Free T4 is measured as a second line test when the TSH result indicates a thyroid function disorder or if excessive thyroxine replacement is suspected. However, FT4 is also measured as a frontline assay for thyroid screening if the patient is pregnant or a paediatric.
No known common methodological interferences. The test cannot be used in patients receiving treatment with lipid-lowering agents containing D-T4. If the thyroid function is to be checked in such patients, the therapy should first be discontinued for 4-6 weeks to allow the physiological state to become re-established. Autoantibodies to thyroid hormones can interfere with the assay. Binding protein anomalies seen with FDH (familial dysalbuminemic hyperthyroxinemia), for example, may cause values which, while characteristic of the condition, deviate from the expected results.
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