What is thyroid-stimulating hormone (TSH)?

Thyroid-stimulating hormone, commonly called TSH and also referred to as thyrotropin, is a hormone that your pituitary gland releases to trigger your thyroid to produce and release its own hormones — thyroxine (T4) and triiodothyronine (T3). These two hormones are essential for maintaining your body’s metabolic rate — the speed at which your body transforms the food you eat into energy and uses it. Thyroxine and triiodothyronine also maintain:

  • Your heart and digestive functions.
  • Muscle control.
  • Brain development.
  • Bone maintenance.

Hormones are chemicals that coordinate different functions in your body by carrying messages through your blood to your organs, muscles and other tissues. These signals tell your body what to do and when to do it.

Your pituitary gland is a small, pea-sized gland located at the base of your brain below your hypothalamus. It makes and releases eight hormones, including TSH. Your pituitary gland consists of two lobes: the posterior (back) lobe and the anterior (front) lobe. The anterior lobe makes TSH.

Your thyroid is a small, butterfly-shaped gland located at the front of your neck under your skin. Your pituitary gland and thyroid are part of your endocrine system.

How are TSH levels controlled?

Multiple hormones and glands in your endocrine system work together to carefully control the level of TSH in your bloodstream through a feedback loop.

To start, your hypothalamus releases thyroid-releasing hormone (TRH) to trigger the release of thyroid-stimulating hormone (TSH) by your pituitary gland.

Your pituitary gland is connected to your hypothalamus through a stalk of blood vessels and nerves. This is called the pituitary stalk. Your hypothalamus is the part of your brain that controls functions like blood pressure, heart rate, body temperature and digestion. Through the stalk, your hypothalamus communicates with your pituitary gland and tells it to release certain hormones. In this case, your hypothalamus releases thyroid releasing hormone (TRH), which stimulates your anterior pituitary lobe to release TSH. Your hypothalamus can also release somatostatin, another hormone, to inhibit (prevent) the release of TSH from your anterior pituitary.

TSH then stimulates cells in your thyroid to release thyroxine or T4 (80%) and triiodothyronine or T3 (20%) into your bloodstream. These two hormones prevent your pituitary gland from producing more TSH if the levels of thyroxine and triiodothyronine are too high, thus completing the cycle. When T4 and T3 levels drop, the cycle starts over again.

Because of this feedback loop, if your thyroid is making too much or too little thyroid hormones, it affects your TSH levels. In addition, if your pituitary gland is producing too much or too little TSH, it impacts the function of your thyroid. It’s more common to have an issue with your thyroid that causes irregular levels of TSH than to have an issue with your pituitary gland that causes irregular TSH levels.

What are normal TSH levels?

Normal levels of TSH vary based on your age. In general, normal ranges of TSH for healthy people who aren’t pregnant include:

  • Infants up to 5 days old: 0.7 – 15.2 micro-international units per milliliter (uIU/mL).
  • Infants 6 to 90 days old: 0.72 – 11.0 uIU/mL.
  • Babies 4 to 12 months old: 0.73 – 8.35 uIU/mL.
  • Children 1 to 6 years old: 0.7 – 5.97 uIU/mL.
  • Children 7 to 11 years old: 0.6 – 4.84 uIU/mL.
  • People 12 to 20 years old: 0.51 – 4.3 uIU/mL.
  • Adults 21 to 99 years old: 0.27 – 4.2 uIU/mL.

Normal value ranges for TSH may vary slightly among different laboratories. Be sure to check your lab report’s reference range on your results. If you have any questions about your results, ask your healthcare provider.

What are normal TSH levels during pregnancy?

It’s especially important for people who are pregnant to have healthy amounts of TSH and thyroid hormones to ensure the healthy development of their babies. TSH levels fluctuate throughout pregnancy. In general, normal TSH levels during pregnancy include:

  • First trimester (9 to 12 weeks): 0.18 – 2.99 (uIU/mL).
  • Second trimester: 0.11 – 3.98 uIU/mL.
  • Third trimester: 0.48 – 4.71 uIU/mL.

