High and Low TSH Levels: What They Mean

It's important to understand the meaning of both high thyroid-stimulating hormone (TSH) and low TSH. This applies whether you have been living with thyroid disease for a long time or are only having the test to screen for a thyroid disorder.

A high TSH level can mean a new diagnosis of hypothyroidism, or an underactive thyroid. A low TSH might mean hyperthyroidism, or an overactive thyroid. That said, there are exceptions to these interpretations, as well as what a "normal" level means for you.

This article covers the causes of both high and low TSH levels along with several reasons TSH levels may change. You will also learn about testing for thyroid disorder and a number factors that may impact your results.

What Is TSH?

TSH—short for thyroid-stimulating hormone—is a hormone that acts as a messenger to the thyroid gland. When released by the pituitary gland—a small gland at the base of the brain—it stimulates the production of more thyroid hormone.

How TSH Levels Change

TSH levels are not very intuitive. Why does a high TSH mean you have an underactive thyroid gland? Why do low levels means the gland is overactive?

Understanding exactly how the thyroid gland works can help.

Your thyroid gland produces thyroid hormone. When it functions properly, your thyroid is part of a feedback loop with your pituitary gland that involves several actions:

First, your pituitary gland senses the level of thyroid hormone that is released into the bloodstream.

Your pituitary then releases the special messenger hormone TSH, which makes the thyroid release more thyroid hormone. From there:

  • If your thyroid doesn't produce enough thyroid hormone, your pituitary triggers your thyroid to make more. (This might happen in cases of illness, stress, or surgery, for example.)
  • If your thyroid is overactive and producing too much thyroid hormone, your pituitary senses that and slows or shuts down TSH production. 

Interpreting TSH Levels

high and low tsh levels

Verywell / Emily Roberts

Understanding when and why thyroid hormone is released (or not) helps clear up what a low TSH and a high TSH level reveal about your thyroid's function.

  • A high TSH suggests your thyroid is underactive (hypothyroid) and not doing its job of producing enough thyroid hormone.
  • A low TSH suggests your thyroid is overactive (hyperthyroid) and producing excess thyroid hormone.

TSH is measured in mU/l. This abbreviation stands for milliunits per litre, with one milliunit equating to one-thousandth of a unit.

The normal range for TSH is generally between 0.5 mU/l and 5.0 mU/l.As with most medical conditions and tests, however, there are exceptions to this rule.

Thyroid hormone levels that are considered normal may be abnormal for you under certain conditions. It's for this reason that healthcare providers have varied opinions about what the optimal TSH level should be.

Controversy Over Optimal TSH

While most laboratories define a normal TSH as between roughly 0.5 mU/l and 5.0 mU/l, some experts argue that the upper limit of a normal TSH should be lower—around 2.5 mU/l.

The reasoning behind this is that the vast majority of adults without thyroid disease have a TSH value between 0.45 and 4.12 mU/l.

In addition, some healthcare providers believe older patients should have a TSH level that is greater than 4.0 mU/l or 5.0 mU/l, since TSH normally increases with age.

Healthcare providers can avoid some of this controversy simply by looking at each person as an individual. For example, a person who still has significant symptoms of hypothyroidism at a TSH of 4.0 mU/l may do better with a goal TSH of around 1.0 mU/l.

In contrast, someone who has health risks—such as heart disease or osteoporosis—may benefit from having a goal TSH that is higher, perhaps around 5.0 mU/l or 6.0 mU/l.

In pregnancy, TSH should not be allowed to rise above 3.0 mU/l for the health of both the baby and mother.

Recap

A normal TSH value is generally defined as between .5 mU/I and 5.0 mU/I. But opinions about this vary, since everyone has a personal "normal" that depends on their age and overall health.

For example, your target level may be higher if you have heart disease or lower if you are pregnant.

Causes of High TSH

A high TSH means different things depending on whether a person has known thyroid disease or not.

In People Without Known Thyroid Disease

A high TSH in people who are not undergoing thyroid disease treatment usually indicates the presence of primary hypothyroidism.

This is by far the most common form of hypothyroidism, and it occurs because the thyroid gland produces an inadequate amount of thyroid hormones. The pituitary gland senses these low levels and increases the production of TSH.

An elevated TSH may also occur with normal thyroid function due to the presence of antibodies, proteins made by the immune system.

In People Being Treated for Thyroid Disease

A high TSH may be found in people being treated for either hypo- or hyperthyroidism.

With hypothyroidism, a high TSH usually means that the dose of thyroid hormone replacement needs to be increased.

In some cases, however, the dose is OK, but the medication is not being fully absorbed. For example, many foods and medications can affect the absorption of Synthroid (levothyroxine) in your system. Learning what to avoid while taking this medication is important for it to work properly.

With hyperthyroidism, a high TSH usually means that the treatment—be it surgery, radioactive iodine, or medications—was so effective in limiting the production of thyroid hormone it caused the person to become hypothyroid.

Causes of Low TSH Results

A low TSH often means that a person has an elevated level of thyroid hormones.

In People Without Known Thyroid Disease

Low TSH is most often associated with hyperthyroidism. This can be due to a number of causes, ranging from autoimmune disease to lumps in the thyroid gland (toxic nodules) to an abnormally enlarged thyroid gland (goiter).

It can also occur during a normal pregnancy as thyroid hormones are affected by "the pregnancy hormone" human chorionic gonadotropin (HCG).

Less often, a lack of TSH produced by the pituitary gland (due to dysfunction) can lead to low levels of thyroid hormone in the blood—a condition known as central hypothyroidism. This is an exception to the general rule that hypothyroidism is associated with a high TSH.

Central hypothyroidism is uncommon and usually associated with a deficiency of other pituitary hormones. This deficiency leads to a number of other symptoms as well.

In People With Thyroid Disease

In people being treated for hypothyroidism, a low TSH level may mean:

  • Overmedication with thyroid hormone replacement
  • Interactions that cause increased absorption or activity despite an optimal dose of medication
  • Central hypothyroidism

In people being treated for hyperthyroidism, a low TSH level usually means that further treatment is needed to reduce thyroid hormone levels.

It could also mean that a person must continue to be monitored to make sure thyroid hormone levels return to normal. This is often the case for people who have temporary thyroiditis related to pregnancy or chemotherapy treatment.

Underactive Thyroid + High Thyroid
  • Treatment may need to be more aggressive

  • Medication may not be absorbing enough

Underactive Thyroid + Low TSH
  • Treatment may be more aggressive than needed

  • Medication may be absorbing too much

Factors That Can Skew TSH Results

There are a number of factors that can affect TSH levels. These are important to consider because lab values don't tell the whole story about someone's condition.

A treatment plan that does not also consider someone's symptoms may prove to be ineffective.

Laboratory Error

If a TSH test result is unexpected, simply repeating the test is often the best course of action.

Errors can occur during the blood draw, in transcribing the results, or due to mix-ups in the lab. Statistically, there is always a risk of lab error, and results should always be interpreted along with clinical symptoms and findings.

Antibodies

Antibodies are thought to interfere with accurate thyroid testing in roughly 1% of people.

In a 2018 review, such interference caused either misdiagnosis or inappropriate treatment in more than 50% of people who have these antibodies:

  • Heterophile antibodies: People exposed to animal-derived drugs and antibody therapies typically have these. A discrepancy between TSH levels and free T4, or between TSH levels and your symptoms, should alert your healthcare provider of a test inaccuracy.
  • Thyroid antibodies: These may also affect TSH levels in people who may or may not have a thyroid condition. Again, a discrepancy between lab values and your symptoms should raise the question of test inaccuracy.
  • Other antibodies: Anti-ruthenium and anti-streptavidin antibodies can also affect TSH testing results.

Health and Habits

A number of other factors can affect TSH test results, including:

  • The time of day that the test is done: If you are tested after fasting—for example, before you have eaten anything for the day—your TSH levels may be higher than if you had eaten just before your test.
  • Illness: Acute or chronic illnesses may put stress on your endocrine system, resulting in a skewed test result.
  • Pregnancy: TSH levels may be lower than the normal range during pregnancy, regardless of whether or not you have a thyroid disorder.
  • Medications: Some medications that are used to treat heart disease and cancer may affect results. Nonsteroidal anti-inflammatory medications, or NSAIDs, like Advil (ibuprofen) can as well.
  • Foods or supplements: Items rich in/derived from iodine or kelp may interfere with testing. The same is true about biotin supplements.
  • Changes in sleep habits: A lack of quality sleep has been linked with thyroid imbalances.

Testing Method

A capillary finger prick test, or blood spot test, is a somewhat controversial alternative to traditional TSH testing.

It uses a finger prick rather than a standard blood draw from your vein to evaluate thyroid function. Blood spot testing kits have also been developed so that you can monitor your TSH level from home.

Proponents of this test believe that it ensures greater accuracy without requiring as much of a sample as a venous blood draw.

However, since the test is not widely available right now, it's unknown exactly how well the test compares with conventional TSH testing.

Recap

No lab test is 100% accurate, including the TSH test. Do your part to help get the most accurate results by being as consistent as possible when it comes to the conditions under which you are tested.

For example, always have your test done at the same time of day, postpone your appointment if you are sick, and be sure you're always getting quality sleep.

When TSH Alone Is Not Enough

During diagnosis, most healthcare providers use the TSH test to evaluate thyroid function and determine the optimal course of treatment. There are times, however, when knowing one's TSH may be insufficient.

For instance, free T4 in addition to TSH is usually tested if a healthcare provider suspects thyroid dysfunction due to disease of the pituitary gland or hypothalamus.

