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Thyroid Hormone Tests

Results continued...

Results are usually available within a few days.

Labs generally measure free T4 (FT4) levels, but also may measure total thyroxine (T4) and T3 uptake (T3U). Results of these thyroid hormone tests may be compared to your thyroid-stimulating hormone (TSH) results.

Thyroid hormone tests2
Total thyroxine (T4):

11.8–22.6 micrograms per deciliter (mcg/dL) or 152–292 nanomoles per liter (nmol/L) in newborns

6.4–13.3 mcg/dL (83–172 nmol/L) in babies and older children

5.4–11.5 mcg/dL (57–148 nmol/L) in adults

Free thyroxine (FT4):

0.7–2.0 ng/dL nanograms per deciliter (ng/dL) or 10–26 picomoles per liter (pmol/L)

Total triiodothyronine (T3):

105–245 ng/dL (1.6–3.8 nmol/L) in children ages 1–14

82–213 ng/dL (1.3–3.28 nmol/L) in adolescents ages 12–23

80–200 ng/dL (1.2–3.1 nmol/L) in adults

Free triiodothyronine (FT3):

260–480 picograms per deciliter (pg/dL) or 4.0–7.4 pmol/L in adults

Free thyroxine index (FTI):

1.5–4.5 (index) in adults

Many conditions can change thyroid hormone levels. Your doctor will talk with you about any abnormal results that may be related to your symptoms and past health.

High values

High thyroid hormone levels (hyperthyroidism) may be caused by:

  • Diseases of the thyroid gland, such as Graves' disease, thyroiditis, or a goiter camera.gif that contains one or more abnormal growths (nodules).
  • Taking too much thyroid medicine.

Low values

Low thyroid hormone levels (hypothyroidism) may be caused by:

  • Thyroid disease, such as thyroiditis.
  • Pituitary gland disease.
  • Destruction of the thyroid gland by surgery or radiation.

What Affects the Test

Reasons you may not be able to have the test or why the results may not be helpful include:

  • Taking certain medicines, such as:
    • Corticosteroids, estrogen, progesterone, or birth control pills.
    • Blood-thinning medicines such as aspirin, heparin, or warfarin.
    • Antiseizure medicines such as phenytoin or carbamazepine.
    • Heart medicines such as amiodarone or propranolol.
    • Lithium.
  • Having recently had an X-ray test that uses contrast material.
  • Being pregnant.

What To Think About

  • Because false-positive results can occur when testing a newborn for congenital hypothyroidism, the thyroid hormone tests may be repeated a few days after initial testing. If the results are still abnormal and congenital hypothyroidism is suspected, additional testing is done.
  • Other blood tests are often used to check how well the thyroid gland is working.
    • Thyroid-stimulating hormone (TSH) test measures the amount of TSH in the blood and is considered the most reliable way to find a thyroid problem. If the TSH test is abnormal, other thyroid hormone tests such as a T3 or T4 may be done. For more information, see the topic Thyroid-Stimulating Hormone.
    • Thyroid antibodies test measures the presence of antibodies against thyroid tissue. Antibodies may mean that you have an autoimmune disease such as Hashimoto's thyroiditis or Graves' disease.
    • Thyroxine-binding globulin (TBG) test. TBG is an important protein in the blood that carries the thyroid hormones T3 and T4. TBG testing is not done very often.
  • Other tests used to investigate problems with the thyroid gland include:

WebMD Medical Reference from Healthwise

Last Updated: August 07, 2012
This information is not intended to replace the advice of a doctor. Healthwise disclaims any liability for the decisions you make based on this information.

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