There seems to be a lot of confusion among the general public and doctors about which thyroid tests to order. Thyroid hormones play a big role in metabolism, regulation of body temperature and helping organs like the heart to function properly. When the thyroid is not functioning properly, many different systems in the body are affected, which may result in a wide variety of symptoms.

When testing thyroid function, many ‘old school’ doctors will just order a TSH (thyroid stimulating hormone) test. TSH is generally produced in the body when thyroid hormones are low, signaling to the body that it needs to release or convert more thyroid hormone. These doctors rely on it solely to determine if thyroid supplementation is needed. The problem is the TSH does not give the whole picture of what’s going on with the thyroid. In fact thyroid supplementation may still be needed, even if the TSH is normal. 

Most doctors now order not only order a TSH but also free T3 and free T4 levels. So what is T4 and T3? T4 is a storage hormone. T4 is converted to T3 when thyroid hormone is needed in the body.  When ordered together, these three  tests  give a better picture of how the thyroid is functioning and lets the doctor know if thyroid supplementation is needed.

So what do these tests mean? First, an elevated TSH with low T3 and T4 typically means there is a problem with the thyroid itself. It is not always clear what causes this but it can sometimes be linked to autoimmune disease or iodine deficiency. A doctor testing for only TSH will likely catch this type of thyroid deficiency. Many times supplementation with thyroid hormones containing only T4 like Synthroid (levothyroxine) will  be adequate and will relieve symptoms. 
Another common problem occurs when the TSH and T4 are normal but the T3 is low. This means that the thyroid is performing properly but the body is not converting T4 to T3 as it should. A doctor testing for only TSH would not catch this type of thyroid deficiency.  This could mean the body is low in enzymes. In this case supplementation with a supplement that contains T3 and T4 like Armour Thyroid (dessicated thyroid) may be necessary.

If you or your doctor suspect that you may be deficient in thyroid hormones, make sure they order a T3 and T4 test in addition to the TSH. At the very least, a T3 should be ordered with a TSH. This will help ensure that a potential thyroid deficiency doesn’t go undetected and will help you chose the right medication. 

Author: Christi Larson, Pharm. D.

Dr. Larson is a pharmacist, author of Empowered Medicine; A Guide for Consumers and creator of Empowered Medicine. You can read more about her by visiting www.EmpoweredMedicine.com and clicking on the 'About Us' tab.  EmpoweredMedicine.com is committed to providing evidence-based medical information from a pharmacist's point of view.