MISLEADING SERUM-FREE THYROXINE RESULTS DURING LOW-MOLECULAR-WEIGHT HEPARIN TREATMENT

Measured free thyroxine concentrations in serum increase markedly afte r intravenous heparin administration, but the effect of heparin admini stered subcutaneously has not been adequately documented. We found in vitro increases of up to 63% in measured FT4 after a single dose of su bcutaneous heparin (enoxaparin, 2000 units) in nine healthy volunteers , and the magnitude of these increases was correlated with initial ser um triglyceride concentrations (r = 0.93, P <0.005) and in vitro free fatty acid release (r = 0.88, P <0.005). In 10 cardiac inpatients rece iving repeated doses of enoxaparin (2000 units twice daily), measured FT4 increased by up to 171% in specimens taken 2-6 h after injection. When specimens were obtained 10 h after injection, the effect appeared to be minimized, with in vitro increases of <40%, but such increases may still be sufficient to cause interpretative errors. If FT4 estimat ion is absolutely necessary in patients receiving enoxaparin, specimen s should be taken greater than or equal to 10 h postdose and analyzed within 24 h.