We report a rare case with pheochromocytoma as the first manifestation of multiple endocrine neoplasia type 2A with RET mutation S891A. Bilateral pheochromocytomas were identified in a 54-year-old woman. Screening for RET revealed a rare S891A mutation located in the intracellular tyrosine kinase domain. This mutation was previously recognized as one of the mutations only in cases manifesting solely medullary thyroid carcinomas (MTCs). Since calcitonin stimulation test indicated positive result, total thyroidectomy was performed 1 year after the bilateral adrenalectomy, and C-cell hyperplasia was diagnosed by histopathological examination. Our report suggests that cases with S891A mutation, akin to those with other RET mutations, require screening for pheochromocytoma. In addition, it is indicated that calcitonin stimulation test should be performed even in the unaffected elder cases with S891A mutation although the mutation is classified as lowest risk group on MTC in guidelines.
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