Background: It was found that calcitonin in vitro can degrade rapidly whether in refrigeration or at room temperature, which increases the difficulty of accurate detecting of calcitonin greatly. This study was designed to solve this problem. Methods: Serum samples with increased calcitonin from patients with medullary thyroid carcinoma were collected and divided into refrigeration and room temperature group. Analyze the degradation rule of calcitonin in the two groups according to the time of blood draw (T0), the time of sample detection (Tx), and the concentration (y) measured at Tx. Results: The degradation rate of calcitonin was significantly different between the two storage conditions. The degradation has plateau stage and its concentration is associated with the original concentration. The degradation rule of the two groups can be expressed by the formula: y = -aln(x) + b. In the formula, b/a are constants associated with storage condition and gender. "x" is the time interval between blood draw and detection, that is, x = Tx - T0. Conclusions: The significance of the presence of immunoreactive isoforms and fragments of calcitonin with different stability in patients with medullary thyroid carcinoma is unclear. To make the inspection report more accurate and reliable, maybe the inspection personnel should report both the instrumental and corrected result, and also indicate the correction formula on the inspection report. (C) 2018 Elsevier Inc. All rights reserved.