Fig. 3.1.4.5
Summary of the heparin-induced changes that can markedly increase the apparent concentration of serum free T4. Heparin acts in vivo (left) to liberate lipoprotein lipase from vascular endothelium. Lipase acts in vitro (right) to increase the concentration of free fatty acids to levels more than 3 mmol/l, resulting in displacement of T4 and T3 from TBG. In vitro generation of free fatty acids is increased by sample storage at room temperature, or incubation at 37°C and by high concentration of serum triglyceride. The T4-displacing effect of free fatty acids is accentuated at low albumin concentrations. NEFA, nonesterified fatty acids; fT3, free T3; fT4, free T4.
