Catalyzes the hydrolysis of 5-hydroxyisourate (HIU) to 2-oxo-4-hydroxy-4-carboxy-5-ureidoimidazoline (OHCU). 5-hydroxyisourate + H2O = 5-hydroxy-2-oxo-4-ureido-2,5-dihydro-1H-imidazole-5-carboxylate + H(+) Homotetramer. Despite its homology to transthyretins, is not able to bind to the thyroid hormones thyroxine (T4) and triiodothyronine (T3). HIU hydrolysis also occurs spontaneously, but more slowly. Belongs to the transthyretin family. 5-hydroxyisourate hydrolase subfamily.