PTH and secondary hyperparathyroidism: For evaluation of PTH resistance and to differentiate PTH resistance from normocalcaemic hyperparathyroidism, renal failure, vitamin D deficiency and any form of secondary hyperparathyroidism, the following laboratory tests should be performed; ionized calcium, total calcium, phosphate, magnesium, PTH, vitamin D (25-hydroxyvitamin D), creatinine, urinary calcium and urinary phosphate excretion.