Always check your lab’s reference range on your results report. If you have questions about your results, ask your healthcare provider.

How do you test TSH levels?

Healthcare providers test TSH levels using a blood test. They take a sample of blood from a vein in your arm and send the sample to a laboratory for testing. You usually don’t need to do anything special to prepare for a TSH blood test.

A TSH blood test is usually the first test providers order if you’re experiencing symptoms related to thyroid issues.

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. A little over 1% of adults in the United States have hyperthyroidism.

Since thyroid hormone suppresses TSH release, high levels of thyroid hormone can cause lower-than-normal TSH levels. Rarely, issues with your pituitary gland, such as a non-functioning pituitary adenoma, can result in low TSH levels as well as low thyroid hormone levels.

Symptoms of low TSH levels

Low TSH levels are usually a sign of hyperthyroidism. Symptoms of hyperthyroidism include:

If you’re experiencing these symptoms, it’s important to talk to your healthcare provider. Hyperthyroidism is treatable.

What happens when TSH levels are too high?

If you have too much TSH, it may indicate that your thyroid isn’t making enough thyroid hormone. This condition is called hypothyroidism, or underactive thyroid. A number of conditions can cause hypothyroidism, including Hashimoto’s disease. About 5% of adults in the United States have hypothyroidism.

Since thyroid hormone suppresses TSH release, too little thyroid hormone can cause your pituitary to make excess TSH. Rarely, issues with your pituitary gland, such as a TSH-secreting pituitary adenoma, or rare genetic conditions can result in higher-than-normal TSH and thyroid hormone levels.

Symptoms of high TSH levels

High TSH levels are usually a sign of hypothyroidism. Symptoms of hypothyroidism include:

It’s important to talk to your healthcare provider if you’re experiencing these symptoms. Hypothyroidism is treatable.

Should I be concerned if my TSH level test results are abnormal?

If your TSH test results are abnormal, it doesn’t always mean you have a medical condition. Your healthcare provider will consider many factors when interpreting your TSH test results, including:

  • Your age: TSH levels tend to be higher in people over the age of 80. Most older people with slightly higher-than-normal TSH levels don’t have any associated health conditions.
  • Pregnancy: Pregnancy causes changes in thyroid hormones. It’s common for TSH to be slightly lower than normal during the first trimester, then slowly increase.
  • Severe illness: People who are very sick with conditions that aren’t related to the thyroid may have a low TSH level temporarily.
  • Other thyroid tests: The results of other thyroid tests, such as free T4 and thyroid antibodies, may affect how your provider interprets TSH test results.

A note from Cleveland Clinic

In most cases, irregular thyroid-stimulating hormone (TSH) levels mean there’s an issue with your thyroid — it’s either producing too much thyroid hormone or too little. The good news is that your thyroid hormone levels, and TSH levels, can be corrected with treatment and medication. If you have any questions about your TSH test results, talk to your healthcare provider. They’re there to help.

Last reviewed by a Cleveland Clinic medical professional on 07/25/2022.

References

  • Lab Tests Online. Thyroid-Stimulating Hormone (TSH). (https://www.testing.com/tests/thyroid-stimulating-hormone-tsh/) Accessed 7/25/2022.
  • National Library of Medicine. TSH (Thyroid-Stimulating Hormone) Test. (https://medlineplus.gov/lab-tests/tsh-thyroid-stimulating-hormone-test/) Accessed 7/25/2022.
  • Pirahanchi Y, Toro F, Jialal I. Physiology, Thyroid Stimulating Hormone. (https://www.ncbi.nlm.nih.gov/books/NBK499850/) [Updated May 9, 2021]. In: StatPearls [Internet]. Treasure Island, FL: StatPearls Publishing; 2021 Jan. Accessed 7/25/2022.
  • Society for Endocrinology. Thyroid Stimulating Hormone. (https://www.yourhormones.info/hormones/thyroid-stimulating-hormone/) Accessed 7/25/2022.

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