RESUMEN
BACKGROUND: We aimed to assess determinants for serum calcitonin (CT) levels and to define reference ranges for different CT immunoassays integrating thyroid ultrasonography (TUS) and conventional clinical and biochemical criteria to select the reference population. METHODS: Five hundred and nineteen thyroid-healthy subjects were included in this prospective cross-sectional population study. Thyroid volume was measured by TUS, while serum CT levels were measured by three different immunoassays. RESULTS: Significant interassay differences were found and the agreement between assays was poor. CT levels were higher in males than in females in all immunoassays. Using the first two assays, both gender and thyroid volume were independent determinants for CT levels. While using the third assay, one thyroid volume was the only determinant for CT levels. CONCLUSIONS: Thyroid volume is a relevant determinant for CT levels. In the clinical practice, the difference of the thyroid sizes in males and females warrants gender-specific reference ranges.