Sea lamprey (Petromyzon marinus) contain four developmentally regulated serum thyroid hormone distributor proteins

► The lamprey albumins SDS-1 and AS are thyroid hormone binding proteins. ► Lamprey serum contains at least 4 serum thyroid hormone binding proteins. ► Lamprey serum thyroid hormone binding proteins are developmentally regulated. ► Serum proteins do not cause the decline in thyroid levels at lamprey metamorphosis. ► First definitive report showing a fish albumin can bind hormone.