TSH Levels: What Normal, Low and High Ranges Mean

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Young Hispanic or middle Eastern man getting thyroid or neck exam from young Caucasian male doctor

Your doctor may order a blood test to check your body’s levels of thyroid stimulating hormone (TSH). The amount of TSH circulating in your bloodstream indicates how well your thyroid gland is functioning. TSH levels that are too high or too low can tell your doctor if you have a thyroid disorder or not. Learn more about what normal, high and low TSH levels mean.

How to interpret a TSH test result

Interpreting the results of a TSH test can be confusing, because high readings indicate a low-performing thyroid. When the thyroid gland is not producing adequate amounts of T3 and T4 hormones, the pituitary gland repeatedly releases TSH into the bloodstream to stimulate the thyroid gland. Thus, high levels of TSH indicate that the pituitary gland is frequently stimulating an underactive thyroid.

The reference range for normal TSH can vary by age and overall health status. In older adults, for example, a higher-than-normal TSH reading may, in fact, be normal. If your TSH lab result is flagged as “abnormal,” talk with your doctor about it, because the reading may not be abnormal for you.

What is a normal TSH level?

TSH levels are measured in ranges. In general, the normal reference range for TSH levels is 0.5 to 5.0 milli-international units per liter (mIU/L) of blood. A TSH reading in this range indicates the thyroid gland is functioning normally.

However, doctors do not all agree on the precise TSH range of a normal-functioning thyroid gland. Some doctors consider a TSH level of 4.5 mIU/L to be an indication of an underactive thyroid. Before diagnosing you with any type of thyroid disorder, your doctor will consider not only your TSH level but any signs or symptoms you’re exhibiting, such as an enlarged thyroid gland.

What does a low TSH level mean?

A TSH level below 0.5 mIU/L can mean that your thyroid gland is overactive. This is hyperthyroidism, which can cause such symptoms as unexplained weight loss, heat intolerance, increased appetite, and bulging eyes.

One of the most common causes of hyperthyroidism is Graves’ disease, an autoimmune disorder in which your body’s immune system mistakenly attacks the thyroid gland. Hyperthyroidism also can be caused by goiter or taking certain medications.

What does a high TSH level mean?

A TSH level above 5.0 mIU/L usually indicates an underactive thyroid gland, or hypothyroidism. High TSH levels can cause symptoms that include:

  • Cold intolerance
  • Difficulty focusing or concentrating (“brain fog”)
  • Difficulty losing weight
  • Irregular menstrual periods

A great many medical issues can cause hypothyroidism. The most common cause is Hashimoto’s disease (an autoimmune disorder). Underactive thyroid usually is treated with supplemental thyroid hormone (levothyroxine).

What should I do about an abnormal TSH test?

If your TSH levels are outside the normal range, talk with your doctor about what may have caused this. Be sure to mention any health symptoms you’re experiencing. Also provide your doctor with a list of all medicines and vitamins, minerals, herbs, or other dietary supplements you take. Some of these products can alter your thyroid function and cause an abnormal test result.

After an abnormal TSH test, your doctor may recommend further testing, such as a thyroid ultrasound, to rule out a serious disorder like thyroid cancer. Most of the time, however, hyperthyroidism and hypothyroidism both can be treated with medication.

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Medical Reviewer: William C. Lloyd III, MD, FACS
Last Review Date: 2020 Jun 12
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  2. Grave’s Disease. MedlinePlus, U.S. National Library of Medicine. https://medlineplus.gov/ency/article/000358.htm
  3. Thyroid Tests. U.S. National Institute of Diabetes and Digestive and Kidney Disorders. https://www.niddk.nih.gov/health-information/diagnostic-tests/thyroid
  4. Lewandowski K. Reference ranges for TSH and thyroid hormones. Thyroid Res. 2015; 8(Suppl 1): A17. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4480274/
  5. Topliss DJ. What happens when laboratory reference ranges change? CMAJ 2020;192(18):E481-E482; DOI: https://doi.org/10.1503/cmaj.200511, Retrieved from https://www.cmaj.ca/content/192/18/E481
  6. Determination of optimal TSH ranges for reflex Free T4 testing. American Thyroid Association. https://www.thyroid.org/patient-thyroid-information/ct-for-patients/february-2018/vol-11-issue-2-p-3-4/
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