Endocrine Abstracts

Parathyroid hormone is a polypeptide consisting of 84 amino acids. Quantification of circulating intact PTH assists in the differential diagnosis of hypercalcemia. It is important to interpret PTH results with caution and to keep in mind Immunoassays interference can also be a reason of falsely high PTH levels. We present an interesting case referred with high parathyroid hormone levels. A 74 years old female attended clinic with raised PTH levels of 11.7 pmol/l, a normal calcium of 2.29 mmol/l, Vitamin D levels of 60 nmol/l and normal kidney functions. She had a family history of Hyperparathyroidism. We repeated her blood tests including Pituitary functions to exclude MEN syndrome, 24 hour urine calcium creatinine ratio, Plasma metanephrines and normetanephrines. All of these results were normal. An US of parathyroid was arranged which showed no parathyroid adenoma. Her Parathyroid hormones were persistently high as shown below: As no obvious cause of high PTH was established, we decided to rule out immunoassay interference. Subsequently her serum sample was sent to another Laboratory using Roche intact PTH assay. Interestingly her PTH was found to be normal at 5.9 pmol/l. This lady had dogs and likely interference was due to Heterophilic antibodies in human serum which can react with reagent immunoglobulins, interfering with in vitro immunoassays.

Conclusion: Our case report highlights the importance of thinking out of the box when High PTH is not fitting in the picture of hyperparathyroidism. Do think about immunoassay interference especially in patients routinely exposed to animals. This will also prevent unnecessary investigations.