High TSH: Causes, Symptoms, and How it Affects T3 and T4

Elevated TSH: Causes, Symptoms, and How it Affects T3 and T4

High level of TSH usually indicates that you have an underactive thyroid and that it doesn’t produce enough thyroid hormones. Thyroid-stimulating hormone (TSH) directly affects how much T3 and T4 hormones the thyroid secretes. If there is not enough T3 and T4 in the blood, your pituitary gland will increase levels of TSH as a result. Elevated TSH levels usually cause symptoms of hypothyroidism like fatigue, constipation, joint pain, a puffy face, or heavy menstrual bleeding (in women).

The reasons for levels of TSH being higher than the normal range can include dysfunction of your thyroid, an autoimmune disease, or a side effect of some medications. Sometimes pregnancy can cause T3 and T4 to increase which will affect levels of TSH. If you are showing signs of an underactive thyroid and your TSH levels are too high, doctors will usually arrange for blood tests to check for levels of free T3 and T4.

To treat symptoms of high TSH, doctors usually prescribe thyroxine (T4) to lower TSH levels and treat hypothyroidism. Blood tests are then regularly taken to monitor levels of TSH and regulate thyroid therapy.

In this article, you will find out about the important role thyroid hormones play in your general health. I will also examine what medical literature says about how high TSH levels affect the thyroid hormones T3 (triiodothyronine) and T4 (thyroxine). This will help you to spot the signs and symptoms of an underactive thyroid.

What Does the Thyroid Do?

Your thyroid is a small, butterfly-shaped gland at the front of your neck that secretes thyroid hormones into your bloodstream.

Doctors from the Department of Health in Australia say that your thyroid regulates your body’s metabolism and affects your nervous and immune systems. This means that your thyroid affects most of the complex processes that happen in your body. Your body needs iodine to create adequate levels of T3 and T4 hormones.1

Some of the functions that your thyroid controls include:

  • Your body weight
  • Menstrual cycles
  • Gastrointestinal system
  • Processes connected to your central nervous system
  • Your heart rate

What is TSH (Thyroid-Stimulating Hormone)?

TSH stands for thyroid stimulating hormone and is a hormone secreted by the pituitary gland in your brain. As its name implies, TSH directly affects the thyroid to produce thyroid hormones.

The book Clinical Methods says that high TSH levels usually indicate hypothyroidism or an underactive thyroid. TSH levels will be elevated above the normal reference range to try and stimulate the thyroid to produce more hormones.2

Doctors arrange for TSH blood tests to monitor thyroid hormone replacement therapy, screen for thyroid issues in newborns, or diagnose signs of hypothyroidism.

What are T3 and FT3?

Triiodothyronine is usually referred to as T3 and is one of the hormones that the thyroid secretes. Doctors from the American Association for Clinical Chemistry say that T3 is more active than the more abundant T4 and is a powerful thyroid hormone in the body. Free T3 (FT3) is the amount of triiodothyronine that is not bound to protein and can still be used by the body. Doctors sometimes run tests for total T3 (TT3) or just FT3 to check thyroid function.3

What are T4 and FT4?

Thyroxine is referred to as T4 and is the main hormone that the thyroid secretes. When T4 in the blood binds to proteins, it becomes T3. Doctors say that blood tests to check if T4 levels are in the normal range are done to test thyroid function. Often, a T4 test is done together with checking for high TSH if a person shows symptoms of hypothyroidism.4

How T3 and T4 affect TSH levels

The amount of T3 and T4 in blood serum directly affects levels of TSH that the pituitary gland produces.

The journal Endocrinology says that there is a negative feedback loop that signals to the pituitary gland to produce more or less TSH. If there are low T4 and T3 in the blood, more thyroid-stimulating hormone (TSH) is produced to boost T4 levels. This is why an underactive thyroid gland is associated with TSH levels above the normal reference range.5

Normal TSH Levels (Normal Thyroid Levels)

What is the normal range of thyroid-stimulating hormone? Dr. Alina G. Sofronescu from the University of Nebraska Medical Center says that normal TSH levels are the following:6

  • 0.4 – 4.2 mIU/L (milliunits per liter)

However, the normal TSH range also varies depending on age and if a woman is pregnant or not. For example, Dr. Sofronescu says that for an elderly person, anything up to 8.9 mIU/L would be not be considered high TSH.

Women in their third trimester of pregnancy could expect to see TSH levels to rise to 5.2 mIU/L and still not be considered as having hypothyroidism. However, some doctors recommend a much lower cut-off point for high TSH levels.

