Misleading serum free thyroxine results during low molecular weight heparin treatment.

Measured free thyroxine concentrations in serum increase markedly after intravenous heparin administration, but the effect of heparin administered subcutaneously has not been adequately documented. We found in vitro increases of up to 63% in measured FT4 after a single dose of subcutaneous heparin (enoxaparin, 2000 units) in nine healthy volunteers, and the magnitude of these increases was correlated with initial serum 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 receiving 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 estimation is absolutely necessary in patients receiving enoxaparin, specimens should be taken > or =10 h postdose and analyzed within 24 h.