Background: Thyrotoxicosis is characterized by excessively high tissue thyroid hormone levels. Untreated or inadequately managed thyrotoxicosis can lead to various complications. Understanding factors influencing the time to achieve thyroid function normalization is essential for improving treatment outcomes and patient care. Objectives: This study aimed to assess the median time to normalization of thyroid function tests and identify factors associated with delayed euthyroidism among thyrotoxic patients attending the Endocrine Clinic at St. Paul’s Hospital Millennium Medical College, Addis Ababa, Ethiopia, in 2023. Methods: A hospital-based retrospective cohort study was conducted involving 181 patients diagnosed with thyrotoxicosis who attended the adult Endocrine Clinic from April 2023 to January 2024. Data were collected using a structured questionnaire and entered into EpiData version 7.1, then exported to STATA version 15 for analysis. Descriptive statistics summarized continuous variables as mean ± SD or median with interquartile range, and categorical variables as frequencies and percentages. The association between independent variables and time to euthyroidism was analyzed using Cox proportional hazards regression, with the log-rank test employed to compare median times across groups. Adjusted hazard ratios (AHR) with 95% confidence intervals (CI) were reported to quantify the strength of associations. Results: Approximately 61% of patients achieved euthyroidism, with a median time of 7 months (Interquartile Range: 3–13 months). Factors significantly associated with delayed normalization included use of Propylthiouracil (AHR = 0.33; 95% CI: 0.12–0.89), poor medication adherence (AHR = 0.24; 95% CI: 0.13–0.42), higher baseline pulse rate (AHR = 0.97; 95% CI: 0.95–0.99), and elevated baseline free thyroxin levels (AHR = 0.99; 95% CI: 0.98–0.995). Conclusion: Although more than half of the patients attained euthyroidism, the process took longer than expected. High baseline FT4 levels, elevated pulse rate, non-adherence to medication, and the use of PTU as the anti-thyroid drug delayed the achievement of euthyroidism. Patients with thyrotoxicosis require attentive and continuous monitoring until thyroid function normalizes to prevent complications.