Salsalate and its metabolite salicylate are inhibitors of thyroid hormone binding to serum transport proteins and reduce serum total thyroxine (T4) and triiodothyronine (T3) in vivo. The present study is the first to examine and compare salsalate and salicylate binding to human thyroxine-binding globulin (TBG), transthyretin (TTR), albumin (ALB), and whole human serum, and displacement by salsalate and salicylate of T4 from isolated TBG, TTR, ALB. Salsalate and salicylate binding were studied by ultrafiltration at salsalate concentrations of 0.24-1.16 mM (62-300 microg/mL) and salicylate concentrations of 0.0375-2.25 mM (6.0-360 microg/mL). T4 displacement by salsalate and salicylate from TBG, TTR, and ALB was studied by equilibrium dialysis at salsalate concentrations of 0 to 1.52 mM (0-393 microg/mL) and salicylate concentrations of 0 to 60 mM (0-9600 microg/mL). In normal human serum, 95% of salsalate and 76% of salicylate were protein-bound. In the presence of isolated ALB, 89% of salsalate and 64% of salicylate were ALB-bound. Both salsalate and salicylate inhibited T4 binding to all 3 major T4 binding proteins. Both displaced proportionately more T4 from TBG compared with TTR and ALB. The principal site to which both salsalate and salicylate in serum is bound (ALB) is different from the principal site from which they displace T4 (TBG). Salsalate potency in displacing T4 from TBG and ALB was approximately 100-fold greater than salicylate potency.