Endocrine Abstracts

Introduction: Immunoassays are important tools in the diagnosis and management of disease; however, they are not free from interference by cross-reacting substances. Discordant clinical evidence and laboratory results raised suspicion of interference in a patient with persistently raised parathyroid hormone.

Case Presentation: Case description: A 56-year-old Caucasian female previously diagnosed with hypothyroidism consistently demonstrated elevated PTH levels with normal renal function, calcium and vitamin D concentrations. There was no clinical evidence of hyperparathyroidism and imaging of thyroid and parathyroid showed no evidence of pathology.

Results: The PTH levels were persistently elevated as measured by 2 different methods. (Roche and Abbott). PTH concentrations increased on dilution (base sample 28.5 pmol/l, 1:2 dilution 47 pmol/l). After treatment of the sample with polyethylene glycol (PEG) the PTH concentration decreased to 10.2 pmol/l. On dilution of the PEG treated sample, the diluted concentration was 12.1 pmol/l, demonstrating an appropriate response to dilution. These results indicated the presence of a Macro-PTH.

Conclusion: Macro-PTH is PTH bound to an immunoglobulin molecule, which prevents its clearance by the kidneys and thus increased half-life. These molecules are usually not biologically active. Interference should be sought when there is lack of clinical correlation with immunoassay hormone levels.