Lab Tests

What do the results of a thyroid function test mean?

A Answers (1)

  • AHealthwise answered

    Thyroid hormone tests are blood tests that check how well the thyroid gland is working.


    The normal values listed here—called a reference range—are just a guide. These ranges vary from lab to lab, and your lab may have a different range for what’s normal. Your lab report should contain the range your lab uses. Also, your doctor will evaluate your results based on your health and other factors. This means that a value that falls outside the normal values listed here may still be normal for you or your lab.

    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 tests

    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 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.

      This information does not replace the advice of a doctor. Healthwise disclaims any warranty or liability for your use of this information. To learn more visit

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