What Do High and Low TSH Levels Mean? (2022)

High thyroid-stimulating hormone (TSH) levels based on a TSH test may mean you have hypothyroidism (underactive thyroid), while low TSH levels suggest hyperthyroidism (overactive thyroid).

TSH values are described as follows:

  • Normal: Between 0.4 and 4.0 mU/L
  • High: Over 4.5 mU/L
  • Low: Under 0.4 mu/L

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.

A high TSH suggests yourthyroid is underactive (hypothyroid) and not doing its job of producing enough thyroid hormone.

A low TSHsuggests yourthyroid is overactive (hyperthyroid) and producing excess thyroid hormone.

How TSH Levels Change

TSH levels seem counterintuitive. Why does a high TSH mean you have an underactive thyroid gland? And why do low levels mean it's overactive?

To answer that, you have to know that 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, 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 revealabout 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.

What Do High and Low TSH Levels Mean? (1)

Controversies Over Optimal TSH

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.

Differences in TSH Levels in Females and Males

Some studies suggest that a normal TSH range in younger females may be closer to 0.4-2.5 mU/L. This is due to the fact that TSH concentrations are higher in females than in males whose normal TSH range is closer to 0.4-4.0 mU/L

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.

(Video) TSH Results / Levels: How to explain in 80 seconds

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

During pregnancy, TSH should not be allowed to rise above 2.4 mU/L in the first trimester or 3.0 mU/L in the second and third. That protects the health of both the baby and mother.

How Do Thyroid Problems Affect Fertility and Pregnancy?

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 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.

All About How to Take Your Thyroid Medication

Causes of Low TSH

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.

What to Know About Hypopituitarism

(Video) Causes of normal T3,T4 levels but low TSH levels - Dr. Anantharaman Ramakrishnan

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 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.

NSAIDs and Your Thyroid

Testing Method

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

(Video) High TSH Levels: Lab Interpretation for New Nurse Practitioners

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.

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.


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're being heard, consider seeking a second opinion.

Frequently Asked Questions

  • What is the normal range for TSH levels?

    The normal range for TSH is between 0.4 mU/L and 4.0 mU/L.

    (Video) Signs of Low Thyroid Level (Hypothyroidism), & Why Symptoms Occur
  • What does it mean when your TSH levels are low?

    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.

  • What causes high TSH levels?

    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.

  • How does estrogen affect TSH levels?

    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.

    Learn More:Thyroid Disease and Menopause: What's the Link?

  • Do diet changes level out TSH?

    They can, yes. Studies show that dietary changes can have a beneficial effect on TSH and thyroid function. While a diet designed specifically for you is best, some researchers have seen success with a Mediterranean-style diet.

    Learn More:Nutrition and Your Thyroid Health

  • What is a normal TSH level in women?

    (Video) SIGNS THAT YOU HAVE A LOW THYROID LEVEL - Hypothyroidism Symptoms

    Generally speaking, a normal TSH range in adults is 0.4-4.0 mU/L. However, some studies suggest that a normal range in females aged 18 to 29 may be closer to 0.4-2.5 mU/L. This is because TSH levels in females are influenced by the hormone estrogen, and, as result, TSH concentrations tend to be higher than in males.


Why would TSH be high and low? ›

These fluctuations can occur as your thyroid disease progresses. But other factors, such as hormonal changes and medication variations, or significant weight gain or loss, can alter your thyroid hormone levels as well; in some cases progression of a thyroid condition (such as a nodule) may be responsible.

Is it better to have low TSH or high TSH? ›

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.

What does a low level of TSH indicate? ›

Low TSH levels indicate hyperthyroidism.

This is also known as an overactive thyroid. If a person's thyroid gland is secreting levels of hormones that are too high, the pituitary gland produces less TSH. Symptoms of hyperthyroidism can include: irregular or rapid heartbeat.

What does a high TSH level indicate? ›

Blood tests

A low level of thyroxine and high level of TSH indicate an underactive thyroid. That's because your pituitary produces more TSH in an effort to stimulate your thyroid gland into producing more thyroid hormone.

Should I worry if my TSH is low? ›

What happens when TSH levels are too low? If you have too little TSH, it's most likely that your thyroid gland is making excess thyroid hormone. This condition is called hyperthyroidism, or overactive thyroid. A variety of conditions lead to hyperthyroidism, including Graves' disease and thyroid nodules.

What are side effects of low TSH? ›

TSH is a hormone made by the pituitary gland that tells the thyroid gland how much thyroid hormone to make. Symptoms of low TSH include weight loss, fatigue, confusion, heat intolerance, fast heartbeat, high blood pressure and other symptoms.


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