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. Induced by H(2)O(2), via the HprR/HprS two-component system, and by Cu(2+), via the CusS/CusR two-component system. HprR and CusR bind to essentially the same sequence within the hiuH promoter region. Despite its homology to transthyretins, is not able to bind to the thyroid hormones thyroxine (T4) and triiodothyronine (T3). Although some crystallographic structures were found to contain zinc, the purified protein does not contain bound metal ions, and does not require zinc or other metal ions for enzyme activity. HIU hydrolysis also occurs spontaneously, but more slowly. Belongs to the transthyretin family. 5-hydroxyisourate hydrolase subfamily.