1 The TSH response to TRH is suppressed 24 h after the commencement of i.v. heparin infusion in euthyroid subjects, as compared to repeat testing, both after 7 days of continuous i.v. heparin and after 7 days without heparin but while on warfarin. 2 This suggests that the rise in serum free thyroid hormones, which heparin produces, has a 'metabolic effect'. Possible clinical effects of this action must be considered. 3 Heparin has no effect on the radio-immunoassay of TSH.