A recombinant soluble human thyroid hormone receptor, comprising an extracellular domain moiety of a human thyroid hormone receptor, or a mutant thereof, being secretory, and having reactivity with an anti-human thyroid stimulating hormone receptor autoantibody; a composition for assaying an anti-human thyroid stimulating hormone receptor antibody, comprising the receptor and a carrier or diluent; a method for assaying an anti-human thyroid stimulating hormone receptor antibody, comprising reacting an anti-human thyroid stimulating hormone receptor antibody with the receptor; and a process for producing a recombinant soluble human thyroid hormone receptor which is secretory and has reactivity with an anti-human thyroid stimulating hormone receptor autoantibody, comprising infecting an insect cell with a recombinant baculovirus introduced with an extracellular domain moiety of a gene encoding a human thyroid hormone receptor or a mutant thereof, and culturing the infected cell.