permanent postoperative hypoparathyroidism results from the unintentional removal or injury of parathyroid glands during thyroid and parathyroid surgery .
permanent hypoparathyroidism is defined as persistent hypocalcemia requiring calcium and vitamin d supplementation 6 months after surgery .
the risk is nominal during a minimally invasive parathyroidectomy for a single adenoma but is greatest after a subtotal or total parathyroidectomy , thyroid resection and nodal dissection for large and extensive thyroid cancers , and reoperative neck operations .
even though the risk of transient hypocalcemia can be high during an extensive neck dissection , the permanent hypoparathyroidism rate is typically around 1% in the hands of experienced endocrine surgeons at high - volume centers .  
the accidental onset of permanent hypoparathyroidism can be agonizing for the patient and the clinician alike . for the patient , its negative impact includes a reduced quality of life , expensive lifelong medication supplementation , frequent laboratory testing , and the potential for frequent hospital admissions .
in addition , the persistent absence of parathyroid hormone ( pth ) has long - term systemic effects on the body , such as the development of osteoporosis ( due to the decreased function of osteocytes ) , premature cataracts , cardiac dysfunction , and neurologic dysfunction [ 1 , 2 ] .   in the late 1960s and 1970s ,
several new techniques were introduced in an attempt to prevent the detrimental health and social consequences of permanent hypoparathyroidism .
for example , intraoperative autotransplantation of parathyroid tissue into the sternocleidomastoid muscle or the brachioradialis muscle was recommended .
however , not all patients at risk of permanent hypoparathyroidism actually develop it , nor do all patients require an immediate autotransplant .
in fact , an unnecessary autotransplant , if performed during parathyroid surgery , could result in persistent hyperparathyroidism .
additionally , the autograft could become autonomously hyperfunctional , posing a diagnostic and treatment dilemma in the future ; wells et al
. overcame that limitation and transformed our approach to prevent hypoparathyroidism by introducing autotransplantation of cryopreserved parathyroid tissue .  
cryopreservation permits parathyroid tissue to be stored for potential reimplantation , thereby avoiding the risk of needlessly implanting parathyroid tissue during initial surgery .
the clinician is able to accurately determine whether any residual parathyroid tissue will recover function or whether a delayed autotransplant will be needed .
the disadvantages of cryopreservation of parathyroid tissue include the potential of graft failure and the risk of graft - dependent hypercalcemia .
this paper provides an up - to - date , comprehensive review of the cryopreservation of parathyroid tissue and its current role in thyroid and parathyroid surgery .
the clear indication for an autotransplant of cryopreserved parathyroid tissue is permanent postoperative hypoparathyroidism . in the hands of experienced endocrine surgeons ,
the risk is particularly low in patients with sporadic primary hyperparathyroidism ( phpt ) , most of whom have a single parathyroid adenoma .  
nonetheless , certain patients have a higher risk of developing permanent hypoparathyroidism after their initial neck surgery ( table 1 ) .
most commonly at risk are patients with multigland parathyroid hyperplasia , especially those with familial primary hyperparathyroidism .
such patients may undergo either a subtotal ( 3.5-gland ) parathyroidectomy or a total parathyroidectomy with an immediate autotransplant .
both a subtotal parathyroidectomy and a total parathyroidectomy can result in aparathyroidism , so cryopreservation of parathyroid tissue is recommended at the time of the initial surgery . in addition , it has been reported that the use of intraoperative parathyroid hormone ( iopth ) monitoring during parathyroid surgery can accurately predict patients at risk of developing postoperative hypocalcemia .
a drop of > 80% of iopth at 10 minutes is a significant factor for postoperative hypocalcemia .
therefore , cryopreservation of parathyroid tissue should be considered during parathyroid surgery when the iopth drop > 80% .
patients with end - stage renal disease are at high risk of developing secondary ( shpt ) and tertiary ( thpt ) hyperparathyroidism .
such patients have persistent stimulation of the parathyroid glands secondary to abnormalities in the metabolism of calcium , phosphorus , and vitamin d , resulting in multigland parathyroid hyperplasia .
they have an inherently high risk of disease recurrence if a subtotal parathyroidectomy is performed but a high risk of permanent hypoparathyroidism if a total parathyroidectomy and an immediate autotransplant are performed .