Some doctors say that even though normal TSH levels could be as high as 4.0 mIU/L, the optimal TSH range should be lowered. According to the International Journal of Laboratory Medicine, the ideal cut off point for TSH should be 2.5 mIU/L. This is because individuals in the “normal” TSH range of 2.5 – 4.0 mIU/L are at risk of developing thyroid disorders.7

Of course, different labs will have slight variations in the normal ranges for TSH, T3, and T4.

What are High TSH Levels?

The normal reference range for the concentration of TSH levels in blood serum is between 0.4 and 4.0 mUI/L. If you have high TSH of over 4.0 mIU/L doctors will probably arrange to check for levels of T4 hormone to help identify the type of hypothyroidism you have.

However, as already mentioned, some doctors say that anything over 2.5 mIU/L should be considered as high TSH.

Symptoms of High TSH

The effects of high TSH will show many symptoms that are associated with hypothyroidism or an underactive thyroid gland. According to doctors from the National Health Service, the many symptoms of high TSH can include:

It is important to get symptoms of an underactive thyroid treated because if left unchecked, hypothyroidism can cause complications like:

  • Puffy face
  • Enlarged thyroid gland (goiter)
  • Forgetfulness
  • Depression

Causes of Elevated TSH Levels

TSH levels that are above the normal reference range usually show that your thyroid isn’t working as it should. This usually results in blood tests showing low T3 and T4 in blood serum.

Let’s look in more detail at the various causes of high TSH levels in your blood.

Autoimmune disease

One of the most common causes of hypothyroidism that causes TSH levels to shoot up is an autoimmune condition.

Doctors from the Mayo Clinic say that Hashimoto’s disease is one such autoimmune disease that affects the thyroid. Your body starts producing antibodies that attack the thyroid gland. Hashimoto’s disease causes inflammation in your thyroid and slows your thyroid down, causing an increase in TSH levels.

The main symptoms of a thyroid disorder caused by an autoimmune disease are the same as for hypothyroidism.

Iodine deficiency

Your thyroid needs sufficient levels of iodine for it to function properly. If your diet lacks iodine, then your thyroid will slow down and secrete less T4 and T3 which will cause higher than normal TSH levels.

According to Professor of Endocrinology, Dr. Philip R. Orlander, apart from Hashimoto’s disease, an iodine deficiency is one of the most common reasons for hypothyroidism.10

A report from 2007 said that iodine is necessary for T3 and T4 synthesis. Adults need between 120 and 150 μg per day to help keep the thyroid healthy. It is possible to get adequate amounts of iodine from a healthy, well-balanced diet. In fact, the report said that taking iodized salt with food may be all that is required to offset an iodine deficiency and lower TSH levels.11

The best foods containing iodine are:

  • Salt that has been iodized
  • Kelp
  • Haddock
  • Bread
  • Milk
  • Vitamin supplements

Treatment for hyperthyroidism

If you have been receiving treatment for low TSH levels and the effects of hyperthyroidism, there is a chance that you could have high TSH.

The journal American Family Physician says that treating hyperthyroidism requires lowering T3 and T4 levels in the blood. This can result in elevated TSH levels when you take a blood test. However, doctors will monitor your medication and adjust it as necessary to bring TSH levels within the optimal range.12


Thyroiditis is inflammation of the thyroid gland that can result in high TSH levels or low TSH levels depending on how the thyroid is affected.

According to the American Thyroid Association, thyroiditis refers to general inflammation of the thyroid gland. As already mentioned, Hashimoto’s disease is one cause of thyroiditis. However, viral or bacterial infections can affect the thyroid, resulting in levels of TSH that are higher than normal.13

Thyroid removal or thyroid deactivation

If you have had surgery to remove your thyroid or radioiodine therapy to deactivate your thyroid, you will receive regular T4 replacement therapy. This usually affects TSH levels secreted by your pituitary gland as your medication is monitored and adjusted.

The journal Rambam Maimonides says that low levels of T3 and T4 exert a negative feedback on TSH and signals to your body to increase TSH levels. When receiving levothyroxine (L-T4) therapy, your TSH levels and T4 levels will be closely monitored to make sure that T4 levels don’t drop dangerously low.14

Radiation therapy

If you have received radiation therapy to treat cancer, there is a possibility that you will have hypothyroidism and high TSH levels.

According to the Asian Pacific Journal of Cancer Prevention, radiation for cancer increases a person’s risk of developing an underactive thyroid. Therefore, doctors advise that blood tests to check TSH levels in blood serum be carried out as part of follow-up treatment.15

Cysts or nodules in the thyroid gland

Nodules or cysts in the thyroid gland can affect its function and cause too little T4 and T3 to be produced. This has the effect of increased secretion of TSH which can cause levels to increase above 4.0 mIU/L.

