Accuracy of free thyroxine measurements across natural ranges of thyroxinebinding to serum proteins

Systemic inaccuracies, proportional to the concentrations of serum proteins and the thyroxine (T-4) they carry, have been reported in direct free T-4 immunoassays. However, analytical recoveries of free T-4 have not been care fully examined in most current methods, and they have not previously been e xamined across the pathophysiological range of serum T-4 binding. In the pr esent study we investigated ranges of serum T-4 binding using free and tota l T-4 measurements from 1359 individuals. Carefully characterized, gravimet rically calibrated, serum-based free T-4 test solutions were then prepared with a constant normal free T-4 concentration (12 ng/L) and varied serum T- 4 binding (approximately 300:1 to 24,000:1, ng protein bound T-4: ng free T -4) These standardized test solutions were analyzed using five T-4 analog b ased free T-4 methods. Analytical recoveries were calculated as ratios of a ctual free T-4 measurements to the target value, and expressed as a percent of the target. Analytical recoveries were directly proportional to the ext ent of serum T-4 binding and ranged 2% to 155%, 25% to 131%, 53% to 106%, 3 7% to 93%, and 37% to 73%, lowest to highest, in different methods. These s ystemic inaccuracies will confound interpretations of free T-4 test results in clinical conditions with altered T-4 binding. Future investigations int o free T-4 status must examine the analytical recovery of the free T-4 meth od(s) used, as they relate to the extent of serum T-4 binding in the clinic al condition(s) studied.