Abstract:The case of a 53‐year‐old man with familial hypoparathyroidism in the presence of circulating immunoreactive PTH is discussed. The patient responded to exogenous PTH by an increase in urinary cAMP excretion and by several post cyclase parameters including an increase in serum calcium and 1,25‐dihydroxyvitamin D, an increase in urinary phosphate excretion and a decrease in urinary calcium. Immunoreactive PTH was detected in this patient's serum by three separate anti‐PTH antisera. This immunoreactive PTH behaved aberrantly with these antisera. Nonparallelism to the standard curve was seen in two radioimmunoassays and the material was detected by an antiserum which preferentially binds bovine PTH. No circulating PTH binding activity was detectable. Family studies confirmed the genetic nature of the abnormality. HPLC studies revealed the presence of an abnormally hydrophobic fraction containing immunoreactive PTH. We believe these findings constitute strong evidence for the presence of an abnormal PTH molecule with reduced biological activity resulting in hypoparathyroidism. (Aust NZ J Med 1989; 19: