Why are thyroid function tests not always reliable?

The TSH level is the best initial test for determining thyroid function. Other tests of thyroid function include free T4, and free T3. Each of these tests has a reference range of what’s considered to be “normal”, and these ranges are not the same at every testing center. Therefore, a lab value may be reported as normal at one facility, and abnormal at a different facility. Additionally, test results can be affected by stress, medications, or other medical illnesses that a patient has and results can vary when checked at different times or different labs.

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Thyroid gland: background, thyroid tests

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