title: Elevation of free thyroxine measurements in patients without thyrotoxicosis.
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.
authors.affiliation: Department of Endocrinology, University of Birmingham, Queen Elizabeth Hospital.
authors.forename: D F
authors.initials: DF
authors.lastname: Wood
authors.forename: A M
authors.initials: AM
authors.lastname: Zalin
authors.forename: W A
authors.initials: WA
authors.lastname: Ratcliffe
authors.forename: M C
authors.initials: MC
authors.lastname: Sheppard
mesh_terms: D000328:Adult; D000368:Aged; D003937:Diagnosis, Differential; D005260:Female; D006801:Humans; D006981:Hyperthyroxinemia / Q000097:blood* / Q000175:diagnosis / Q000235:genetics; D008297:Male; D008875:Middle Aged; D011237:Predictive Value of Tests; D013960:Thyroid Function Tests; D013974:Thyroxine / Q000097:blood*
chemical_list: D013974:Thyroxine