Background: Subclinical hypothyroidism is defined as high levels of serum thyroid stimulating hormone (TSH, > 0.17 關IU/mL) associated with normal levels of thyroxine (T4, 4.6-12.5 關g/dL ) and has been known to be a potential risk factor of coronary atherosclerosis. However, the impact of subclinical hypothyroidism on the endothelial dysfunction has not been elucidated yet. Therefore, we evaluated the impact of subclinical hypothyroidism on coronary artery spasm (CAS) with the intracoronary acetylcholine (Ach) provocation test. Methods: A total 1047 consecutive patients without significant coronary artery disease who underwent the Ach provocation test by injecting incremental doses of 20, 50, 100 ug into the left coronary artery were enrolled between November 2004 and March 2009. Significant CAS was defined as focal or diffuse severe transient luminal narrowing (>70%) with /without chest pain or ST-T change on ECG. The Ach provocation test results and its associated parameters were compared between the subclinical hypothyroidism group (n=25) and control group (n=964). Results The baseline clinical and angiographic characteristics were similar between the two groups. After Ach injection, the rate of positive provocation test result and associated clinical and angiographic parameters including ischemic ST-T change, ischemic chest pain, response to lower Ach dose, extent and severity of spasm were similar between the two groups (Table). Conclusion: In our study, patients with subclinical hypothyroidism did not show any difference in the incidence of clinically significant CAS and associated parameters as assessed with Ach provocation test when compared with those of patients with euthyroid status.