Objective <p>Subacute thyroiditis (SAT) is an inflammatory disease of thyroid but it may result in permanent hypothyroidism in some patients. The aim is to explore the prognostic factors that predict the development of permanent hypothyroidism.</p> Design <p>A retrospective study was conducted in an endocrinology outpatient clinic.</p> Patients <p>A total of 144 patients with the diagnosis of SAT were included in this study.</p> <p>Measurements: Patients’ medical charts were reviewed to note demographic data, laboratory and ultrasonography (USG) findings, medication use, and permanent hypothyroidism. The relationship between these variables and the development of permanent hypothyroidism was investigated.</p> Results <p>Approximately one-quarter of patients (27 out of 144 patients, 18.8%) developed permanent hypothyroidism within a median of 3 (range, 1–12) months during a median follow-up of 12 (range, 6–60) months. The presence of bilateral involvement on thyroid ultrasound (OR, 6.4; 95% CI 1.8–22.4; P = 0.0037), TSH &gt; 5.33 mIU/L (OR, 64; 95% CI 16.9–242.4; P &lt; 0.0001) and fT4 &lt; 0.72 mIU/L (OR, 44.9; 95% CI 14.2–141.9; P &lt; 0.0001) at the third month of follow-up were significantly associated with the development of permanent hypothyroidism.</p> Conclusion <p>Patients with bilateral involvement on thyroid USG along with TSH &gt; 5.33 mIU/L and fT4 &lt; 0.72 mIU/L at the third month should be under close monitoring for the development of permanent hypothyroidism.</p>