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Free T3 values were variably reduced in sick sera by the seven
methods used by Sapin et al (9) (Figure 4). Free T3 estimates
were lowest in patients with the most reduced total T4 and total
T3 concentrations by all methods (9), presumably reflecting
the greater severity of illness. Free T3 values determined by
tracer or direct equilibrium dialysis, tracer ultrafiltration, and direct
Sephadex LH- 20 method in sick sera did not correlate with total T3
concentrations, and were reduced in all studies (1,9,19,21,23,26) except
three (22,27,28).
Faber et al. (22,27) used minimally diluted sera but did not demonstrate that radioactivity in ultrafiltrates was T3 rather than labeled contaminants; mean free T3 values in sick sera were 104% and 92% of normal. The direct equilibrium dialysis method of Chopra et al. (26,28) required addition of unlabelled T3 to sick serum samples (exogenous to endogenous T3 concentration ratio of 4:1), to allow detectable levels of T3 to be measured in dialysate. Using this method, mean free T3 estimates varied from 46% to 96% of normal in sick sera in two different reports (26,28). Surks et al (19) avoided the most obvious problems, by using undiluted sera at pH 7.4 and 37 degrees Celsius, and identified T3 in ultrafiltrates by paper chromatography, and observed free T3 values in critically ill patients to be modestly reduced to 80% of normal values. Indeed, free T3 availability to tissue sites of action may be normal in nonthyroidal disorders as indicated by normal T3 nuclear receptor proteins and normal thyroid hormone-regulated mRNAs (14). |
