the pathologic causes of hyperparathyroidism can be divided into single adenoma of the parathyroid ( 89% ) , multiple hyperplasia of the parathyroid ( 6% ) , and parathyroid carcinoma ( 0.1 to 4% ) . the diagnosis of parathyroid carcinoma is made by first referring to serum calcium or intact parathyroid hormone ( ipth ) levels , then locating the enlarged parathyroid with ultrasonography , scintigraphy , or computed tomography and confirming the parathyroid tumor with fine needle aspiration biopsy . however , if asymptomatic , most parathyroid tumors are adhered to the thyroid gland , and therefore , making the distinction between parathyroid and thyroid tumor is difficult under ultrasonography , and the differential diagnosis is also difficult with cytologic findings alone without clinical signs .   stated that vesicle formation and presence of colloid or macrophages suggests thyroid lesions . however , studies on numerous parathyroid lesions have reported that the amount of colloid , vesicular structures , and presence or absence of macrophages , do not greatly help in distinguishing thyroid tumors from parathyroid tumors . for this reason , the criteria for diagnosis of parathyroid carcinoma include 1 ) direct invasion into neighboring organs , such as thyroid gland , neck muscles , nerve , and esophagus , 2 ) distant metastasis to lung or liver , 3 ) cervical lymph node metastasis , and 4 ) histological findings including fibrosis , trabecular growth , necrosis , and nuclear atypia ( pleomorphism , large nuclei , macronucleoli , mitosis ) . the recurrence of parathyroid carcinoma occurs within 2 to 3 years after removal , but the disease - free survival period is known to vary greatly from case to case . therefore , various studies have attempted subgrouping to classify high - risk and low - risk categories , histologically to reflect variable prognosis of the carcinoma . among such studies , the study by bondeson et al .   argues that macronucleoli , necrosis , and mitotic activity in excess of 5 per 50 high - power fields reflect bad prognosis . in addition , a report by kameyama and takami   argues that the criteria suggested by bondeson et al . did not reflect the patients ' prognosisappropriately and classified parathyroid carcinoma into either minimally invasive or widely invasive carcinoma , similar to the classification of follicular carcinoma . serum calcium levels typically returns to normal within 24 to 48 hours after excision of the parathyroid gland due to primary hyperparathyroidism , but hypocalcemia occurs in 10 to 30% of the cases . this is known to be due to the time required for normal parathyroid glands to recover their sensitivity to calcium after surgery , which takes several days . however , persistent hypocalcemia requires differential diagnosis of hypoparathyroidism due to surgery , concomitant hypomagnesemia , and hungry bone syndrome . in other words , after the excision of the parathyroid gland due to primary hyperparathyrodism of any cause , serum ipth levels normalize and serum calcium and phosphate rapidly migrate to the bone , leading to hypocalcemia . this syndrome is known as hungry bone syndrome , known to occur in about 12.6% of operations on the parathyroid gland . we experienced 1 case of minimally invasive parathyroid cancer accompanied by postoperative hungry bone syndrome , and attempt to report this case along with a review of related literatures . a 29-year - old female without any notable past history visited for further evaluation of elevated alkaline phosphatase ( alp ) and a mass was found at the lower right thyroid gland during a regular checkup . a relatively firm mass of about 3  2 cm was found at the right anterior portion of the neck upon examination . the mass was movable , and there was no palpable lymph node . a clinical laboratory study performed before the operation resulted in the following : blood urea nitrogen 6.3 mg / dl , creatinine 0.8 mg / dl , serum calcium 11.1 mg / dl , phosphorous 2.1 mg / dl , alp 1,072 u / l , and ipth 1,476 pg / dl . a round , hypoechogenic mass sized about 2  1.7  3 cm with well - defined margin separate from the right lower thyroid gland was observed under ultrasonography , and accompanied by signs of increased blood flow ( fig . some lymph nodes appearing to be positive reactive changes were observed inferior to the mass at level vi of right neck . fine needle aspiration revealed hurthle cell change of cell clusters , implying the possibility of hurthle cell neoplasm of the thyroid gland or a parathyroid neoplasm . the immunohistochemistry resultsshowed ipth and the clinical pattern confirmed the suspicion of parathyroid tumor , and therefore , a diagnosis of parathyroid tumor was made ( fig . sincecytologic study alone was unable to determine the benign or malignant nature of the tumor , a need for a frozen section of the specimen upon surgical excision was suggested . imaging studies showed enhancement upon contrast injection , making the determination of the exact nature of the tumor more difficult ( fig . the right lower parathyroid gland did not show any direct invasion to neighboring organs on surgical examination , and a round shaped mass of about 3  2 cm with a smooth capsule easily separable from the thyroid gland and the surrounding muscles was found . a right lower parathyroidectomy was performed , and an excisional biopsy of the surrounding lymph nodes was also performed simultaneously for frozen - sectionevaluation . the pathologic result of the frozen section reported high possibility of benign parathyroidoma , and examination of the excised lymph nodes revealed reactive hyperplasia . therefore , no further operation was performed . the serum ipth level immediately after the excision was 199.7 pg / dl , which became 18.54 pg / dl 1 hour later and 43.84 pg / dl 3 hours later . positive chevostek sign was observed , but the patient did not show any spasmodic muscle contraction . serum calcium , phosphorous , and ipth at this period was measured to be 7.2 mg / dl , 1.9 mg / dl , and 102.5 pg / ml , respectively . intravenous calcium chloride solution and 4 tablets of oral calcium agent ( dicamax 1000 tab , dalim biotech , seoul , korea ) divided into 2 doses twice a day , were concomitantly administered thereafter . the serum calcium level was maintained at below normal value despite a continuous supply of calcium , and serum calcium and ipth 8 days after operation was 7.1 mg / dl and 226.1 pg / dl , respectively . dicamax1000 tab was replaced by healthcal tab ( dong wha pharm co. , seoul , korea ) , with a daily dosage of 6 tablets divided into 3 doses three times a day . the patient was discharged 18 days after operation since she did not show any particular symptoms , and laboratory study performed 174 days after surgery reported ipth at 65.73 pg / dl , alp at 172 u / l , serum calcium level at 9.1 mg / dl , and phosphorous at 4.3 mg / dl ( fig . healthcal tab was replaced by dicamax 1000 tab with daily dose of 2 tablets divided into 2 doses twice a day . the final pathologic diagnosis of the patient was minimally invasive parathyroid carcinoma , and no metastasis to surrounding lymph nodes was found ( fig . various studies describe cytologic findings that distinguish parathyroid lesions from thyroid lesions , but the distinguishing points vary from paper to paper , and are sometimes even contradictory [ 4 - 6,12 ] . in short , there is no single cytologic finding that can definitively diagnose a parathyroid lesion as of yet . there have been reports of mixing fine needle - aspirated samples of parathyroid with liquid and measuring the concentration of parathyroid hormone of the solution being helpful in making the diagnosis in such cases . in the case of this patient , the clinical signs clearly pointed toward parathyroid tumor , and the measurement of parathyroid hormone levels in fine - needle aspirated samples is thought to help in making the diagnosis in cases of asymptomatic parathyroid tumors . it is especially of major importance to distinguish hungry bone syndrome from surgical hypoparathyroidism in order to start early the appropriate treatment given for a long period . therefore , we could establishthe cause of hypocalcemia in this patient as hungry bone syndrome . hungry bone syndrome was first reported in 1948 after studying patients who showed hypocalcemia , muscle spasm , and hypophosphatemia after receiving parathyroidectomy . extensive remineralization of the bone , reflecting deposition of calcium and phosphate within the bone parenchyme , was discovered upon bone tissue biopsy , and this finding was named ' hungry bone syndrome ' . the treatment of hungry bone syndrome usually involves supplementation of inadequate minerals . intravenous or oral calcium agents are administered in cases of hypocalcemia , and intravenous or oral magnesium agents are supplied in cases of hypomagnesemia as well . this patient similarly showed hypocalcemia and hypomagnesemia , and therefore , she received supplementary minerals , and a normalized serum calcium level was achieved14 days after operation . braiser and nussbaum   suggested relatively large tumor size , blood urea nitrogen , alp , osteitisfibrosacystica , and old age as risk factors that may cause hungry bone syndrome in patients with primary hyperparathyroidism who receiveparathyroidectomy . when taking into consideration the tumor size and preoperative alp level , it is thought that the possibility of occurrence of hungry bone syndrome after operation in this patient could have been predicted beforehand . in addition , it is thought that radiographic studies , such as osteitisfibrosacystica , that observe the changes in bone related to hungry bone syndrome may have needed to be performed in more detail . mizrachi et al .   stated that of all patients with primary hyperparathyroidism due to parathyroid adenoma , 25% experience a postoperative rise of ipth and that patients with preoperative ipth greater than 225 pg / ml can be predicted to maintain high levels of ipth after operation . mittendorf and mchenry   recorded that this postoperative increase of pth is due to remineralization of the bone , and stated that most postoperative increases of ipth dropped to normal levels after an average of 16 months . since this patient recorded preoperative ipth greater than 225 pg / ml , a persistent elevation in postoperative ipth could be predicted , and a nearly normal level was recorded after about 6 months of follow - up study . the cause of primary hyperparathyroidism in this case was parathyroid carcinoma , which prompted the possibility of metastatic lesions in distant areas and caused much concern ; however , both preoperative and postoperative imaging studies revealed no abnormal findings , so close follow - up monitoring is thought to be needed . in this patient , a focal , very limited capsular invasion and invasion into one small vessel was histologically observed , which prompted the diagnosis of minimally invasive parathyroid carcinoma according to the aforementioned report by kameyama and takami . their report also stated that all of 12 patients who had no recurrence had minimally invasive parathyroid carcinoma . examination of preoperative laboratory results related to postoperative hungry bone syndrome can help in predicting the possibility of hungry bone syndrome . there is a high possibility that a postoperative increase of ipth may be caused by remineralization of the bone associated with hungry bone syndrome ; also , close follow - up monitoring , including blood tests and imaging studies , is thought to be warranted even in minimally invasive parathyroid carcinomas that are reported to show good prognosis . a case of hungry bone syndrome , accompanied by persisting hypocalcemia and elevated ipth after operation on postoperatively diagnosed minimally invasive parathyroid carcinoma has been reported as such .