A best evidence topic in cardiac surgery was written according to a structured protocol. The question addressed was: 'should cardiac surgery be delayed in patients with uncorrected hypothyroidism?' A total of 1412 papers were found using the reported search, of which 7 represented the best evidence to answer the clinical question. The authors, journal, date and country of publication, patient group studied, study type, relevant outcomes and results of these papers are tabulated. There was limited high-quality evidence with the majority of the studies being retrospective. One propensity-matched analysis and 6 cohort studies provided the evidence that there was no significant difference in the rate of major adverse cardiac events including mortality based on thyroid status. However, hypothyroidism and subclinical hypothyroidism were associated with higher rates of postoperative atrial fibrillation. Based on the available evidence, we conclude that cardiac surgery should not be delayed to allow achievement of euthyroid status.