The journal Medical Clinics of North America reports that testing for high levels of TSH is essential in monitoring thyroid nodules. Blood serum tests for TSH are usually combined with ultrasound scans to check for cancerous cells in the thyroid. Doctors say that constant monitoring is essential because low T4 in the blood along with normal or high TSH can increase the risk that nodules become malignant.16

Other Causes of High TSH Levels

There are other reasons why elevated TSH levels show up in blood test results. There are less common than autoimmune diseases or iodine deficiencies that impact on TSH levels.

Side effect of medication

The journal Medicina says that some types of medication can cause hypothyroidism along with its associated symptoms. For example, lithium used to treat various psychiatric disorders can increase TSH levels and cause your thyroid to become underactive.17

Issues with the pituitary gland

Although rare, problems with the pituitary gland’s function can affect TSH levels. Researchers from Johns Hopkins Medicine report that secondary hypothyroidism can occur if your pituitary gland doesn’t work properly.18


TSH levels can fluctuate during pregnancy. Therefore doctors regularly monitor blood work during pregnancy to ensure that TSH levels don’t increase which would signify that your body isn’t producing enough T4 hormones.

According to the National Institutes of Health, the signs of hypothyroidism are the same for pregnant women as those who are not pregnant. Your thyroid needs to produce enough T4 hormone for you and your baby. This is essential for your baby’s brain development. If you are pregnant and have mild hypothyroidism, your obstetrician will recommend the best course of treatment.19

How TSH Affects T3 and T4 Levels

Levels of thyroid-stimulating hormone directly influence the amount of T4 and T3 your thyroid secretes.

The journal Acta Endocrinologica says that there is a positive relation between TSH serum levels and serum levels of T3 and T4.20

TSH stimulates your thyroid gland to produce T3 and T4 hormones. Obviously when TSH is high, your thyroid will be stimulated to generate more thyroid hormones. When TSH levels are low, your thyroid will generate less T3 and T4.

Normal T3 and T4 Levels

What are normal T3 and T4 levels when your thyroid is functioning properly?

Dr. Jayita Poduval who specializes in neck and throat disorders says that normal free T3 (FT3) levels are 125 – 250 ng/dL in children and 80 – 200 ng/dL in adults.21

The reference range for free T4 (FT4) is 0.7 – 1 ng/dL in adults and 0.8 – 2 ng/dL in children and adolescents.22

High TSH in Pregnancy

It is normal during pregnancy for T4 levels to increase which causes TSH levels to drop. According to Dr. Sandra Fryhofer on Medscape, T3 and T4 thyroid hormones can increase by up to 50% during pregnancy. Even though some labs put the normal TSH range for pregnancy as high as 5 mIU/L, Dr. Fryhofer recommends a much lower level.

The recommendations for TSH during pregnancy are as follows:

  • First trimester: 0.1 – 2.5 mIU/L
  • Second trimester: 0.2 – 3.0 mIU/L
  • Third trimester: 0.3 – 3.0 mIU/L

If TSH levels are higher than 2.5 mIU/L during pregnancy, Dr. Fryhofer recommends testing for levels of T4 to check for hypothyroidism in pregnancy.

High TSH but Normal Free T4

What does it mean if your TSH levels are too high but you have normal FT4?

The journal American Family Physician says that levels of TSH over 4.0 mIU/L and free T4 levels within the normal range could be an indicator of subclinical hypothyroidism. This means that a person is at risk of developing hypothyroidism but is not yet showing any noticeable symptoms.12

High TSH but Normal T3 and T4

Just as with high TSH and normal FT4, if your blood test results for both T4 and T3 show normal levels but there is too much TSH in the blood, you could have subclinical hypothyroidism. Doctors will usually monitor your thyroid hormone levels to monitor thyroid function.

High TSH and High T4

It would be very unusual for both thyroid-stimulating hormone to be above the normal range and T4 levels to be too high. Usually, T4 is directly connected with TSH levels. If T4 drops, TSH levels will increase because of an underactive thyroid. On the other hand, if T4 levels increase too much, negative feedback to the pituitary gland will signal to it to reduce TSH hormone.

High TSH and Low T4

Expert in endocrinology, Dr. Philip R. Orlander says that high TSH and T4 levels below the normal range are clear signs of primary hypothyroidism. Usually, in the early stages of hypothyroidism, TSH is high and T4 is normal. However, as the disease progresses, T4 will drop below the normal range but T3 will be normal. However, in time both T3 and T4 will be below normal as TSH increases and hypothyroidism develops.24

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