ZRT Total T4 Blood Spot Test

Thyroxine (T4) is the main hormone produced by the thyroid. A Total T4 test measures the overall level of T4 output by the thyroid gland. 


Price: $60.00

Thyroxine (T4) is the main hormone produced by the thyroid. Total T4 measures the overall level of T4 output by the thyroid gland.

Most of the thyroxine (T4) in the blood is attached to a protein called thyroxine-binding globulin.  Less than 1% of the T4 is unattached.  A total T4 blood test measures both bound and free thyroxine. Free thyroxine (fT4) affects tissue function in the body, while total thyroxine (T4) helps transport those molecules and allows cells to access T4.

A total T4 or free T4 test is primarily ordered in response to an abnormal TSH test result. Sometimes the T4 will be ordered along with a TSH to give the doctor a more complete evaluation of the adequacy of the thyroid hormone feedback system. These tests are usually ordered when a person has symptoms of hyperthyroidism or hypothyroidism.  

Why Test Total T4?

In general, high free or total T4 results may indicate an overactive thyroid gland (hyperthyroidism), and low free or total T4 results may indicate an underactive thyroid gland (hypothyroidism). The test results alone are not diagnostic but will prompt the doctor to perform additional testing to investigate the cause of the excess or deficiency. Both decreased and increased T4 results are associated with a variety of temporary and chronic thyroid conditions. Low T4 results in conjunction with a low TSH level or high T4 results along with a high TSH may indicate a pituitary gland condition.
Many medications—including estrogen, certain types of birth control pills, and large doses of aspirin—can affect total T4 test results and their use should be discussed with the doctor prior to testing. In general, free T4 levels are not affected by these medications.

The lab will email you within 3-5 business days after they receive the samples for on-line access to your results.

Once you have your results, read our Interpreting Results article and consider consulting with a practitioner for next steps.  Find out what you need to know and start your path towards recovery today.

Total T4 Yes
Sample Type
Blood Spot Yes