Background: The completeness of REarranged during Transfection (RET) testing in patients with medullary thyroid carcinoma (MTC) was recently reported as 60%. However, the completeness on a population level is unknown. Similarly, it is unknown if the first Danish guidelines from 2002, recommending RET testing in all MTC patients, improved completeness in Denmark. We conducted a nationwide retrospective cohort study aiming to evaluate the completeness of RET testing in the Danish MTC cohort. Additionally, we aimed to assess the completeness before and after publication of the first Danish guidelines and characterize MTC patients who had not been tested. Methods: The study included 200 patients identified from the nationwide Danish MTC cohort 1997–2013. To identify RET tested MTC patients before December 31, 2014, the MTC cohort was cross-checked with the nationwide Danish RET cohort 1994–2014. To characterize MTC patients who had not been RET tested, we reviewed their medical records and compared them with MTC patients who had been tested. Results: Completeness of RET testing in the overall MTC cohort was 87% (95% CI: 0.81–0.91; 173/200). In the adjusted MTC cohort, after excluding patients diagnosed with hereditary MTC by screening, completeness was 83% (95% CI: 0.76–0.88; 131/158). Completeness was 88% (95% CI: 0.75–0.95; 42/48) and 81% (95% CI: 0.72–0.88) (89/110) before and after publication of the first Danish guidelines, respectively. Patients not RET tested had a higher median age at diagnosis compared to those RET tested. Median time to death was shorter in those not tested relative to those tested. Conclusion: The completeness of RET testing in MTC patients in Denmark seems to be higher than reported in other cohorts. No improvement in completeness was detected after publication of the first Danish guidelines. In addition, data indicate that advanced age and low life expectancy at MTC diagnosis may serve as prognostic indicators to identify patients having a higher likelihood of missing the compulsory RET test.