Subacute thyroiditis (SAT) is a self-limiting, inflammatory thyroid disease possibly caused by viral infection. In recent years, the incidence of SAT is increasing, especially during the pandemic of the COVID-19. This study aimed to evaluate the efficacy, safety, and recovery time of capsular thyroid injection therapy under ultrasound guidance for SAT.A total of 73 patients with SAT were divided into two groups. Patients in group A (n = 48) received an ultrasound-guided capsular injection consisting of dexamethasone (DEX) and lidocaine in the thyroid lesion area, while patients in group B (n = 25) received oral prednisolone (PSL). The two groups were compared for pain relief and treatment duration, the recovery time of thyroid function, recurrence rates, hypothyroidism incidence, and drug-related side effects.The follow-up time was 1 year. In group A, the duration of pain relief, treatment, and recovery time of thyroid function were significantly shorter than that in group B (P < .05), and no statistically significant differences in recurrence rate or incidence of hypothyroidism were observed (P > .05). Weight gain was significantly higher in group A at the end of treatment (P < .001).Compared with oral PSL treatment, ultrasound-guided local injection of DEX and lidocaine into the capsular thyroid is a safe and effective procedure that can significantly reduce the treatment time of SAT.