While there is considerable agreement on how the thyroid works, on how thyroid problems develop and even on many thyroid treatment plans, there is not as much agreement on the tests needed and the interpretation of the tests.  Here are the primary tests.

Overview of the thyroid system (See Wikipedia:...

Overview of the thyroid system (See Wikipedia:Thyroid). To discuss image, please see Talk:Human body diagrams (Photo credit: Wikipedia)


 
Thyroid-Stimulating Hormone Test (TSH)

One of the most common tests is the thyroid-stimulating hormone (TSH) blood test.  If the thyroid is not producing sufficient thyroid hormones, the hypothalamus signals the pituitary gland to release TSH to cause the thyroid to start producing hormones.   When there are sufficient thyroid hormones in the blood, then no TSH is released.  The TSH test measures the amount of thyroid-stimulating hormone that is released by the pituitary gland into the blood.   

The TSH test is normally stated in milliunits per liter of blood.  A unit is a standard of measurement and a milliunit is one-thousandth of a unit. A liter is a measure of volume that is a little bigger than a quart.
In theory, the TSH test should be reliable. If the TSH amount is low, this is an indicator of hyperthyroidism (overactive thyroid).  If the TSH amount is too high, this is an indicator of hypothyroidism (underactive thyroid). However, many doctors disagree on what level is too low and what is too high.  

For example, many doctors believe that the “normal” TSH reference scale is between 0.5 to 5.0.  However, the American Association of Clinical Endocrinologists states that the correct range is 0.3 to 3.0.  It states,

“…In the future, it is likely that the upper limit … will be reduced to 2.5 mIU/L because >95% of rigorously screened normal euthyroid (normal thyroid) volunteers have serum TSH values between 0.4 and 2.5 mIU/L….”

For example, if your TSH level was 0.5 and your doctor used the American Association of Clinical Endocrinologists reference range, then the TSH would be considered normal.   However, most labs provide a table that shows the range of TSH results and the middle of the curve is often between .4 to 4.0.  if the doctor used a lab’s reference range, then the TSH test would indicate that your thyroid is hyperactive but this may not be correct.  
Whether high or low, an abnormal TSH indicates an excess or deficiency in the amount of thyroid hormone available to the body, but it does not indicate the reason why.  An abnormal TSH test result is usually followed by additional testing to investigate the cause of the increase or decrease.  

ADDITIONAL TESTS

The following are the main tests now being used:

T4 Test – T4 is produced by the thyroid gland; it enters the bloodstream and some of the T4 is absorbed into the cells.  It is found in the blood in two forms:  T4 that binds to proteins (believed to be 99% of the T4 produced by the thyroid), and  free T4 (T4 not bound to proteins) which is the only form of T4 that can be absorbed into the cells.

There is a total T4 test which measures the protein-bound T4 and the free T4.  Then there is the free T4 test which is called the Free T4 (“FT4”) and the Free T4 Index (“FT4I or FTI”). Generally, individuals who have hyperthyroidism will have an elevated FT4 or FTI and individuals with hypothyroidism will have a low level of FT4 or FTI. 

T3 Test – T3 is much more powerful than T4, and most of the T3 used by the cells occurs when T4 in the cell is converted into T3. Normally, individuals who are hyperthyroid will have an elevated T3 level. In some individuals with a low TSH, only the T3 is elevated and the FT4 or FTI is normal.

Reverse T3 Test – Many of us experience stress and find that we have problems with our adrenal glands producing too much cortisol.  In times of stress, cortisol is released and this affects the conversion of T4 to T3 which results in the creation of what is called reverse T3, a biologically inactive form of T3.  It does not have the characteristics of T3 but it will bind to the T3 receptors in the cells and prevent T3 from activating the receptors.  In fact, some doctors now believe that the T3/reverse T3 ratio is currently one of the best indicators of hypothyroidism.  
 
Thyroid Antibody Tests – Rather than attacking foreign substances like bacteria and viruses, autoimmune disorders cause the body to produce antibodies that attack the organs of the body.  Antibodies that attack the thyroid can either stimulate the activities of the thyroid or they can damage the thyroid gland.  Thyroid peroxidase and thyroglobulin are the two common antibodies that cause thyroid problems.  In cases where the other tests are not conclusive, measuring the levels of these two thyroid antibodies may help diagnose the cause of thyroid problems. 
 
Radioactive Iodine Uptake – T4 contains iodine.  This means that the thyroid gland must absorb iodine from the bloodstream to enable it to make the appropriate amount of T4.  In this test, the individual swallows a small amount of radioactive iodine. The radioactive iodine can be tracked and the amount of the radioactive iodine taken into the thyroid gland can be measured.  This measure is called radioactive iodine uptake (“RAIU”).  A very high RAIU level is associated with hyperthyroidism and a low RAIU is associated with hypothyroidism.