Figure 1. Regulation of Bone Formation and Resorption. Osteoblasts are the primary bone-forming cells. Osteoblast anabolic activity is stimulated by testosterone and growth hormone and inhibited by cortisol and hyperglycemia. Osteoclasts mediate bone resorption. Thyroid hormone, parathyroid hormone, and inflammatory cytokines increase bone resorption, while estradiol inhibits osteoclast function. Osteocytes embedded within the bone matrix secrete sclerostin which inhibits osteoblast function; mechanical loading decreases sclerostin production thereby “releasing the brake” on osteoblast activity. Calcium deficiency, often as a result of vitamin D deficiency, leads to poorly mineralized bone matrix as well as secondary hyperparathyroidism.