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Thyroid Function Testing: A Brief Update



Balancing It All Up - What Test to Use When, cont.

Slide 14

April 2009

Anti-TSH-receptor autoantibodies, measured either by a binding assay or a bioassay, can also be useful in some select situations. Those would include the differential diagnosis of Graves’ disease versus a thyrotoxic phase of subacute thyroiditis. Or, for example, also Graves’ disease on top of a toxic goiter. Many patients with a toxic goiter may also develop Graves’ disease. The main applications, however, are in pregnancy. First of all, a differential diagnosis between excessive vomiting of pregnancy versus first trimester Graves’ disease. Sometimes, Graves’ disease can manifest for the first time in the first trimester, this is also the time when excessive vomiting and nausea can occur and, those by themselves, through mechanisms which are not entirely clear, but may involve very high levels of HCG cross-reacting on the TSH receptor, can lead to apparent thyrotoxicosis. So here, detecting or not detecting anti-TSH receptor autoantibodies can give a definitive diagnosis. A second pregnancy-related scenario centers on the risk of a baby of a women who had Graves’ disease in the past, to be afflicted by neonatal Graves’ disease. Neonatal Graves’ disease is caused by autoantibodies that the mother still has in her system passing through the placenta to the baby causing transient, but sometimes severe, thyrotoxicosis. As most of those mothers either would have had their Graves’ disease treated, or the Graves’ disease would naturally go into seeming remission in the second trimester, it is very difficult to use a mother’s thyroid-function testing to judge whether there is circulating autoantibodies, which could affect the baby. Hence, measurement of TSH-receptor autoantibodies can be very helpful. If those are significantly elevated, either by a binding or activity assay, then this means the baby is at risk of neonatal thyrotoxicosis and expert obstetric care is required.

What Test to Use When, cont.

 


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