Summary
As T3 thyrotoxicosis may be difficult to be assessed in the low T3 syndrome and in
advanced age, the matrix influence on different T3 parameters was evaluated in identical
serum specimens of euthyroid non critically ill patients aged up to 95. The values
obtained by the conventional TT3 RIA and the FT3 analog single step method with known
albumin interference showed a strong relation to age (r = 0.4877 and r = 0.6929, respectively;
both p < 0.001). Free T3 measurement by a labeled monoclonal antibody technique was
independent of age effects (r = 0.0324, p = 0.748). Upon analyzing the binding parameters,
albumin decreased with age and correlated closely with the age dependent T3 parameters
(TT3: r = 0.6610, apparent FT3 by analog: r = 0.7664; both p < 0.001). In identical
specimens, FT3 obtained by labeled monoclonal antibody is not influenced by albumin
changes (r = 0.1986, p = 0.063). Consequently, TT3 cannot exactly predict the thyrometabolic
status without corrections regarding binding protein variation including albumin,
which is the underlying cause of the age dependence. The FT3 assay using labeled monoclonal
antibody represents a precise and convenient technique and gives reliable results
independent of changes of binding protein.
Key words
Albumin - age factors - binding proteins - low T3 syndrome - triiodothyronine
