Thyroid Hormone Tests
Results are usually available within a few days.
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
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
nanograms per deciliter (ng/dL) or 10–26 picomoles per liter (pmol/L)
| Total triiodothyronine
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
260–480 picograms per deciliter (pg/dL) or 4.0–7.4 pmol/L in adults
| Free thyroxine index
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 thyroid hormone levels (hyperthyroidism) may be caused by:
- Diseases of the thyroid gland, such as
thyroiditis, or a
goiter that contains one or more abnormal growths
- Taking too much thyroid medicine.
Low thyroid hormone levels (hypothyroidism) may be caused by:
- Thyroid disease, such as
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:
estrogen, progesterone, or birth control
- Blood-thinning medicines such as aspirin, heparin, or
- Antiseizure medicines such as phenytoin or
- Heart medicines such as amiodarone or
- Having recently had an X-ray test that uses
- Being pregnant.
What To Think About
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
- 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'
- 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: