Thyroid tests in people who are taking quetiapine.

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Quetiapine is a commonly-prescribed drug with a good safety margin that is prescribed in psychotic illnesses, agitation and anxiety. Thyroid function is commonly checked in patients with psychotic disorders and signs of anxiety. We have observed that some patients taking quetiapine have low results on measure of free thyroid hormone level in blood, even though there is no other evidence that their thyroid function is impaired. We suspect this is because the quetiapine is interfering with the laboratory test, but it might actually be a biological effect of the quetiapine and potentially important to quetiapine’s adverse effects on metabolism. In this project, we will first ask 20 patients who are already taking quetiapine to provide us with one specimen of blood each to check thyroid function. In a subsequent part of the study (which has a different registration on the ANZ Clinical Trials Registry), we will also ask a further 20 patients in whom their treating psychiatrist has already made the clinical decision to commence quetiapine as part of their management plan, to provide us with a specimen of blood to check thyroid function on 4 occasions (prior to quetiapine commencement, 1 day following commencement of quetiapine, and at weeks 3 and 6 ). We will firstly check to see whether our impression, that free thyroid hormone tests are reading lower in patients on quetiapine by the current test, is correct. The current test is an immunoassay. Then we will use the same blood specimens to measure free thyroid hormone level by another newer, highly specific test (LCMSMS) that is not open to interference in the way immunoassays may be. A comparison of the results will tell us whether the problem is simply immunoassay interference from quetiapine.

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