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Thyroid status is most reliably assessed from the levels of free thyroid hormone since levels of total thyroid hormone are influenced by the level of thyroxine binding globulin and may lead to erroneous diagnoses of hyper- or hypothyroidism.
Analogue immunoassay is the most widely used method but has the disadvantage that the analogue - a labelled derivative of T3 or T4 - variably binds to albumin. Other methods which are not influenced by albumin and which are becoming increasingly available are two-step immunoassays and labelled monoclonal antibody techniques.