Persistent Hyperprocalcitoninemia as the Initial Diagnostic Clue to Sporadic Medullary Thyroid Carcinoma

Medullary thyroid carcinoma (MTC) is a rare neuroendocrine tumour arising from parafollicular C cells, and serum calcitonin remains its most reliable biochemical marker. Procalcitonin (PCT), widely used in the evaluation of bacterial infections, is usually undetectable under non-infectious conditions. Persistent elevation of PCT in the absence of clinical or microbiological evidence of infection is uncommon and may represent an early clue to underlying endocrine malignancy. We report the case of a 48-year-old woman who presented with markedly elevated PCT levels (>20 ng/ml) detected during routine laboratory testing despite being asymptomatic and having normal inflammatory markers. Extensive infectious work-up was negative, and antibiotic therapy did not reduce PCT levels. Further endocrine evaluation revealed a calcitonin level >2000 ng/l, elevated carcinoembryonic antigen, and a suspicious thyroid nodule on ultrasonography. Fine-needle aspiration with calcitonin washout confirmed the diagnosis of MTC. The patient underwent total thyroidectomy with central neck dissection, and histopathology demonstrated a 4.5 cm sporadic MTC without nodal metastasis. Postoperative PCT and calcitonin levels declined markedly, supporting their parallel behaviour in relation to disease activity. This case highlights the clinical importance of considering MTC in patients with unexplained hyperprocalcitoninemia and supports the potential role of PCT as an adjunct biomarker in diagnostic evaluation and postoperative monitoring once infection has been excluded.

Persistent elevation of procalcitonin in the absence of infection should raise suspicion for medullary thyroid carcinoma and other neuroendocrine malignancies.Procalcitonin may serve as a useful adjunct biomarker alongside calcitonin in the diagnostic evaluation and postoperative follow-up of medullary thyroid carcinoma.Awareness of non-infectious causes of hyperprocalcitoninemia can prevent unnecessary antimicrobial therapy and reduce diagnostic delay in internal medicine practice.