TSH Levels: What's Normal and What's Not

Medically Reviewed By Kelly Wood, MD
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The normal range for thyroid-stimulating hormone (TSH) is 0.35 to 4.5 milli-international units per liter (mIU/l) of blood. TSH is a hormone produced by your pituitary gland that regulates the production of hormones by your thyroid gland. The levels of TSH in your body can tell you how well your thyroid gland is functioning. A variety of factors can cause atypical TSH levels, such as thyroid disease, the effects of certain medications, autoimmune conditions, and more.

Read on to learn more about normal TSH levels and the symptoms, tests, and treatments associated with unusual TSH levels.

What is a normal TSH level?

Woman getting blood taken for testing
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Doctors measure TSH levels in ranges. In general, the normal reference range for TSH levels is 0.35 to 4.5 mIU/l of blood. A TSH reading in this range indicates that the thyroid gland is functioning normally.

However, some people may fall outside the normal reference range and still have thyroid glands that are functioning as they should be. Before diagnosing any thyroid disorder, your doctor will consider your TSH level as well as any signs or symptoms you are experiencing, such as an enlarged thyroid gland.

What does a low TSH level mean?

A TSH level below 0.35 mIU/l can mean that your thyroid gland is overactive. An overactive thyroid is known as “hyperthyroidism.”

What are the symptoms of a low TSH level?

Common symptoms of low TSH levels can include:

What causes a low TSH level?

One of the most common causes of hyperthyroidism is Graves’ disease. This is an autoimmune condition in which your body’s immune system mistakenly attacks the thyroid gland.

Other causes of hyperthyroidism include thyroiditis and certain medications.

What are the treatments for a low TSH level?

Treatments for a low TSH level often include medications such as methimazole and radioiodine therapy. Surgery to remove your thyroid gland is a less common treatment option for hyperthyroidism due to Graves’ disease.

What does a high TSH level mean?

A TSH level above 4.5 mIU/l usually indicates an underactive thyroid gland. This is known as “hypothyroidism.”

What are the symptoms of a high TSH level?

High TSH levels can cause symptoms that include:

What causes a high TSH level?

Many medical issues can cause hypothyroidism. One common cause is an autoimmune condition called Hashimoto’s disease. Thyroiditis and the surgical removal of all or part of the thyroid gland can also cause high TSH levels.

What are the treatments for a high TSH level?

The most common treatment option for an underactive thyroid is levothyroxine, which is a supplemental thyroid hormone.

How do doctors measure TSH levels?

Your doctor can order several common tests to assess your TSH levels. The first will usually be a TSH blood test. The results of this test will tell you if your TSH levels are normal, low, or high.

Tests that your doctor can use to confirm unusual TSH levels include the following:

  • A T4 test will show the levels of T4 in your system. T4 is one of the two main thyroid hormones. If your T4 level is too high, it might indicate hyperthyroidism. If your T4 level is too low, it could indicate hypothyroidism.
  • A T3 test can help doctors further understand the results of your T4 test. T3 is the other main thyroid hormone. If your T4 level is typical but your doctor still suspects that you might have hyperthyroidism, the T3 test can help confirm the diagnosis. T3 is high in hyperthyroidism.

Thyroid antibody test

In addition, your doctor may order a thyroid antibody test to measure the levels of thyroid antibodies in your body. This test can help diagnose an autoimmune thyroid condition, such as Graves’ disease or Hashimoto’s disease, that may be causing atypical TSH levels. 

How should I prepare for a TSH test?

Your doctor or healthcare team will provide detailed preparation instructions before your test. The only medication that healthcare professionals recommend stopping for at least 3–5 days before a TSH test is biotin, as it can interfere with the results.

What should I do about an unusual TSH test result?

If your TSH levels are outside the normal range, your doctor will discuss possible causes. They may ask about your family history and any health symptoms you are experiencing.

Tell your doctor about any medications, vitamins, minerals, herbs, or other dietary supplements you take. Some of these products can alter your thyroid function and cause an atypical test result.

After an unexpected TSH test result, your doctor may recommend further testing — such as a thyroid ultrasound — to rule out a serious condition, such as thyroid nodules. However, treatments for hyperthyroidism and hypothyroidism are very effective most of the time.

Summary

Your TSH levels are an important indication of how your thyroid gland is functioning. A low level may indicate hyperthyroidism, and a high level may indicate hypothyroidism.

The symptoms and causes of unusual TSH levels can vary. It is important to talk with your doctor about any symptoms you are experiencing and any medications or supplements you are taking.

Doctors can order one or more blood tests to determine your TSH level. These tests will help determine your diagnosis and treatment, which can involve medications or surgery.

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Medical Reviewer: Kelly Wood, MD
Last Review Date: 2022 Mar 24
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