moreover , such patients may require multiple operations ; to prevent aparathyroidism as a consequence of the initial or subsequent operations , cryopreservation of parathyroid tissue is recommended .
the risk of aparathyroidism is nominal with only a thyroidectomy but increases with more extensive surgical resections .
routine central neck dissections for thyroid cancer have a complication rate of permanent hypoparathyroidism of up to 14% .
given such risks , the role of prophylactic central neck dissections for papillary thyroid cancer continues to be debated .
an immediate intraoperative autotransplant is preferred during a neck dissection for thyroid cancer , yet cryopreservation of parathyroid tissue fragments is also warranted given the multiple operations that may be required in the future .
subsequent operations increase the risk of permanent aparathyroidism , an outcome that cryopreservation of parathyroid tissue may prevent .
in contrast to initial neck operations , the risk of permanent hypocalcemia after a redo neck operation is not minuscule , but rather as high as 30% [ 6 , 10 ] . the risk of aparathyroidism is higher during a redo neck operation because the viability of the remaining parathyroid glands can not be adequately determined . during the initial surgery
, parathyroid glands left in situ may have unknowingly been devascularized or damaged . during a redo neck operation , removal or inadvertent injury of any remaining functional gland
this scenario can be particularly problematic in patients who develop hyperparathyroidism after previously undergoing a thyroidectomy ; during a redo neck operation , any parathyroid gland removed must be assumed to be the last viable parathyroid gland and should be autotransplanted immediately to optimize transplantation success .
a fragment of the parathyroid tissue may be cryopreserved for a possible delayed autotransplantation if future neck reoperations result in aparathyroidism .  
other common reasons for parathyroid reoperations include persistent hyperparathyroidism ( hypercalcemia < 6 months after the initial surgery ) and recurrent hyperparathyroidism ( hypercalcemia > 6 months after the initial surgery ) .
operative success for redo parathyroid surgery is < 90% and is fraught with an 18% rate of causing permanent hypoparathyroidism .
immediate autotransplantation of parathyroid tissue to prevent hypoparathyroidism impedes determination of surgical outcome , which is unpredictable during redo parathyroid surgery . accurately assessing the number of parathyroid glands previously resected , or the viability of the remaining parathyroid glands , is difficult .
cryopreservation of parathyroid tissue , rather than an immediate autotransplant , allows the surgeon to appropriately predict surgical outcome , the functionality of any remaining parathyroid tissue , and the need for a delayed parathyroid autotransplant .
operative failure and disease recurrence are infrequent in patients with sporadic phpt ; reoperations are more commonly needed in those with shpt and thpt .
up to 15% of patients with shpt and thpt tend to have more than four parathyroid glands , increasing the risk of operative failure .
cryopreservation of their parathyroid tissue is especially important , because the long - term renal effect of prolonged shpt and thpt predisposes patients to transient hypocalcemia from bone hunger . before any parathyroid autotransplant , it is critical to differentiate transient from permanent hypoparathyroidism
. similarly , patients with thyroid cancer who require repeating operations are at risk of hypoparathyroidism , because their remaining parathyroid gland function can not properly be assessed . furthermore ,
if parathyroid glands are immediately autotransplanted into the neck muscles during the initial thyroid surgery , the function of those glands can not be properly evaluated . during reoperations for thyroid cancer ,
a portion of any inadvertently removed or devascularized parathyroid glands should be sent for cryopreservation before any immediate autotransplant .
patients who develop aparathyroidism during subsequent operations might later benefit from a delayed autotransplant of the cryopreserved parathyroid remnant .
is dissected into 30 to 40 pieces of 1  1  1  mm .
the supernatant is decanted ; about 10 tissue fragments are transferred into each sterile freezing vial to be resuspended in the freezing media .
several media have been proposed in the literature since the initial report by wells et al . .
the typical freezing medium contains roswell park memorial institute ( rpmi ) 1640 solution [ 2 , 4 , 6 , 10 , 1316 ] .
