What are Thyroid Function Tests?

Thyroid function tests are blood tests taken to determine if the thyroid gland is producing high enough quantities of certain important hormones, or if it is creating too many of these hormones. There are three main hormones associated with the thyroid, and tests may be done to indicate levels of both. The thyroid produces thyroxine, or T4, which is then converted by the liver into T3. Thyroid stimulating hormone (TSH) is the third, and it controls the rate at which thyroxine is produced. A deficiency or overabundance of any of these hormones can be indicative of a thyroid problem.

The thyroid is a small gland found in the base of the neck, and it has a characteristic butterfly shape. It is responsible for controlling or aiding in many bodily functions, including the metabolism and temperature regulation. Sometimes the thyroid becomes sluggish, as with hypothyroidism, and causes symptoms like fatigue or weight gain. Other times it becomes overactive, also called hyperthyroidism, and may cause weight loss or dizziness.

One of the first thyroid function tests which may be performed is one that measures the levels of TSH. Too much or too little of this hormone generally indicates hypothyroidism or hyperthyroidism, although this test is one of the least accurate. Many times, levels will appear normal even if one of the other hormones is lower than normal. If a patient is exhibiting all symptoms of a thyroid disorder but TSH levels appear adequate, additional testing may be performed.

Individual thyroid function tests for T4 and T3 hormones may be conducted next, as this will give a better indication of how well the thyroid is functioning. Although there are still errors in these tests, they are much more accurate than the basic TSH scan.

The primary problem with thyroid function tests is that doctors have different interpretations as to what constitutes normal. Hormone levels may be different for everyone, and one person may have a certain level and feel fine, while someone else may have the same levels and exhibit symptoms of a disorder. For this reason, many doctors will prescribe hormonal replacement medications even with test results within the normal range because there may be a different “normal” for every individual.

Other thyroid function tests may check for antibodies produced by the body’s immune system. In some disorders, primarily Hashimoto’s disease, the thyroid gland is attacked by the immune system as if it were an invading party. This is one form of an auto-immune disorder. Treatment for this may include hormonal replacements. Sometimes the thyroid gland will even have to be removed or destroyed, with hormone injections given to resume its functions.