I often get asked, “Do I have a low thyroid problem if I am always tired and/or gaining weight?” There are many symptoms of hypothyroidism, an underactive thyroid, including fatigue and weight gain. However, they are non-specific, meaning they can have many different causes. Evaluating the thyroid with blood tests is the only way to determine if it is the cause. See the related article on thyroid symptoms. The most common methods of testing thyroid function are reliable most of the time, but that is not always the case. I will discuss the different ways to test the various thyroid hormone levels. See the table below for a description of some of the various tests available.
Thyroid hormones are typically measured with immunoassays, which rely on antibodies in the testing mechanism. These antibodies are prone to interference for multiple reasons, such as use of biotin and/or the presence of other “interfering” antibodies. Free T3 and free T4 can give misleading results, and it is very important to correlate these levels with the TSH. A better way of measuring free T3 and free T4 is by equilibrium dialysis and mass spectrometry, the gold standards for measuring these hormones. If levels and symptoms seem “off” then it is appropriate to check the levels with these methods. However, many physicians don’t even know they can do this, so this is where a specialist comes in. We understand the nuances of testing thyroid levels and correlating the appropriate levels, especially since it is not always as cut and dry as checking a simple TSH with a free T4 and/or free T3 (if the TSH is abnormal). To make matters more puzzling, the TSH can be falsely low or high as well.
An example of misleading results is a treated hypothyroid patient with symptoms of too much thyroid hormone (overtreatment) but has persistently elevated TSH and “normal” free T4 levels. I have seen patients continually have their thyroid hormone dose increased in this very situation, despite symptoms of already being overtreated. It results in a patient that gets more and more overtreated, which can be very dangerous. I have checked free T4 by equilibrium dialysis in patients like this and it returns very HIGH, which confirms the overtreatment. When this happens, we must rely on the gold standard tests to ensure proper dosing with thyroid hormone.
Another example of misleading results that I see is a patient with seemingly normal thyroid levels but continued symptoms of hypothyroidism. Testing with the gold standard tests then reveals that the levels are truly low and thyroid hormone replacement is warranted. However, if the gold standard tests are normal as well, then the symptoms are NOT due to the thyroid. In this case, one must start to search for other causes of the symptoms. You do not want to be treated with thyroid hormone if you do not need it, as it can be dangerous! It can lead to abnormal and possibly fatal heart rhythms, as well as osteoporosis among other issues.
On a side note, I sometimes get asked about the reverse T3 test. This is NOT appropriate to test on a routine basis. It is only used in very specific clinical situations, such as critically ill patients in the hospital. Ordering it routinely is unnecessary.
Most of the time, thyroid dysfunction is diagnosed in a straightforward fashion, and hypothyroidism can typically be managed by your primary care provider. Hyperthyroidism, an overactive thyroid, is more complex and you should see a specialist for this condition. Furthermore, if thyroid test results appear to be confusing along with abnormal symptoms, then it is time to see a thyroid specialist to sort it out.
To answer the original oft asked question, certain non-specific symptoms do not always indicate thyroid dysfunction. Testing must prove this is the case, and it must be appropriate testing with results that are clear and reliable.