Secondary causes of an elevated PTH—such as renal compromise (creatinine clearance of less than 60 mL/min), 25-hydroxyvitamin D insufficiency (less than 30 ng/mL or 80 nmol/L), primary hypercalciuria, malabsorption syndromes, and medications such as lithium or thiazides—must be excluded. The gene discussed is PTH; the disease is malabsorption syndrome.