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Thyroid Blood Tests

The thyroid is a gland located in the neck. It’s job is to take iodine from the blood and combine it with an amino acid (one of the building blocks of protein) to form thyroid hormones. One of the hormones, thyroxine, is responsible for your metabolism. Thyroid function tests help to determine if your thyroid is not working correctly:

  • hyperthyroid – an over-working thyroid
  • hypothyroid – poor thyroid function

Abnormal thyroid function is common. It is seen in two to three percent of the entire population. When the thyroid is not working properly, it can cause changes in other blood tests as well.

Thyroid Stimulating Hormone (TSH)

An indicator of thyroid function.

Normal range for an adult: 0.4 – 5.5 mU/mL

Preparation
  • This test may be measured any time of the day without fasting.
  • The brain regulates the amount of thyroid hormones in the blood.
  • When the hormone levels are low, the brain sends a message to send out TSH. This causes the thyroid gland to send out more hormones.
  • If blood levels of thyroid hormone are high, the brain senses this and sends a message to stop producing TSH. TSH is a very good test to check for hypothyroidism. TSH is increased with hypothyroid and decreased with hyperthyroid.
  • Values may be lowered with use of aspirin, corticosteroids and heparin therapy.
  • Values may be raised with use of lithium, potassium, iodide and TSH injections.

Thyroxine (T4)

Normal range for an adult: 5 – 11 µg/dL

Preparation
  • This test may be measured any time of the day without fasting.
  • Thyroxine is a hormone produced by the thyroid. It is drawn to assess thyroid function. Low T4 is seen with hypothyroidism. High T4 is seen with hyperthyroidism.
  • The T4 blood test can also be used to monitor the effectiveness of medications used to treat thyroid disease. Estrogen, anticonvulsants, aspirin use and anticoagulants may affect T4 levels. They are increased with pregnancy.

Microsomal Thyroid Antibodies (TPO)

Desirable level for an adult: 0.0 - 5.0 IU/mL

Preparation
  • This test may be measured any time of the day without fasting.
  • Thyroid antibodies are present if hypothyroid is related to thyroiditis.
  • Hypothyroid can be caused by primary thyroid disease. It also can be related to other health problems, such as glucocorticoid or amiodarone use, osteoporosis, pregnancy, insulin dependent diabetes and liver disease.
Reviewed: 08/10

This information is about testing and procedures and may include instructions specific to Cleveland Clinic. Please consult your physician for information pertaining to your testing.

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This information is provided by Cleveland Clinic and is not intended to replace the medical advice of your doctor or health care provider. Please consult your health care provider for advice about a specific medical condition.

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