TSH Levels: What Do High and Low Levels Mean?

"Normal" results depend on factors like male or female sex and age

Normal thyroid-stimulating hormone (TSH) levels generally fall between 0.4 and 4.0 milliunits per liter (mU/L).

TSH levels higher than 4.5 mU/L usually indicate an underactive thyroid (hypothyroidism), and low TSH levels—below 0.4 mU/L—indicate an overactive thyroid (hyperthyroidism). 

But, there are exceptions to these interpretations as well as variations as to what a "normal" TSH level is. This is because TSH levels can vary by age, male and female sex, and other factors. In the end, the results need to be interpreted alongside other thyroid function tests to make the correct diagnosis.

This article looks at the causes of high and low TSH levels, what they mean, and the various factors that can influence test results.

What Is TSH?

TSH is a hormone that acts as a messenger to the thyroid gland. When released by the pituitary gland—a small gland located at the base of the brain—TSH stimulates the thyroid to produce more thyroid hormone.

How TSH Levels Change

TSH levels seem counterintuitive. A high TSH suggests an underactive thyroid gland, while low TSH levels indicate an overactive thyroid.

TSH and thyroid hormones are different things. The thyroid hormones are:

Your body uses thyroid hormones to regulate all kinds of processes. That includes how fast you burn calories, your heart rate, and many other automatic functions.

When thyroid hormones are low, your pituitary makes more TSH. That signals the thyroid to increase production. When levels are high, the pituitary releases less TSH. That signals the thyroid to slow down. This is why high TSH indicates low thyroid function and vice versa.

Interpreting TSH Levels

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.

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

The normal range for TSH is generally between 0.4 mU/L and 4.0 mU/L. As with most medical conditions and tests, however, there are exceptions to this rule.

high and low tsh levels

Verywell / Emily Roberts

Normal TSH

The official normal range for TSH is between 0.4 mU/L and 4.0 mU/L. However, many non-modifiable factors influence a person's TSH levels and dictate what a "normal" value is in an individual. This not only includes variations by a person's biological sex or age but fluctuations that regularly occur during different seasons of the year.

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. This is especially true in younger females.

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.

TSH Levels by Sex

TSH levels also vary by sex. This is because TSH levels are influenced by the hormone estrogen.

Some studies suggest that a normal TSH range in younger females aged 18 to 29 may be closer to 0.4-2.5 mU/L. In males, the normal TSH range is closer to 0.4-4.0 mU/L.

Low estrogen impairs the thyroid gland's ability to make thyroid hormones. So TSH rises to get the thyroid to produce more. High estrogen leads to increased thyroid hormone production and lower TSH. In menopause, estrogen levels are lower, which sometimes triggers thyroid disease.

TSH Levels During Pregnancy

During pregnancy, TSH levels vary by trimester. According to the American Endocrine Society, TSH levels during pregnancy should be: 

  • First trimester: 0.1 to 2.5 mU/L
  • Second trimester: 0.2 to 3.0 mU/L
  • Third trimesters: 0.3 to 3.0 mU/L

Higher TSH levels (TSH levels > 4.5 mU/L) in early pregnancy are associated with increased risk for miscarriage.

TSH Levels by Age

In addition, some healthcare providers believe that 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.

Many healthcare providers sidestep this controversy by simply 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

Causes of High TSH

A high TSH level—above 4.5 mU/L—indicates an underactive thyroid, also known as hypothyroidism. This means your body is not producing enough thyroid hormone. 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 okay, 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 that it caused the person to become hypothyroid.

Causes of Low TSH

A low TSH often means that a person has an elevated level of thyroid hormones. A low TSH level—below 0.4 mU/L—indicates an overactive thyroid, also known as hyperthyroidism. This means your body is producing an excess amount of thyroid hormone.

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 TSH
  • 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 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 iodine or derived from 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.

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 (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. Your healthcare provider will consider these factors along with any symptoms you're experiencing to determine your optimal range.

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. Your healthcare provider may order additional tests to help determine how your thyroid gland is performing.


5 Common Misconceptions About Thyroid Disease

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By Mary Shomon
Mary Shomon is a writer and hormonal health and thyroid advocate. She is the author of "The Thyroid Diet Revolution."