Impact of subclinical hypothyroidism on outcomes after coronary artery bypass grafting: a systematic review and meta-analysis

INTRODUCTION: Coronary artery bypass grafting (CABG) is a surgical procedure that restores blood flow to heart muscle by bypassing the blocked or narrowed coronary arteries. On the other hand, subclinical hypothyroidism (SCH) is characterized by an elevated serum concentration of thyroid stimulating hormone with normal levels of serum free thyroxine. With limited research into the impact of SCH on postoperative CABG outcomes, this systematic review and meta-analysis was performed. EVIDENCE ACQUISITION: An electronic search of PubMed, Cochrane Library, and Scopus was performed from inception to April 2023. After the inclusion of five studies, a total of 2,786 patients were pooled in this quantitative synthesis. EVIDENCE SYNTHESIS: It was observed that SCH significantly increased cardiovascular mortality (OR: 2.80; 95% CI: 1.37, 5.72; P=0.005), and all-cause mortality (OR: 2.62; 95% CI: 1.80, 3.80; P<0.00001). However, no significant differences were observed for secondary outcomes, including major adverse cardiac events, incidence of postoperative stroke, and incidence of postoperative myocardial infarction. CONCLUSIONS: To the best of our knowledge, this is the first meta-analysis conducted that evaluates the impact of SCH on outcomes after CABG. The preoperative assessment of thyroid function may be considered prior to cardiovascular procedures, particularly within CABG. However, future comprehensive studies, with individual participant-level data, are necessary in order to arrive at a valid conclusion and recommendation.