Simple Blood Test Measures Thyroxine Levels

The test was developed by Steven Soldin, Ph.D. FACB, professor at Georgetown University Medical Center (GUMC; Washington, DC, USA).

When the thyroid gland produces too little thyroxine, the condition is known as hypothyroidism, whereas when the gland produces too much, the condition is called hyperthyroidism. Thyroxine regulates the body's metabolism, and hypothyroidism, associated with fatigue and weight gain, is much more common than hyperthyroidism, which is characterized by weight loss.

In order to treat these conditions, physicians need to know how much synthetic thyroxine to either give patients or how much natural hormone should be blocked, and there have long been concerns that the common immunoassay test now in use worldwide is neither specific nor very accurate. To date, the immunoassay test has been used to measure those levels in women known to have abnormal levels of thyroid function based on a screening test.

In a study initiated by the Georgetown University scientists, investigators tested the method they had developed and found that it was far superior to the currently used immunoassay and just as good as an expensive, time-consuming, but very accurate laboratory analysis that is less commonly used. The scientists estimated the ability of their new test to measure thyroxine in blood donated by two groups of women: a group of 98 pregnant women and a control group of 29 women who were not pregnant. Measuring the hormone in pregnant women can be problematic because some conditions (e.g., high levels of binding proteins and certain antibodies) affect immunoassay performance.

Three different tests were used in the study--the immunoassay, the Georgetown tandem mass spectrometry, and the "gold standard” laboratory test known as equilibrium dialysis--to measure the hormone in blood samples donated by the women. They found that, across all stages of pregnancy, there was almost total agreement between mass spectrometry and equilibrium dialysis, but immunoassay results differed significantly.

Thyroxine output dramatically changes during pregnancy in order to support development of the fetus. The hormone helps control metabolism and physical development, and because a fetus does not develop its own supply until the second trimester, it is crucial that pregnant women have adequate supplies in the first trimester.

One major use of such a test would be to help guide treatment of people with hypothyroidism or hyperthyroidism. Physicians diagnose these conditions using a test that measures thyroid-stimulating hormone (TSH), a hormone released by the pituitary gland, which stimulates the thyroid gland to secrete thyroxine. When TSH is high, levels of thyroxine are low, and vice versa.

After the condition is diagnosed, doctors use the immunoassay to determine what level of thyroxine should be supplemented or repressed. "We think our treatment of hyperthyroidism and hypothyroidism would be much more accurate if we combined TSH testing with tandem mass spectrometry instead of with the immunoassay,” said one of the study's investigators, Jacqueline Jonklaas, M.D., Ph.D


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