Description
Also know as T4.
Thyroid hormone testing checks how well the thyroid gland is working. The Thyroid gland makes hormones that regulate the way the body uses energy.
The thyroid gland makes two thyroid hormones, thyroxine (T4) and triiodothyronine (T3).
These hormones can be found in two forms in the blood stream: free, or attached to a protein (Thyroxine Binding Globulin). The greatest percentage of thyroid hormones are attached to TBG, while only a small part of them are free on the blood.
Thyroid hormones are important for normal development of the brain, especially during the first 3 years of life. Older children also need thyroid hormones to develop normally, and adults need the hormones to regulate the way the body uses energy (metabolism).
Its concentration is measured by means of analysis of a blood sample drawn from the vein in the arm.
Purpose of the test
Thyroxine (T4) test is measured in order to check the levels of this hormone in blood in order to help diagnose thyroid disorders such as hyperthyroidism and hypothyroidism.
Also this test is done in order to check how well treatment for thyroid disease is working.
Finally newborns are screened with test in order to detect congenite diseases such an underactive thyroid.
Reference range values
ng/dL (SI: pmol/L = 12.9 x ng/dL)
18-150Y 0.8 – 1.9
Abnormal findings
If after a TSH test result high and a T4 test result normal, a T3 test result is normal, it can indicate a mild form of hypothyroidism.
If after a TSH test result high and a T4 test result low, a T3 test result is normal or low, it can indicate hypothyroidism.
If after a TSH test result low and a T4 test result normal, a T3 test result is normal, it can indicate a mild form of hyperthyroidism.
If after a TSH test result low and a T4 test result normal or high, a T3 test result is normal or high, it can indicate hyperthyroidism.
If after a TSH test result low and a T4 test result normal or low, a T3 test result is normal or low, it can indicate a nonthyroidal illness, for example a rare pituitary hypothyroidism.