Likewise, if the TSH is normal, but a person still has symptoms of being hyperthyroid or hypothyroid, free T4 may be checked.

TSH is also not necessarily sufficient to monitor hypothyroidism during pregnancy; T4 and free T4 are often recommended.

Depending on the clinical situation, other thyroid tests that may be done include triiodothyronine (T3), free T3, reverse T3, and thyroid antibody tests.

Summary

A TSH level that is optimal for all people cannot quite be established. That's because TSH levels vary depending on numerous factors, including your age, your thyroid condition, and possibly even how well you sleep at night or when you last ate.

To gauge the optimal level for you, your healthcare provider should consider these factors along with any symptoms you are experiencing.

You can also improve the accuracy of your tests by keeping any factors that may influence your results as consistent as possible.

Frequently Asked Questions

What is the normal range for TSH levels?

The normal range for TSH is between 0.5 mU/l and 5.0 mU/l.

What does it mean when your TSH levels are low? 

A low TSH level—below 0.5 mU/l—indicates an overactive thyroid, also known as hyperthyroidism. This means your body is producing an excess amount of thyroid hormone.

What causes high TSH levels?

A high TSH level—above 5.0 mU/l—indicates an underactive thyroid, also known as hypothyroidism. This means your body is not producing enough thyroid hormone.

2:19

5 Common Misconceptions About Thyroid Disease

A Word From Verywell

The TSH test is the gold standard in diagnosing and monitoring thyroid disease. But as with any medical test, there are exceptions to what the test results may mean, as well as situations in which the test may be inaccurate.

It is entirely possible that you may still have symptoms despite a normal TSH level. If this is the case, speak up. Even a "normal" TSH may be abnormal for you.

And if you don't feel you are being heard, consider seeking a second opinion.

Was this page helpful?
Article Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Sheehan MT. Biochemical testing of the thyroid: TSH is the best and, oftentimes, only test needed - A review for primary care. Clin Med Res. 2016 May;14(2):83-92. doi:10.3121/cmr.2016.1309

  2. Biondi B. Normal TSH reference range: What has changed in the last decade?. JCEM. 2013 Sep;98(9):3584-3587. doi:10.1210/jc.2013-2760

  3. Leng O, Razvi S. Hypothyroidism in the older population. Thyroid Res. 2019 Feb;12(2):1-10. doi:10.1186/s13044-019-0063-3

  4. Stagnaro-green A, Abalovich M, Alexander E, et al. Guidelines of the American Thyroid Association for the diagnosis and management of thyroid disease during pregnancy and postpartum. Thyroid. 2011 Jul;21(10):1081-125. doi:10.1089/thy.2011.0087

  5. Chaker L. Hypothyroidism. Lancet. 2017 Sep;390(10101):1550-1562. doi:10.1016/S0140-6736(17)30703-1

  6. Mcmillan M, Rotenberg KS, Vora K, et al. Comorbidities, concomitant medications, and diet as factors affecting levothyroxine therapy: results of the CONTROL Surveillance Project. Drugs in R&D. 2016 Mar;16(1):53-68. doi:10.1007/s40268-015-0116-6

  7. De Leo S. Hyperthyroidism. Lancet. 2016 Aug;388(10047):906-918. doi:10.1016/S0140-6736(16)00278-6.

  8. Ross DS. 2016 American Thyroid Association guidelines for diagnosis and management of hyperthyroidism and other causes of thyrotoxicosis. Thyroid. 2016 Oct;26(10):1343-1421. doi:10.1089/thy.2016.0229

  9. Koulouri O, Moran C, Halsall D, Chatterjee K, Gurnell M. Pitfalls in the measurement and interpretation of thyroid function tests. Best Pract Res Clin Endocrinol Metab. 2013 Dec;27(6):745-62. doi:10.1016/j.beem.2013.10.003

  10. Favresse J, Burlacu MC, Maiter D, Gruson D. Interferences with thyroid function immunoassays: Clinical implications and detection algorithm. Endocrine Rev. 2018 Jul;39(5):830-850. doi:10.1210/er.2018-00119

  11. Nair R, Mahadevan S, Muralidharan RS, Madhavan S. Does fasting or postprandial state affect thyroid function testing?. Indian J Endocrinol Metab. 2014 Sep;18(5):705-707. doi:10.4103/2230-8210.139237

  12. National Institute of Diabetes and Digestive and Kidney Diseases. Thyroid tests. Reviewed May 2017.

  13. Ardabilygazir A, Afshariyamchlou S, Mir D, Sachmechi I. Effect of high-dose biotin on thyroid function tests: Case report and literature reviewCureus. 2018 Jun;10(6):e2845. doi:10.7759/cureus.2845

  14. Song L, Lei J, Jiang K, et al. The association between subclinical hypothyroidism and sleep quality: A population-based studyRisk Manag Healthc Policy. 2019 Dec;12(1):369-374. doi:10.2147/RMHP.S234552