In a normally functioning body, Thyroxine (T4) and Triiodothyronine (T3) are hormones produced by the thyroid gland. They help control the rate at which the body uses energy and are regulated by a feedback system. Thyrotropin or Thyroid-Stimulating Hormone (TSH) from the pituitary gland stimulates the production and release of T4, (primarily,) and T3 by the thyroid. Most of the T4 and T3 circulates, bound to protein, in the blood. A small percentage is free (not free, not bound).
According to the New York Times, “Thyroid disorders…are among the most undiagnosed and misdiagnosed problems in the United States. Doctors generally don’t examine the thyroid. Disorders come on slowly, and the symptoms can be ambiguous.” Although the test itself is easily incorporated into a routine blood panel, many doctors choose not to pursue testing and this is a distinct problem, especially given the number of people suffering from thyroid disorders.
Although, the test may be routine in nature. The testing itself relies upon a number of advanced techniques. What follows is a general review of the most common forms of thyroid testing. For more specifics, please visit the American Thyroid Association.
The best way to initially test thyroid function is to measure the TSH level. A high TSH level suggests that the thyroid gland is failing because of a problem that is directly affecting the thyroid; this is called primary hypothyroidism. The opposite, in which the TSH level is low, usually indicates that the person has an overactive thyroid; this is called hyperthyroidism. Occasionally, a low TSH may result from an abnormality in the pituitary gland, which prevents it from making enough TSH to stimulate the thyroid; this is known as secondary hypothyroidism. In most healthy individuals, a normal TSH value means that the thyroid is functioning normally.
T4 circulates in the blood in two forms, as mentioned in the opening paragraph:
1) Bound to proteins that prevent it from entering the various tissues that need thyroid hormone.
2) Free, which does enter the various target tissues.
The free T4 fraction is the most important factor used to determine how the thyroid is functioning; tests to measure this are called the Free T4 (FT4) and the Free T4 Index (FT4I or FTI). People who have hyperthyroidism will have an elevated FT4 or FTI. However, patients with hypothyroidism will have a low level of FT4 or FTI.
T3 Tests T3 tests are often useful when diagnosing hyperthyroidism and when deciding the degree of the hyperthyroidism. T3 testing is underutilized, but very helpful in determining response to thyroid therapy. As well, low levels of free T3 may suggest poor conversion of T4 to T3 in the body. Many patients have normal TSH and T4 levels, but they show symptoms of hypothyroidism possibly due to low serum T3. This conversion is dependent upon the enzyme 5’ diodinase which in part is selenium dependent. Aging and high corstisol levels (stress) can also cause poor conversion from T4 to T3. As T3 is the active form, low T3 can cause symptoms related to hypothyroidism. Many of these patients benefit from inclusion of T3 as part of their thyroid hormone replacement.
Thyroid Antibody Tests
Our body’s immune system generally protects us from unwelcome visitors, such as including bacteria and viruses, by destroying them with antibodies, which are produced by blood cells known as lymphocytes. In many patients with hypothyroidism or hyperthyroidism, lymphocytes make antibodies against their thyroid. These antibodies either stimulate or damage the gland. Two common antibodies that cause thyroid problems are thyroid peroxidase and thyroglobulin. Measuring levels of thyroid antibodies may help aid in the diagnosis of thyroid issues such as Hashimoto’s Thyroiditis.
Reverse T3 Tests
Reverse T3 (RT3 or REVT3) is a biologically inactive form of T3. Normally, when the liver converts T4 to T3, it also produces a certain amount of RT3. When the body is under stress, such as during a serious illness, it tries to prevent many tissues that depend on T3 from being metabolically active by producing more RT3 than T3. This is believed to be a way of conserving energy until the stress is relieved and it causes a syndrome called non-thyroidal illness (NTI). Testing measures the amount of RT3 present in the body to determine a thyroid problem at a cellular level, or cellular hypothyroidism.
For additional details about the advanced testing techniques used by your doctor, have a conversation with him or her. Medical health professionals are happy to share their expertise and to help you feel more comfortable with testing.