Extremely Low Doses of Heparin Release Lipase Activity into the Plasma and Can Thereby Cause Artifactual Elevations in the Serum-Free Thyroxine Concentration as Measured by Equilibrium Dialysis

Heparin can cause an artifactual elevation in the concentration of unbound (free) thyroxine (T₄) in the plasma, particularly when measured by equilibrium dialysis. The lipase released into the plasma by heparin acts on substrate (triglycerides; TG) in the plasma in vitro to release nonesterified (free) fatty acids (FFA), which, in high concentrations, inhibit the binding of T₄ to its plasma binding proteins. This artifact occurs only in the presence of sufficient substrate (serum TG greater than approximately 180 mg/dL), and is most pronounced in methods requiring long incubation times. We observed this artifact in a patient receiving intralipid and subcutaneous (sc) heparin. Plasma-free T₄, when measured by equilibrium dialysis, was elevated, but was normalized when the in vitro generation of FFA during equilibrium dialysis was prevented by prior treatment of the sample with protamine to inhibit lipoprotein lipase and with an antibody to hepatic triglyceride lipase. This observation caused us to investigate formally whether heparin, at standard sc doses or at iv doses even lower than those that are commonly used to flush iv lines (100-300 U), could also cause this artifact. We gave increasing doses of heparin at weekly intervals to each of three normal volunteers and measured FFA generation in their plasma (supplemented with 250 mg/dL triglycerides) under conditions simulating equilibrium dialysis. We found that, indeed, iv doses of heparin as low as 0.08 U/kg (5.6 U in a 70-kg subject) as well as a standard dose of sc heparin (5000 U) could release significant lipase activity into the plasma and, in the setting of sufficient substrate, cause enough in vitro generation of FFA to artifactually increase the serum-free T₄ concentration when measured by equilibrium dialysis. These results indicate that equilibrium dialysis may not always be the best method for assessing serum-free T₄ concentrations in hospitalized patients, and should be taken into account when interpreting previous studies demonstrating inhibitors of T₄-serum protein binding in sera from hospitalized patients.