PTH and secondary hyperparathyroidism: Previous literature has reported increased serum levels of parathyroid hormone in subjects having low serum vitamin D levels < 20ng/ml causing decrease in calcium absorption and serum calcium concentration, which is maintained as the expense of increased parathyroid secretion, referred to as secondary hyperparathyroidism.20 Several observational studies have reported the association of serum vitamin D levels with osteoporotic fractures.