Free thyroxine measured in undiluted serum by dialysis and ultrafiltration: Effects of non-thyroidal illness, and an acute load of salicylate or heparin

Abstract

In vitro dilution of serum during processing of a free T4 assay explains to some extent the divergent results obtained in non-thyroidal illness. If serum from such patients contains low affinity T4 protein binding inhibitors, as has been suggested, in vitro dilution will result in spuriously reduced serum free T4 measurements. If these inhibitors cross the dialysis membrane in an equilibrium dialysis assay, their inhibitory effect will be weakened, and in vitro free T4 levels will decrease, even in undiluted serum. In contrast, ultrafiltration methods on undiluted serum seem accurate. We have compared a new, commercially available dialysis technique with an in-house ultrafiltration method for free T4 measurements in undiluted serum. Control subjects ( n = 41) had 14% higher free T4 ( P < 0.02) by ultrafiltration. Nonthyroidally ill patients not receiving glucocorticoids or dopamine ( n = 54) had unaltered free T4 levels, 28.4 ± 10.3 pmol/l (dialysis) and 31.0 ± 10.3 pmol/l (ultrafiltration). Dopamine infusion in somatic ill patients ( n= 11) resulted in reduced free T4 in both assays but only significantly for dialysis, and subjects with familial dysalbuminemic hyperthyroxinemia ( n = 8) had unaltered free T4 levels in both assays. Salicylate (1.5 g) given orally 09:00 h. ( n = 5) resulted within 30 min. in increased ( P < 0.01) free T4 as measured by both techniques, although more pronounced and sustained as measured by ultrafiltration. Serum TSH decreased concomitantly ( P < 0.01). These findings were confirmed when salicylate was administered at 13:00 h. ( n = 8). The dialysis procedure resulted in a decrease in serum salicylate of 14% ( P < 0.001). Heparin (5,000 units) given intravenously at 09:00 h. ( n = 7) tended to increase ( P < 0.10) free T4 after 15 min. in both assays, and decreased TSH ( P < 0.001). In conclusion the two methods demonstrated qualitatively, but not always quantitatively quite similar changes in situations with (1) the possible existence of circulating low affinity T4 binding inhibitors (somatic illness), (2) pituitary suppression (dopamine), (3) protein binding capacity changes (dysalbuminemic hyperthyroxinemia), and (4) acute protein binding displacement by salicylate or heparin. The dialysis method seems therefore to be a valuable reference assay for the measurement of serum free T4, although from a theoretical point of view it seems not ideal.