Thiol protease involved in osteoclastic bone resorption. Displays potent endoprotease activity against fibrinogen at acid pH. May play an important role in extracellular matrix degradation (By similarity). Involved in the release of thyroid hormone thyroxine (T4) by limited proteolysis of TG/thyroglobulin in the thyroid follicle lumen (PubMed:12782676). Broad proteolytic activity. With small-molecule substrates and inhibitors, the major determinant of specificity is P2, which is preferably Leu, Met > Phe, and not Arg. Localizes to the lumen of thyroid follicles and to the apical membrane of thyroid epithelial cells. Predominantly expressed in bones (PubMed:8814310). Expressed in thyroid epithelial cells (PubMed:12782676). Expressed in new born and adults. Enlarged thyroid follicles, reduced extension of the thyroid epithelium, and slight increase in the levels of Tg/thyroglobulin in the thyroid follicles. Loss of localization of CTSB/cathepsin B and L to the apical membrane of thyroid epithelial cells. Serum levels of thyroid hormone thyroxine (T4) are normal. However, reduction in T4 levels is more severe in CTSK and CTSL double knockout mice compared to CTSL double knockout mice. Belongs to the peptidase C1 family.