Free Thyroxine (FT4) and Free Triiodothyronine (FT3) Estimate Tests

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Introduction

Thyroxine in blood is more tightly bound to serum proteins than is T3, consequently the free T4 (FT4) bioavailable fraction is less than free T3 (0.02% versus 0.2%, FT4 versus FT3, respectively). Unfortunately, the physical techniques used for separating the minute free hormone fractions from the predominant protein-bound moieties are technically demanding, inconvenient to use and relatively expensive for routine clinical laboratory use. Those methods that employ physical separation of free from bound hormone (i.e. equilibrium dialysis, ultrafiltration and gel filtration) are typically only available in reference laboratories. Routine clinical laboratories typically use a variety of free hormone tests that estimate the free hormone concentration in the presence of protein-bound hormone. These free hormone estimate tests employ either a two-test strategy to calculate a free hormone "index" [see Section-3 B2] or a variety of ligand assay approaches.[14,145,147] In reality, despite manufacturers claims, most if not all FT4 and FT3 estimate tests are binding-protein dependent to some extent.[148,149] This binding protein dependence negatively impacts the diagnostic accuracy of the free hormone methods that are subject to a variety of interference's that can cause misinterpretation or inappropriately abnormal results ( Table 1 ). Such interferences include sensitivity to abnormal binding proteins, in-vivo or in-vitro effects of various drugs [Section-3 B3(c)vi], high FFA levels and endogenous or exogenous inhibitors of hormone binding to proteins that are present in certain pathological conditions.[60]

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