Background Iodinated contrast media (ICM) is a source of excess iodine which may induce thyroid dysfunction. We conducted a prospective cohort study to assess the effects of ICM on urinary iodine clearance and serum thyroid function tests (TFTs) in adults. Methods In this prospective cohort study of 54 adults undergoing elective CT scans at an academic medical center, serial urinary iodine concentrations (UIC) and serum TFTs were obtained until UIC normalized following ICM administration. Thyroid volume/nodularity were assessed by ultrasound. Associations between covariates and peak UIC, duration for UIC to peak and normalize, and thyroid dysfunction risk were assessed. Results Mean±SD iodine administered was 34.6±6.0g. Baseline median(range) UIC was 105.1(17.0-866.1)µg/L and serum thyroid-stimulating hormone (TSH) concentration was 1.26(0.5-11.2)mIU/L. Mean±SD time to achieve peak UIC [median(range): 3,519(233-157,500)µg/L] and normalized UIC were 1.1±0.5 and 5.2±4.0 weeks, respectively. Four subjects had elevated baseline TSH and one had missing baseline TSH values; of the remaining 49 subjects, 11(22%) developed an abnormal TSH within 1-4 weeks (six elevated and five decreased). Administered iodine amount correlated with peak UIC following ICM administration (p<0.001). Increasing age and administered iodine amount predicted peak UIC (p=0.024 and p<0.001, respectively). Age, gender, race, smoking status, family history of thyroid disease, personal or family history of thyroid autoimmunity, thyroid volume, presence of thyroid nodules ≥1cm, iodine dose, baseline UIC, and baseline TFTs were not predictive of durations to achieve peak or normalized UIC. Conclusion Peak UIC occurred at 1.1 weeks and normalized by 5.2 weeks following ICM administration for outpatient CT scans. Because thyroid dysfunction developed in 22% individuals following a single ICM dose, monitoring of thyroid function should be considered in at-risk patients.