Comparison of a new ultrafiltration method for serum free T4 and free T3 with two RIA kits in eight groups of patients.

We measured serum free thyroxine (FT4) and free T3 (FT3) concentrations by a newly developed ultrafiltration (UF) method and by the Diagnostic Products Corporation (DPC) and Amersham Corporation (Amerlex) RIA methods in 173 patients divided into eight groups: 1) normal controls, II) normal elderly subjects, III) hyperthyroidism, IV) hypothyroidism, V) pregnancy, VI) critically ill, VII) chronic renal failure, and VIII) heparin therapy. The results were also compared with the free T4 index and free T3 index. FT4 (UF) and FT3 (UF) were generally normal in the elderly subjects. All methods showed elevated values in hyperthyroid patients. FT4 was normal in 7-23% of hypothyroid patients (depending on the method), and FT3 was normal in 15-27% of hypothyroid patients. Mean FT4 (UF) and FT3 (UF) were increased in pregnancy; in contrast the Amerlex RIA results were reduced in pregnancy. In critically ill patients, mean FT4 and FT3 were reduced by all methods. In heparinized patients, FT4 (UF) and FT3 (UF) were normal, but the RIA kits, especially Amerlex, gave very low values. Although the various methods gave considerably different absolute values of FT4 and FT3, there was a high degree of correlation between the ultrafiltration FT4 and FT3 and the results by the other methods.