# Small Incidental Pheochromocytoma Presenting With Normal or Borderline High 24-hour Urine Fractionated Metanephrines

## TL;DR

A 73-year-old man with a small adrenal tumor had normal urine metanephrine levels but elevated plasma metanephrine, leading to successful surgery for pheochromocytoma.

## Contribution

Highlights that plasma metanephrine testing is more reliable than urine tests for diagnosing small pheochromocytomas.

## Key findings

- Plasma metanephrine measured by LC-MS/MS was 3.3-fold elevated despite normal urine metanephrine.
- Postoperative normalization of plasma metanephrine and normetanephrine confirmed successful pheochromocytoma resection.
- Elevated plasma metanephrine by 2-fold or more under optimal sampling confirms pheochromocytoma, even with normal urine tests.

## Abstract

A 73-year-old man was found to have a 2-cm lipid-poor right adrenal incidentaloma on computed tomography imaging for hematuria. Twenty-four-hour urine metanephrine was 1.1-fold elevated, then normal on repeat measurement. Paired with the second urine collection, plasma metanephrine measured by liquid chromatography tandem mass spectrometry after a 30-minute supine rest was 3.3-fold elevated. Plasma normetanephrine was 1.2-fold elevated. The 24-hour urine catecholamines and normetanephrine, measured twice, were normal. He received low-dose phenoxybenzamine and underwent successful resection of right pheochromocytoma. Postoperatively, both plasma metanephrine and normetanephrine levels normalized, using an age-appropriate upper reference limit for plasma normetanephrine. Patients who harbor small lipid-poor adrenal incidentalomas have a relatively high risk (>5%) of having pheochromocytoma, indistinguishable from adenomas or carcinomas on computed tomography scan. In such cases when 24-hour urine fractionated metanephrines are normal, plasma free metanephrines measured by liquid chromatography tandem mass spectrometry under optimal sampling conditions that are 2-fold or more elevated confirm the diagnosis of pheochromocytoma. Preoperative alpha blockade followed by surgical resection is then appropriate, rather than continued monitoring with repeat urine measurements.