Abnormal serum free thyroid hormone levels due to heparin administration

Fractionated or unfractionated heparin may produce artefactual elevation in measured concentrations of free thyroid hormones. Although the specific ca use is unknown, it may be a consequence of displacement of thyroid hormones from their binding sites by free fatty acids liberated in vitro. We descri be four cases of heparin-induced abnormalities in free thyroid hormone meas urements where some diagnostic confusion was generated. Increasing physicia n awareness of this poorly appreciated entity may avert diagnostic confusio n and unnecessary investigation.