Persistent elevation of procalcitonin following resolution of presumed community-acquired pneumonia revealing sporadic medullary thyroid carcinoma.

Procalcitonin (PCT) is widely utilized as a biomarker of bacterial infection and sepsis and is expected to decline following successful treatment. Persistent elevation in the absence of ongoing infection should prompt consideration of alternative diagnoses. Medullary thyroid carcinoma (MTC), a rare neuroendocrine malignancy arising from parafollicular C-cells, is increasingly recognized as a non-infectious cause of elevated PCT. A 67-year-old woman presented with palpitations, exertional dyspnea, and dizziness. Initial investigations demonstrated a markedly elevated PCT level of 11.7 ng/mL and chest radiographic findings suggestive of community-acquired pneumonia. She responded well to antibiotic therapy; however, serial monitoring over seven months revealed persistently elevated PCT ranging from 9.5-12.9 ng/mL, despite normal C-reactive protein and no identifiable infectious focus. Computed tomography identified an incidental thyroid nodule, and a dedicated thyroid ultrasound demonstrated a Thyroid Imaging Reporting and Data System (TI-RADS) 4 lesion measuring approximately 1.6 cm. Fine needle aspiration cytology was Bethesda category V, suspicious for MTC. Serum calcitonin was markedly elevated at 899 pg/mL. The patient underwent total thyroidectomy with bilateral central neck dissection and histopathological examination confirmed a 1.8 cm unifocal MTC (pT1bN0) with lymphatic invasion and minimal extrathyroidal extension. Postoperatively, calcitonin decreased to 71.4 pg/mL. This case highlights that persistent elevation of PCT following apparent resolution of infection served as the clinical trigger for further diagnostic evaluation, ultimately revealing occult MTC. Clinicians should investigate persistent biomarker abnormalities when clinical and biochemical trajectories diverge, as this may facilitate early diagnosis of an underlying malignancy.