TSH Levels: What Do High and Low Levels Mean?

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). 

However, there are exceptions to these interpretations and 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:

• T3 (triiodothyronine)
• T4 (thyroxine)

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, the pituitary makes more TSH, which signals the thyroid to increase production. When levels are high, the pituitary releases less TSH, which 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 hormones are 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. However, as with most medical conditions and tests, there are exceptions to this rule.

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 their "normal" value. These include variations by a person's biological sex or age and 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. This is because most adults without thyroid disease have a TSH value between 0.45 and 4.12 mU/L, especially in younger females.

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 the hormone estrogen influences TSH levels.

Some studies suggest a normal TSH range in younger females aged 18 to 29 may be closer to 0.4-2.5 mU/L. The normal TSH range in males 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 trimester: 0.3 to 3.0 mU/L

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

TSH Levels by Age

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

Many healthcare providers sidestep this controversy by looking at each person individually. 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 higher goal TSH, 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.

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 and 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 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 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 caused by a number of factors, 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 several 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 monitoring to ensure thyroid hormone levels return to normal. This is often the case for people who have temporary thyroiditis related to pregnancy or chemotherapy treatment.

Factors That Can Skew TSH Results

A number of factors can affect TSH levels. These are important 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 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 can interfere with accurate thyroid testing in some people. In one review of studies, 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

Several other factors can affect TSH test results, including:

• The time of day 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 used to treat heart disease and cancer may affect results. Nonsteroidal anti-inflammatory medications (NSAIDs), like Advil (ibuprofen), can also affect results.
• 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

Most healthcare providers use the TSH test during diagnosis to evaluate thyroid function and determine the optimal course of treatment. However, knowing one's TSH may be insufficient at times.

For instance, free T4 and TSH are 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 hyperthyroidism or hypothyroidism, free T4 may be checked.

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

Other thyroid tests that may be performed, depending on the clinical situation, include (T3), free T3, reverse T3, and thyroid antibody tests.

Summary

An optimal TSH level for all people cannot reasonably be established. That's because TSH levels vary depending on numerous factors, including age, thyroid condition, and possibly even how well you sleep at night or when you last ate. Your healthcare provider will consider these factors and any symptoms you're experiencing to determine your optimal range.

The TSH test is the gold standard in diagnosing and monitoring thyroid disease. However, as with any medical test, there are exceptions to what the test results may mean and situations in which the test may be inaccurate. Your healthcare provider may order additional tests to help determine how your thyroid gland is performing.