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 affects tissue function in the body, but bound thyroxine does not.
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.
Levels of free T4 in hypothyroidism may remain within normal range for a period of time while total T4 levels are low in the body's attempt to maintain homeostasis. So, a hypothyroid diagnosis could be delayed if only free T4 is being looked at. These tests are usually ordered when a person has symptoms of hyperthyroidism or hypothyroidism.
Thyroxine (T4) is the predominant hormone produced by the thyroid gland. It is an inactive prohormone, which converts to triiodothyronine (T3) within cells.
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.