The aim of the study was to investigate the influence of severe hyperthyroidism on plasma high-density lipoprotein cholesterol (HDL-C). Recently, it was shown in mice that increasing doses of triiodothyronine (T₃) upregulate hepatic expression of scavenger receptor-BI (SR-BI), resulting in increased clearance of plasma HDL-C. Here we show that severe hyperthyroidism in mice did not affect hepatic expression of SR-BI, but reduced hepatic expression of ATP-binding cassette transporter 1 (ABCA1), accompanied by a 40%-reduction of HDL-C. Sterol content of bile, liver and feces was markedly increased, accompanied by upregulation of hepatic CYP7A1, and ATP-binding cassette half-transporter ABCG5, which is known to promote biliary sterol secretion upon dimerization with ABCG8. Both control and hyperthyroid mice exerted identical plasma clearance of intravenously injected [³H] HDL-C, supporting the view that severe hyperthyroidism does not affect HDL-C clearance, but rather its formation via hepatic ABCA1.