Objectives: This study aimed to examine possible associations between previously undiagnosed subclinical hypothyroidism and short-term outcomes and mortality in a sample of Iraqi patients undergoing primary percutaneous coro-nary intervention for ST-segment elevation myocardial infarction.Design: This is a prospective observational cohort study.Setting: The study was conducted in a single tertiary referral centre in Baghdad, Iraq.Participants: Thyroid-stimulating hormone and free T4 levels were measured in 257 patients hospitalised with ST-elevation myocardial infarction who underwent primary percutaneous coronary intervention between January 2020 and March 2022.Main outcome measures: Adverse cardiovascular and renal events during hospitalisation and 30-day mortality were observed.Results: Previously undiagnosed subclinical hypothyroidism was detected in 36/257 (14%) ST-elevation myocardial infarction patients and observed more commonly in females than males. Patients with subclinical hypothyroidism had significantly worse short-term outcomes, including higher rates of suboptimal TIMI Flow (< III) (p =0.014), left ventricular ejection fraction ≤ 40% (p=0.035), Killip class >I (p=0.042), cardiogenic shock (p =0.016), cardiac arrest in the hospital (p= 0.01), and acute kidney injury (p= 0.044). Additionally, 30-day mortality was significantly higher in patients with subclinical hypothyroidism (p= 0.029).Conclusion: Subclinical hypothyroidism previously undiagnosed and untreated had a significant association with adverse short-term outcomes and higher short-term mortality within 30 days compared to euthyroid patients undergoing primary percutaneous coronary intervention. Routine thyroid function testing during these patients' hospitalisation may be warranted.