T4
Formal name: thyroxine
The Test
How is it used?
T4 can tell whether the thyroid is performing properly. It aids in the diagnosis of hypothyroidism or hyperthyroidism. The test may also be used to help evaluate a patient with an enlarged thyroid gland, called a goitre. It may also aid in the diagnosis of female infertility problems.
When is it requested?
A total T4 or free T4 test is usually used in response to an abnormal TSH test result.
What does the test result mean?
High free or total T4 results may indicate an overactive thyroid gland (hyperthyroidism).
Low free or total T4 results may indicate an underactive thyroid gland (hypothyroidism).
Low free or total T4 results may indicate an underactive thyroid gland (hypothyroidism).
Is there anything else I should know?
Many medicines including oestrogen, certain types of contraceptive birth control pills, those drugs used to help control epilepsy and large doses of aspirin can interfere with total T4 test results, so tell your doctor about any drugs you are taking. In general, free T4 levels are not affected by these medications.
In addition, total T4 levels may be affected by contrast material used for certain X-ray imaging tests. Total T4 is now rarely measured in the UK having been replaced by Free T4 (FT4).
In addition, total T4 levels may be affected by contrast material used for certain X-ray imaging tests. Total T4 is now rarely measured in the UK having been replaced by Free T4 (FT4).
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