Plasma concentrations of proteins secreted by the liver (prealbumin, haptoglobin, transferrin, ceruloplasmin, alpha 1-antitrypsin, antithrombin III, and T4-binding globulin) and proteins mainly derived from endothelium [fibronectin, angiotensin-converting enzyme (ACE), and factor VIII-related antigen (F VIII R:Ag)] were measured in 27 hyperthyroid and 30 normal women. Significantly increased plasma concentrations (P less than 0.01) of endothelium-associated proteins, including fibronectin, ACE, and F VIII R:Ag, were found in hyperthyroid patients, while levels of proteins of primarily hepatic origin were normal. To determine whether the increase in endothelium-associated proteins in hyperthyroidism was directly related to elevated thyroid hormone levels, seven normal women were given T3 (25 micrograms, three times daily) for 2 weeks. These women had a consistent rise (P less than 0.05) in plasma concentrations of fibronectin, ACE, F VIII R:Ag, and tissue plasminogen activator. The rise in endothelium-associated proteins persisted for 10 days after cessation of T3. Plasma concentrations of hepatically synthesized proteins did not change. We conclude that thyroid hormone action either promotes endothelial protein synthesis or impairs its clearance.