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 unb ound (free) thyroxine (T-4) in the plasma, particularly when measured by equilibrium dialysis. The lipase released into the plasma by hepari n acts on substrate (triglycerides; TG) in the plasma in vitro to rele ase nonesterified (free) fatty acids (FFA), which, in high concentrati ons, inhibit the binding of T-4 to its plasma binding proteins. This a rtifact occurs only in the presence of sufficient substrate (serum TG greater than approximately 180 mg/dL), and is most pronounced in metho ds requiring long incubation times. We observed this artifact in a pat ient receiving intralipid and subcutaneous (sc) heparin. Plasma-free T -4, when measured by equilibrium dialysis, was elevated, but was norma lized when the in vitro generation of FFA during equilibrium dialysis was prevented by prior treatment of the sample with protamine to inhib it lipoprotein lipase and with an antibody to hepatic triglyceride lip ase. This observation caused us to investigate formally whether hepari n, 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 art ifact. We gave increasing doses of heparin at weekly intervals to each of three normal volunteers and measured FFA generation in their plasm a (supplemented with 250 mg/dL triglycerides) under conditions simulat ing equilibrium dialysis. We found that, indeed, iv doses of heparin a s low as 0.08 U/kg (5.6 U in a 70-kg subject) as well as a standard do se of sc heparin (5000 U) could release significant lipase activity in to the plasma and, in the setting of sufficient substrate, cause enoug h in vitro generation of FFA to artifactually increase the serum-free T-4 concentration when measured by equilibrium dialysis. These results indicate that equilibrium dialysis may not always be the best method for assessing serum-free T-4 concentrations in hospitalized patients, and should be taken into account when interpreting previous studies de monstrating inhibitors of T-4-serum protein binding in sera from hospi talized patients.