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Low TSH level in the blood

Blood test
Low TSH level in the blood
Last update: 22/10/2020

What is a low TSH level in the blood called?

  • TSH Low

What is the normal TSH level in the blood?

If you need to know which are the TSH reference ranges or you require more information about the role of TSH in the blood, you can visit normal TSH level in the blood

What does a low total TSH (thyroid-stimulating hormone) level in the blood mean?

A TSH (thyroid-stimulating hormone) level in the blood lower than normal range may be a sign of primary hyperthyroidism, a disorder where when the thyroid gland produces too much of the thyroid hormone. For that reason, thyroid hormones T3 (triiodothyronine) and T4 (thyroxine) will be higher than normal.

However, there is a rare situation where the TSH values are low due to a failure of the pituitary gland. In this case, T3 and T4 values will be also lower than normal and it is related to a disorder called secondary hypothyroidism.

TSH (thyroid-stimulating hormone) is also called thyrotropin or thyrotropic hormone.

In the International System of Units (SI), TSH in the blood is measured in mU/L. The numeric values in this case are the same in µUI/ml or in mU/L.

Low TSH blood levels mean:

  • Mild TSH decrease (0.1 - 0.4 µUI/ml in adults):

    The TSH level in the blood is a bit low.

    If T4 value are in the normal range it is usually due to subclinical hyperthyroidism and no symptom will be present.

  • Marked TSH decrease (< 0.1 µUI/ml in adults):

    The TSH level in the blood is very low, almost non-existent and it may be a sign of suffering from primary hyperthyroidism.

    The diagnosis primary hyperthyroidism is conclusive with T4 or T3 hormone levels above the normal range.

    The main symptoms of hyperthyroidism are irritability, weight loss, sweating, palpitations or constipation.

    You should visit your doctor and he will prescribe you the proper treatment.

Which factors can reduce the total TSH level in the blood?

Some particular health situation or drugs may reduce your TSH level in the blood:

  • Iodine excess intake
  • Pregnancy (first trimester)
  • Drugs

Which diseases can reduce your total TSH level in the blood?

The following diseases can explain a TSH level in the blood lower than normal:

  • Secondary hypothyroidism
  • Sheehan's syndrome

What can I do to increase the total TSH level in the blood?

If the TSH level in the blood is low, the following tips can help you:

  • Reduce your iodine intake if it is excessive.
  • Talk to your doctor about the possible interference of the drugs that you are taking.
  • If your doctor considers it appropriate he will prescribe you antithyroid drugs such as propylthiouracil or thiamazole to reduce T3 and T4 hormones and that way the TSH will increase.

Where can I find more information about TSH level in the blood?

You can visit our pages about:

Which values are considered a low TSH level in the blood?

The following values are considered to be above the normal range:

IMPORTANT: These levels are expressed in µUI/ml. They are an example of a healthy man of about 45 years old with no known disease and not taking any medication. The ranges can be different depending on the laboratory or on your personal circumstances.

Mild TSH decrease
0.39 µUI/ml0.38 µUI/ml0.37 µUI/ml0.36 µUI/ml0.35 µUI/ml0.34 µUI/ml0.33 µUI/ml0.32 µUI/ml
0.31 µUI/ml0.3 µUI/ml0.29 µUI/ml0.28 µUI/ml0.27 µUI/ml0.26 µUI/ml0.25 µUI/ml0.24 µUI/ml
0.23 µUI/ml0.22 µUI/ml0.21 µUI/ml0.2 µUI/ml0.19 µUI/ml0.18 µUI/ml0.17 µUI/ml0.16 µUI/ml
0.15 µUI/ml0.14 µUI/ml0.13 µUI/ml0.12 µUI/ml0.11 µUI/ml0.1 µUI/ml  
Marked TSH decrease
0.09 µUI/ml0.08 µUI/ml0.07 µUI/ml0.06 µUI/ml0.05 µUI/ml0.04 µUI/ml0.03 µUI/ml0.02 µUI/ml
0.01 µUI/ml0 µUI/ml      
Medically reviewed by our Medical staff on 22/10/2020


  • Approved Guideline – Procedures for the Handling and Processing of Blood Specimens, H18-A3. 2004. Clinical and Laboratory Standards Institute.
  • HHS Publication, 6th ed., June 2020. Biosafety in Microbiological and Biomedical Laboratories. Available on:
  • Concise Book of Medical Laboratory Technology: Methods and Interpretations. 2nd Edition. 2015. Ramnik Sood. ISBN: 978-93-5152-333-8. Pag. 783.
  • Tietz. Fundamentals of Clinical Chemistry. Carl A. Burtis, Edward R. Ashwood, David E. Bruns, Barbara G. Sawyer. WB Saunders Company, 2008. Pag 766. ISBN: 978-0-7216-3865-2.
  • Laboratory tests and diagnostic procedures with nursing diagnoses (8th ed), Jane Vincent Corbett, Angela Denise Banks, ISBN: 978-0-13-237332-6, Pag. 400.

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