Data search and study selection

We searched the electronic databases of PubMed/MEDLINE, EMBASE and Cochrane Library systematically (updated on December 10, 2020) with the search strategies given in Supplementary Table 2; without language and time restriction in any of these databases. Furthermore, references of retrieved studies were searched for eligible studies. Electronic archives of medical societies (Endocrine Society (https://www.endocrine.org/meetings/endo-annual-meetings; accessed December 18, 2020) and Deutsche Gesellschaft für Nuklearmedizin e.V. (https://www.nuklearmedizin.de/jahrestagungen/abstr_online2020/abstract_search.php?navId=227; accessed December 18, 2020) were also searched. Studies meeting the following inclusion criteria were included: routine calcitonin measurement in serum (with or without a pentagastrin or calcium stimulation test) was performed routinely in all included patients with nodular thyroid disease, diagnosed by palpation or ultrasonography. Exclusion criteria were: data for 2 × 2 table not provided; preoperative measurement of calcitonin in serum; calcitonin measurement as screening test, as screening is epidemiologically defined as testing in healthy people (people with thyroid nodules are not healthy); inclusion of patients with familial history of medullary thyroid carcinoma (MTC); incomplete surgery (e.g. lobectomy); case reports or case series; case–control study; duplication of a study (in case of duplication, inclusion of the study with the longest follow-up); only meeting communication, not published as full-text article.