Both in vivo and in vitro studies show that cathepsin K protein is predominantly found within the thyroid follicular lumen and involved in liberating thyroxine from thyroglobulin; however, research on cathepsin K in para-follicular cells is absent.[2,7,8] Thus, it will be difficult to speculate whether CTSK-deficiency or loss of function is directly affecting in the pathogenesis of MTC. Here, TG is linked to medullary thyroid gland carcinoma.