Free thyroid hormone measurement - A critical appraisal

Whether assessed on theoretical or clinical criteria, there is currently no method of free T-4 estimation that is universally valid. When the concentration of TBG is abnormal, each of numerous free T-4 methods gives results that are more sensitive and specific than measurement of total T-4. Assay artefacts, either preanalytical or analytical, arise when albumin binding of assay tracer is abnormal, in the presence of medications that displace T-4 from TBG, during critical illness, and in pregnancy. Thyroid function should be assessed from the free T-4-TSH relationship, rather than from estimates of free T-4 alone. Anomalous free T-4 values can be evaluated by considering possible methodologic limitations and by correlation with the total T-4 concentration.