Context Thyroid hormones are responsible for different metabolic processes. Hormonal imbalance is a suggested risk factor for metabolic syndrome (MS) development. Aim We aimed to explore the frequency of undiagnosed thyroid dysfunction (TD) among patients with MS in medical ICU and its relation to each component of MS, morbidity, and mortality. Settings and design We carried out a cross-sectional cohort study on patients who were admitted to medical ICU of Zagazig University Hospitals. Patients and methods Of 941 medical ICU patients admitted over 6 months, 106 patients had MS. We measured thyroid stimulating hormone (TSH), free T3, and free T4 for those patients within 48 h of admission and acute physiology and chronic health evaluation II score calculation within 24 h of admission, and then we followed them up for short-term in-hospital mortality. Results The frequency of TD in patients with MS was seen in 32 of 106 patients (90.6% of them were hypothyroid). We found an increased relative risk (RR) of 1.92-fold owing to the female sex. Logistic regression analysis revealed that obesity, hypertension, and hypercholesterolemia were the factors most associated with TD (P<0.001), with an increased RR by 1.25-, 1.45-, and 1.37-fold, respectively. Acute physiology and chronic health evaluation II score and mortality RR increased by 1.05 and 1.26-fold, respectively. Conclusion MS in women may carry a high risk for TD, especially hypothyroidism, with poor medical ICU prognosis.