PTH and hypervitaminosis A: In addition, hypervitaminosis A leads to the weakening and loss of bone through several mechanisms including direct effects of vitamin A on bone cells by suppressing the osteoblast activity and stimulating osteoclast formation [19,30] causing bone loss through reduction of radial bone growth [22], suppression of expression of PTH in vivo and its reduction in serum [23,31] and increased level of oxidative stress [24,31].