Elevation of free thyroxine measurements in patients without thyrotoxicosis

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 alb...

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Bibliographic Details
Published in:Quarterly journal of medicine 1987-10, Vol.65 (246), p.863-870
Main Authors: Wood, D F, Zalin, A M, Ratcliffe, W A, Sheppard, M C
Format: Article
Language:English
Subjects:
Adult
Aged
Diagnosis, Differential
Female
Humans
Hyperthyroxinemia - blood
Hyperthyroxinemia - diagnosis
Hyperthyroxinemia - genetics
Male
Middle Aged
Predictive Value of Tests
Thyroid Function Tests
Thyroxine - blood
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Summary: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.
ISSN:0033-5622