1 History report The patient, male, 65 years old, was admitted to the hospital on October 7, 2008 after finding a right neck mass for 4 years and hoarseness of voice for 3 weeks. On admission, he had difficulty swallowing, foreign body sensation, choking and coughing with water, no pain and discomfort, no symptoms such as panic and shortness of breath, excessive sweating, irritability, wasting and weakness, no urinary stones, bone pain and other symptoms. He was admitted to the hospital for physical examination: a tough mass of about 6cm×5cm×4cm in size was palpable in front of the right neck, smooth, with clear borders, and could move up and down with swallowing. Blood T3, T4 and Tsh were normal, and blood calcium and phosphorus were normal. Ultrasound: 6cm×5cm×4cm liquid dark area in the lower pole of the right lobe of the thyroid gland with intact envelope and clear borders, and small pieces of normal thyroid echogenicity in the upper pole. CT (Figure 1) suggested a thyroid cyst. Clinical diagnosis: thyroid cyst. Intraoperative findings: the thyroid gland was normal bilaterally. A cystic mass of approximately 6 cm × 5 cm × 4 cm in size was seen behind the inferior pole of the right thyroid gland, with a smooth, thin, translucent wall containing aqueous cystic fluid, which was clear and transparent. Intraoperative diagnosis: lymphatic duct hydatid cyst. Postoperative pathological examination: fibrous connective tissue cyst wall lined with a single layer of cuboidal epithelium, the outer layer of the cyst wall was vasodilated and congested, and parathyroid tissue was visible in some areas of the cyst wall. A parathyroid cyst was considered. The postoperative treatment with sedative antibiotics resulted in good recovery and relief of hoarseness. 2 Discussion Parathyroid cysts are rare clinically, accounting for 0.6% of all thyroid and parathyroid disorders. 1880 Ivor sandstrom first discovered it in autopsy, 1905 Goris reported the first clinical case and treated it surgically, so far about 250 cases were reported in the world literature [1] and 140 cases were reported in China. The disease predominates in women, with a male to female incidence ratio of about 1:2.5.[2] There are two theories on the formation of parathyroid cysts, one that several microscopic cysts fuse with each other and the other that a storage cyst occurs in the remnants of the III pharyngeal bursa (Kursteiner’s duct) during embryonic life. They occur bilaterally in the posterior inferior thyroid gland, more commonly on the left side, and occasionally in the mediastinum. Parathyroid cysts are classified as functional or non-functional depending on whether they are associated with hypercalcemia. Non-functional patients account for the majority of patients and often have no obvious clinical symptoms. During physical examination, cystic masses can be detected near the lower pole of the thyroid gland. Depending on the size and location of the mass, symptoms of mass compression such as dysphagia, dyspnea, hoarseness, etc. may occasionally occur. Patients with functional parathyroid cysts often have skeletal, urinary, and digestive symptoms of hyperparathyroidism. Parathyroid cysts are highly misdiagnosed preoperatively. Due to the close anatomical relationship between the thyroid and parathyroid glands, it is often difficult to differentiate them from thyroid tumors on physical examination, ultrasound, nuclear or CT scan and serological examination. The preoperative misdiagnosis rate of this disease is almost 100%. Fine-needle aspiration has diagnostic and therapeutic roles and provides a basis for diagnosis if colorless or yellow clear aqueous fluid is drawn, parathyroid cells are examined by puncture fluid cytology or parathyroid hormone in the puncture fluid is higher than plasma levels [3]. Pathological examination is of diagnostic significance: parathyroid cysts have paper-thin walls lined with a single layer of cuboidal epithelium and nested parathyroid principal cells distributed between the cyst walls. Once a parathyroid cyst is diagnosed, the preferred treatment is surgical excision. Parathyroid cysts have an intact envelope that can be easily separated and removed. Most parathyroid cysts are located in the lower posterior part of the thyroid gland, close to the intersection of the recurrent laryngeal nerve and the inferior thyroid artery, and should be treated to avoid accidental injury to the recurrent laryngeal nerve.[4] In this case, the patient had significant symptoms of recurrent laryngeal nerve compression, which resolved immediately after surgery. There are no reports of parathyroid cyst malignancy. It has been suggested that fine-needle aspiration or sclerotherapy injections such as tetracycline can cure most nonfunctional parathyroid cysts. However, in cases of recurrence after several punctures or with symptoms of pressure, surgical removal is necessary. It is also important to note that sclerosing agents injected outside the cyst can cause fibrous degeneration and involve the recurrent laryngeal nerve, posing a risk of vocal cord paralysis. In functional parathyroid cysts, puncture and aspiration are not recommended. There are reports of hyperparathyroidism crisis after puncture and aspiration, which is caused by the sudden direct entry of PYH secreted by the parathyroid cells in the cyst wall into the blood circulation through the surrounding abundant capillaries, and also by the reabsorption of PTH into the blood in the cystic cavity. Therefore, surgical resection is the only proven method for functional parathyroid cysts [5].