PTH and Obesity: This reduction in the levels of serum calcidiol coinciding with increased adiposity is assumed to be due to enhanced sequestration of vitamin D in fat, a factor leading to decreased bioavailability.2, 3, 11, 16, 28 Consequently, lower serum calcidiol in obesity stimulates a rise in PTH, which in turn stimulates the renal hydroxylation of calcitriol and this condition elevates calcium influx into adipocytes.