A fluid cyst in the thyroid gland found during a thyroid ultrasound is called a thyroid cyst, which is also an indication of hypothyroidism. The causes of thyroid cysts are many, such as low or high iodine intake, hormonal drugs, and micronutrient deficiencies. Iodine intake is an important cause of thyroid cysts. If iodine intake decreases, the concentration of thyroxine in the blood will decrease, causing the anterior pituitary gland to secrete more thyrotropin through neurohumoral regulation, resulting in cysts or enlargement of the thyroid gland. Drinking water with too much iodine for a long time can also lead to the development of thyroid cysts. Women have a high metabolism during breastfeeding, so they may have excessive thyroid secretion for a long time, which is also a cause of thyroid cysts. Some hyperthyroid patients use hormonal drugs such as lithium carbonate and thioborate in high doses when treating hyperthyroidism, which can also lead to thyroid cysts. In addition, thyroid cysts can occur when the body is deficient in trace elements, such as magnesium and fluorine. Thyroid cysts are caused by a variety of factors, so individuals should develop good habits in their daily lives and control their iodine intake in order to prevent this disease from occurring.