thyroid function, and TSH can affect thyroid hormone levels in the blood. TSH levels that are below the normal range often indicate hyperthyroidism which means that your thyroid is overproducing T3 and T4. If your lab test results show high TSH levels, this may indicate that your thyroid is slow and that there are not enough thyroid hormones.

High levels of T3 and T4

Having lab tests showing elevated T3 and T4 levels can mean that you have hyperthyroidism. If this happens, doctors will generally test your FT4 levels and may also run FT3 tests if your TSH readings are low.

Symptoms of elevated T3

A significantly higher than normal T3 can be a sign of severe hyperthyroidism. Graves’ disease is a common cause of severe hyperthyroidism. This can cause symptoms such as:

  • Bulging eyes.
  • Thick, red skin on the tops of the feet.
  • Inflamed eyes with a scratchy sensation.
  • Increased sensitivity to light.

Symptoms of elevated T4

If your T4 tests are too high, it may mean that you have hyperthyroidism. Classic signs of an overactive thyroid gland secreting too much T4 hormone can include any of the following:

  • Increased irritability and restlessness.
  • Poor sleep habits and difficulty falling asleep.
  • Liquid stools due to a rapid functioning of the digestive system.
  • Weight loss without trying.
  • Skin itch.
  • Hair thinning.
  • Increased sweating.
  • Light menstrual periods.
  • Short of breath.

Low levels of T3 and T4

When you’re T3 and T4 levels fall below the normal range, you are likely to show signs of hypothyroidism. Mild hypothyroidism can even occur if your TSH readings are within the normal range.

Symptoms of low T4 levels

Classic symptoms of an underactive thyroid that doesn’t make enough thyroid hormone can include:

  • Feeling of weakness and fatigue.
  • Dry skin and hair.
  • Weight gain.
  • Problems with memory and concentration.
  • Tingling in the fingers or sensation of “needles” in the feet.
  • Cold intolerance.

Symptoms of low T3 levels

Generally, low T3 levels on a thyroid function test correspond with low T4 readings and elevated TSH levels. Therefore, the T3 below the normal range usually means that a person has hypothyroidism.

It would be very rare for blood tests to show a low T3 with a low TSH and a high T4.

How TSH is related to T3 and T4 levels

All thyroid hormones and TSH (thyroid stimulating hormone) must work in balance. This forms what doctors call a feedback loop where thyroid hormone levels in the blood tell the brain to make more or less TSH.

For example, if an overactive thyroid produces too much T4 or T3, the feedback loop will tell your brain to produce less TSH. The same is true if your thyroid is not working properly and produces too little hormone.

The basic relationship between TSH and thyroid hormone levels are as follows:

  • Hypothyroidism: high TSH, low T4, and low or normal T3
  • Hyperthyroidism: low TSH, high or normal T4 and high or normal T3
  • Subclinical thyroid disease: T4 and T3 levels are normal, but TSH is too high or too low
  • Non-thyroid disease: low TSH, low or normal T4, and low or normal T3

High (elevated) TSH and normal T4

Laboratory test results showing TSH levels greater than 4.0 mU / L and T4 within the normal range may indicate subclinical hypothyroidism.

Subclinical hypothyroidism may show that the person is at increased risk of developing an underactive thyroid disorder. This often happens in older women and there may be no visible symptoms. Doctors treat subclinical hypothyroidism on an individual basis since thyroid hormone replacement can lead to osteoporosis.

High TSH and low T4

Blood test results showing TSH levels higher than the normal range and FT4 less than 0.7 ng / dL mean you have an underactive thyroid.

Some of the causes of hypothyroidism can include:

  • Hashimoto’s thyroiditis (Hashimoto’s disease) which is an autoimmune disorder that attacks the thyroid gland.
  • Inflammation of the thyroid gland.
  • Radiation treatment for cancer.
  • A lack of iodine in the diet.
  • Pregnancy can also cause high TSH and low T4 readings.

Low TSH and high T4

A test result that indicates TSH of less than 0.4 mIU / L and FT4 of more than 1.8 ng / dL means that your thyroid is overactive. Hyperthyroidism is less common than an underactive thyroid, however, similar to hypothyroidism, it affects women more than men.

Some conditions that are related to hyperthyroidism can include:

  • Antibodies in your blood that affect your thyroid (such as in Graves’ disease).
  • Non-cancerous growths on the thyroid that cause it to swell and make more T4.
  • An autoimmune condition.
  • Getting too much thyroid replacement hormone.

Normal TSH and low T4

In some cases, TSH test results are within the normal range, but low T4 results may occur. This can often occur in people with chronic illnesses. The disease can affect a person’s metabolism while at the same time affecting their thyroid hormone levels. This means that the pituitary gland secretes TSH within the normal range and that the thyroid is working properly. However, thyroid hormones are lacking in the body.

Also, if you’ve recently been treated for hyperthyroidism, you may have normal TSH but low T4.

Treatment of hypothyroidism with replacement of T4 and T3

Doctors treat the symptoms of hypothyroidism by prescribing oral thyroid hormone replacement therapy. Some treatments for an underactive thyroid involve the prescription of T4 replacements such as levothyroxine sodium. Other treatments involve a combination of T4 and T3 replacement therapy.

According to certain studies, there is a trend towards the use of combined T4 and T3 replacement therapies. Some doctors prefer this because combined T4 and T3 thyroid treatments may have less impact on quality of life and better treat hypothyroidism.

Tests to measure T4 and T3

Tests to measure thyroid dysfunction may include one or more of the following tests:

  • Test to see if free thyroxine (FT4) is within the range of 0.7 to 1.8 ng / dL.
  • Test to see if free triiodothyronine (FT3) is within the range of 260 – 480 pg / dL.
  • TSH and FT4 test.
  • Thyroid antibody test to diagnose Graves’ disease or Hashimoto’s disease.

Doctors recommend testing for T4 and T3 as part of a thyroid function test if you have any of the following symptoms:

  • Fast or irregular heartbeat.
  • Thickening around the base of the neck.
  • High cholesterol.
  • Osteoporosis.
  • Being upset after having a baby.


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