Abstract
The recent development of single step analogue assay techniques to measure free thyroxine in serum has meant that this estimation has become widely used as a first line test of thyroid function. However these assays are subject to in-vitro interference in two ways: where there is a variant serum albumin in familial dysalbuminaemic hyperthyroxinaemia, or where there are circulating thyroid hormone binding antibodies. In both these situations a spuriously high result for free thyroxine is obtained. This may have serious implications for patient management and we describe 10 patients in whom the finding of a misleadingly high free thyroxine concentration led to confusion in diagnosis and, in eight of the 10, to inappropriate antithyroid treatment. Clinicians should be aware of these technical problems and where a result for the free thyroxine concentration seems inappropriate to the patient's clinical state then measurement of thyrotrophin (TSH) by a sensitive immunometric method should be performed. If the results remain confusing the presence of a variant serum albumin or thyroid hormone binding antibodies should be sought.
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