Endocrine Abstracts

Introduction: High parathyroid hormone (PTH) usually associated with parathyroid disorder, but sometimes the elevation is false due to assay interference, such as macro-PTH or heterophile antibodies. In macro-PTH, PTH binds to immunoglobulins, slowing its clearance from the blood. Although these complexes are typically biologically inactive, they can produce misleading laboratory results, complicating both diagnosis and clinical management.

Case presentation: A 35-year-old woman was referred for evaluation of markedly elevated PTH, ranging from 20.2 to 25.6 pmol/l, despite mostly normal serum calcium, which was mildly elevated only once at 2.61 mmol/l. Vitamin D was 69 nmol/l, and eGFR was 85 mL/min/1.73m². She reported non-specific symptoms including episodic weakness, tremors, muscle and bone pain, polydipsia, neck swelling and intermittent hoarseness. Renal ultrasound revealed no calculi, while thyroid ultrasound identified a benign U2 nodule. Twenty-four-hour urinary calcium excretion was 7.32 mmol/24h (reference 2.5–7.5), and DEXA scanning demonstrated osteopenia. Given the discordance between PTH and calcium levels, assay interference was suspected. Polyethylene glycol (PEG) precipitation was performed, revealing a monomeric PTH of 2.97 pmol/l with a low recovery of 24%. To confirm test specificity, a parallel PEG assay was performed on a patient with elevated PTH due to confirmed parathyroid pathology, yielding 76% recovery. Subsequent testing using an alternative assay produced a normal PTH of 22.4 ng/l (reference 15–68).

Results: The initial elevated PTH readings were found to be falsely raised due to immunoassay interference. PEG precipitation and confirmatory testing with an alternative assay demonstrated normal PTH levels, consistent with the presence of macro-PTH and effectively excluding hyperparathyroidism.

Conclusion: Discordantly elevated PTH with normal biochemical and imaging findings should prompt evaluation for assay interference, including macro-PTH. PEG precipitation offers a useful confirmatory approach. Integrating clinical and laboratory input is essential to prevent diagnostic error and unnecessary investigations.