TSH levels <0.4 mU/L indicate hyperthyroidism and >4.5 mU/L indicate hypothyroidism. Normal levels are between 0.4 and 4.0.
Thyroid Stimulating Hormone

Thyroid stimulating hormone (TSH) is a hormone that coordinates with the thyroid gland. It is released by the pituitary gland and stimulates the thyroid gland to produce more thyroid hormone. Low TSH levels indicate an overactive thyroid (hyperthyroidism), whereas high TSH levels indicate an underactive thyroid (hypothyroidism).1

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TSH Levels

When you suffer from hypothyroidism, your TSH levels will be high because your body can’t make enough of the thyroid hormones T3 and T4.2 When your pituitary gland gets the signal that your body needs more thyroid hormones, it overproduces TSH in an attempt to stimulate your thyroid gland to release thyroid hormones.1

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T4 Levels

Sometimes it's necessary to test the amount of T4 in your blood and check for thyroid antibodies to confirm a diagnosis.3 If the thyroid gland appears to have an abnormal shape, a thyroid scan or ultrasound may be performed.4

A TSH Blood Test Can Help

Hypothyroidism can be diagnosed with a TSH blood test. TSH stands for “thyroid stimulating hormone” – a common hormone found in the blood. If you have high TSH levels, it is possible that you also have hypothyroidism.5

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Diagnosing hypothyroidism with a TSH blood test

If you’re experiencing hypothyroidism symptoms, you should talk to your doctor about getting a TSH blood test. If he or she agrees that hypothyroidism is a possibility, you will likely be referred to a laboratory where blood tests can be done.

Following a TSH blood test, your doctor may also want to test your T4 hormone levels.

Another test, which looks at thyroid antibodies can also be of assistance in arriving at a diagnosis.3 Finally, if the shape of the thyroid gland appears abnormal to your doctor, he or she may perform a thyroid scan or ultrasound.4

In most cases of hypothyroidism, there are high TSH levels and lower levels of thyroid hormones3
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Which Women Should Have A TSH Blood Test Done During Their Pregnancy?

Certain women should have a TSH blood test early in their pregnancy to detect thyroid disease.6

A TSH blood test will reveal if your TSH levels are high. High TSH levels can be indicative of hypothyroidism.7

They include women with:

  • Age above 30 years8
  • A history of autoimmune disease8 or prior thyroid surgery9
  • A family history of thyroid disease8
  • A goitre (an enlarged thyroid)9
  • Thyroid antibodies10
  • Hyperemesis gravidarum and clinical features suggestive of hyperthyroidism8
  • Type I diabetes9
  • Previous radiation therapy to the head or neck9
  • A history of miscarriage or preterm delivery10
  • Decreased fertility9
  • A BMI greater than 40 (being very overweight)10
  • A need for amiodarone (a drug for irregular heartbeats), lithium10 or recent administration of iodinated contrast agent (often used during a CT scan)11
  • Living in an area of known moderate to severe iodine insufficiency8

Abbreviations: BMI, Body Mass Index; CT, Computed Tomography; mU/L, Milliunits Per Liter; T3, Triiodothyronine; T4 Thyroxine

References:

1. Shomon M. TSH levels: what do high and low levels mean? Verywell Health. Accessed May 23, 2023. https://www.verywellhealth.com/understanding-thyroid- blood-tests-low-or-high-tsh-3233198 2. Koulouri O, Gurnell M. How to interpret thyroid function tests. Clin Med (Lond). 2013;13(3):282-286. 3. Thyroid function tests. American Thyroid Association. Accessed May 23, 2023. https://www.th roid.org/thyroid-function-tests/ 4. Chaudhary V, Bano S. Thyroid ultrasound. Indian J Endocrinol Metab. 2013;17(2):219-227. 5. Hypothyroidism (underactive thyroid). Mayo Clinic. Accessed May 22, 2023. https://www.mayoclinic.org/diseases- conditions/hypothyroidism/diagnosis-treatment/drc-20350289 6. Hypothyroidism and pregnancy. John Hopkins Medicine. Accessed August 7, 2023. https://www.hopkinsmedicine.org/health/conditions-and-diseases/staying-healthy-during-pregnancy/hypothyroidism-and-pregnancy 7. Pregnancy and thyroid disease. National Institutes of Health. Accessed May 17, 2023. https://www.niddk.nih.gov/-/media/Files/Endocrine-Diseases/Pregnancy_Thyroid_Disease_508.pdf 8. Smith A, Eccles-Smith J, D’Emden M, Lust K. Thyroid disorders in pregnancy and postpartum. Aust Prescr. 2017;40(6):214-219. 9. Sahay RK, Nagesh VS. Hypothyroidism in pregnancy. Indian J Endocrinol Metab. 2012;16(3):364-370. 10. Deshauer S, Wyne A. Subclinical hypothyroidism in pregnancy. CMAJ. 2017;189(28):E941. 11. Lee SY, Rhee CM, Leung AM, Braverman LE, Brent GA, Pearce EN. A review: radiographic iodinated contrast media-induced thyroid dysfunction. J Clin Endocrinol Metab. 2015;100(2):376-383.