Thiol protease which is believed to participate in intracellular degradation and turnover of proteins (PubMed:12220505). Cleaves matrix extracellular phosphoglycoprotein MEPE (PubMed:12220505). Involved in the solubilization of cross-linked TG/thyroglobulin in the thyroid follicle lumen (By similarity). Has also been implicated in tumor invasion and metastasis (PubMed:3972105). {ECO:0000250|UniProtKB:P10605, ECO:0000269|PubMed:12220505, ECO:0000269|PubMed:3972105}. This is the function of ENSG00000164733 (CTSB, cathepsin B).