Thyroid cysts generally do not have any clinical symptoms, but if untreated and continuously growing, there is a risk of hemorrhage within the cyst. Therefore, when a thyroid cyst becomes painful, it may be due to excessive internal pressure and bleeding inside the cyst, causing pain. At this point, it is necessary to go to the hospital emergency department for a comprehensive thyroid ultrasound to observe the size of the cyst, and to provide emergency surgical treatment or local puncture to aspirate the blood and relieve pain.