Great Smokies Medical Center of Asheville

Laboratory Testing of Hypothyroidism

Doctors have traditionally measured blood levels of thyroid stimulating hormone (TSH), T3 Uptake, Total T4 and Total T7 to both diagnose thyroid imbalances and monitor the effectiveness of thyroid hormone therapy regimens. The problem with this laboratory assessment is that most of the total T3 and T4 are bound to proteins that render them unavailable for use by the body.

Alternatively, doctors can measure Free T3 and Free T4 that reveal the parts of Total T3 and Total T4 that are not bound to proteins and are thus biologically active. These more sensitive tests enable a doctor to more accurately and safely customize dosing of T3 and T4 to optimize functioning of the patient.

We recommend having a TSH, Free T3, Free T4, and thyroid antibodies drawn as baseline to test for hypothyroidism. To monitor thyroid replacement therapy in a person taking thyroid hormone, the doctor can usually monitor with a simple free T3 or T4. The time of day that blood specimens are drawn is important. Have your thyroid blood level tests drawn about eight hours after taking your morning dose of thyroid upon waking and while fasting. That is about 3:00 p.m. to 4:00 p.m. for most people.

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