High and Low TSH Levels: What They Mean

Interpretations, Variations, and Controversies

It's important to understand the meaning of both high thyroid-stimulating hormone (TSH) and low TSH—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, while a low TSH might mean hyperthyroidism. 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. Further down the page, 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 pituitary hormone that acts as a messenger to the thyroid gland. When released by the pituitary gland, it stimulates the production of more thyroid hormone.

How TSH Levels Change

TSH levels are confusing and not necessarily intuitive. For example, many people question why high TSH levels can mean the thyroid is under-active whereas low TSH levels can mean it's 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:

  1. First, your pituitary gland senses the level of thyroid hormone that is released into the bloodstream.
  2. Your pituitary releases the special messenger hormone TSH, which stimulates the thyroid to release more thyroid hormone.
  3. When your thyroid, for whatever reason—illness, stress, surgery, or obstruction, for example—doesn't produce enough thyroid hormone, your pituitary triggers your thyroid to make more.
  4. If your thyroid is overactive and producing too much thyroid hormone, your pituitary senses that there is too much of the hormone circulating and slows or shuts down TSH production. 

Interpreting TSH Levels

Once you understand these thyroid basics, it's easier to understand what a low TSH and a high TSH level reveal about your thyroid's function.

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

high and low tsh levels

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  • A high TSH suggests your thyroid is under-active (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.

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 physicians 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 physicians 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.

Doctors can avert 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.


Opinions vary about what is a normal TSH level, since a person's "normal" level depends on their age and certain conditions. A normal TSH value is generally defined as between .5 mU/I and 5.0 mU/I. That number may be higher if you are elderly or have heart disease, and lower if you are pregnant or have hypothyroidism.

Causes of High TSH

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

In People Without 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 will sense these low levels and increase the production of TSH.

An elevated TSH may also occur with normal thyroid function due to the presence of antibodies and more.

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 needs to be increased.

In some cases, however, the dose is already optimal, but the medication is not being fully absorbed. For example, many foods and medications can affect the absorption of 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 effective in limiting the production of TSH, allowing a person to be 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

While often associated with hyperthyroidism, a low TSH could also be a sign of central hypothyroidism.

Hyperthyroidism can be transient and permanent due to a number of causes, ranging from autoimmune disease, to toxic nodules or goiters, to pregnancy-related thyroiditis.

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

That said, central hypothyroidism is uncommon and usually associated with a deficiency of other pituitary hormones. Subsequently, 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 transient thyroiditis related to pregnancy or chemotherapy treatment.


If you are being treated for hyperthyroidism but you have high TSH, it's possible you need more aggressive treatment or your medication isn't absorbing enough. On the other hand, if you are being treated for hypothyroidism but you have low TSH, it's possible your medication is more aggressive than necessary or it's absorbing into your body too much.

Factors That May Affect Your TSH Results

There are a number of factors that can affect TSH levels. It's important to be aware of these, as a treatment that is dictated solely by lab values—as opposed to also considering an individual's symptoms—can result in an ineffective plan.

Laboratory Error

If a TSH level is surprising, sometimes simply repeating the test is the best course. 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 are thought to interfere with accurate thyroid testing in roughly 1% of people.

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

  • Heterophile antibodies: People exposed to animal-derived pharmaceuticals and antibody therapies typically have these antibodies. A discrepancy between TSH levels and free T4, or between TSH levels and your symptoms, should alert your doctor 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: Other antibodies important in TSH testing interference include anti-ruthenium antibodies and anti-streptavidin antibodies.

Other Factors

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 they would be had you 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. Non-steroidal anti-inflammatory medications like Advil (ibuprofen) can as well.
  • Foods or supplements: Items rich in/derived from iodine or kelpas well as biotin supplements may interfere with testing.
  • Changes in sleep habits: A lack of quality sleep has been linked with thyroid imbalances.

In order to get the most accurate results, it's important to be consistent. For example, always have your test done at the same time of day.

Capillary Finger Prick Tests

A somewhat controversial test available in some places uses a finger prick rather than a standard blood draw from your vein to evaluate thyroid function.

Proponents of this test—known also as a blood spot test—believe that it ensures greater accuracy without requiring as much blood as a venous blood draw. Blood spot testing kits have also been developed so that you can monitor your TSH level from home.

Since the test is not currently widely available, it's unknown exactly how well the test compares with conventional TSH testing.


5 Common Misconceptions About Thyroid Disease

When TSH Alone is Not Enough

During diagnosis, most doctors use the TSH test to evaluate thyroid function and determine the optimal course of treatment. There are times, however, when a TSH may be insufficient.

For instance, free T4 in addition to TSH is usually tested if a doctor suspects thyroid dysfunction arising from 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, and a T4 and free T4 are often recommended. Depending on the clinical situation, other thyroid tests that may be evaluated, including triiodothyronine (T3), free T3, reverse T3, and thyroid antibody tests.


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 doctor 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—from when you eat and how long you sleep, to the medications you take.

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 under active thyroid, also known as hypothyroidism. This means your body is not producing enough thyroid hormone.

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. If your TSH levels don't seem to reflect how you are feeling, speak up. Even a "normal" TSH may be abnormal for you.

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