most institutions use an 80% rpmi 1640 solution [ 2 , 4 , 10 , 13 ] , but some use either a 60% rpmi solution 1640   or no rpmi 1640 solution .   in addition , some authors recommend supplementing the rpmi 1640 solution with 2  mm of glutamine [ 11 , 16 ] and 5  g / ml of penicillin streptomycin   or 50  g / ml of gentamicin .
dimethyl sulfoxide ( dmso ) in a 10% concentration is added as a cytoplasmic stabilizer [ 2 , 4 , 10 , 11 , 15 ] .
other reported concentrations of dmso range from 7.5% to 20% [ 16 , 18 ] .
the last component of the storage medium is either a 10% to 30% autologous serum [ 2 , 10 , 11 , 15 , 16 ] or a 10% to 20% fetal bovine serum [ 4 , 11 ] .
when the rpmi 1640 solution is excluded , a 90% fetal bovine serum is used .  
the goal of the freezing process is to preserve cellular function by maintaining cellular integrity through the temperature change . to accomplish that goal ,
a cooling technique developed at the mayo clinic entails placing the vials in an ice chest filled with dry ice prechilled to 55c to 60c for 1 hour to allow cooling by 1c per minute .
other authors recommend placing the vials in a 60c ethyl alcohol bath   or in either a 70c   or a 80c   freezer overnight .
the vials can also be placed in a programmable freezer and cooled by 1c per minute until a temperature of 80c is reached .
once the vials are cooled , they are transferred into a liquid nitrogen freezer and stored at any of several recommended temperatures : 170c [ 15 , 16 ] , 180c [ 4 , 17 ] , 190c , or 196c .
the vials containing the parathyroid tissue designated for delayed autotransplantation are removed from the liquid nitrogen freezer and placed in a warm water bath .
the vials are shaken at 37c [ 11 , 17 ] to 42c   until the parathyroid tissue fragments are thawed .
the fragments are washed in rpmi 1640 solution at 37c three times [ 10 , 11 ] .
other authors recommend rinsing the fragments five times in 1  ml of rpmi 1640 solution with a 20% autologous serum .
the rpmi wash is performed to rinse the dmso , which is toxic to cells at room temperature .
the patient 's nondominant forearm is chosen for the parathyroid tissue reimplantation under local anesthesia .
dissection is continued until a flexor muscle , preferably the brachioradialis muscle , is exposed .
one to three parathyroid graft fragments are transplanted into separate muscle pockets . to maximize the chances of graft function ,
care must be taken to not cause an intramuscular hematoma , which can compromise graft function .
functionality is determined not only clinically but also biochemically ( by sampling blood from the grafted and nongrafted antecubital veins , to determine the pth level at both sites ) .
the parathyroid graft is reported as fully functional , partially functional , or nonfunctional ( table 2 ) .  
clinically , the graft is considered fully functional when the patient remains eucalcemic and asymptomatic  after all the calcium and vitamin d supplementation has been discontinued .
a graft is also considered fully functional when the pth ratio between the grafted and nongrafted arms is greater than 1.5again , after all the calcium and vitamin d supplementation has been discontinued .
the graft is considered partially functional when the patient continues to require calcium supplementation , with or without vitamin d supplementation , with a pth ratio of greater than 1.5 .  
the graft is considered nonfunctional when the patient is hypocalcemic and requires calcium supplementation , with or without vitamin d supplementation , with a pth ratio of less than 1.5 . in renal patients ,
the functionality of the parathyroid graft is determined by their pth levels , independent of calcium and vitamin d supplementation ( since most renal patients require supplementation ) ( table 2 ) .
the potential benefits of cryopreservation are limited by the reduced functionality of cryopreserved grafts , as compared with immediately autotransplanted grafts . cryopreserved grafts retain functionality in < 70% of patients ; fresh autografts > 90% [ 2 , 15 ] .
the cryopreservation process may induce cellular necrosis and impair cellular viability and , ultimately , cellular function .
earlier studies found no difference in cell viability and secretory capacity between fresh and cryopreserved parathyroid tissue grafts [ 6 , 19 ] .
yet other studies demonstrated that , even though the percentage of viable cells did not necessarily differ between fresh and cryopreserved tissue , cryopreservation decreased the total number of live cells by > 70% .
that effect on live cell yield was the same whether the parathyroid tissue was frozen as tissue fragments or as dispersed cells .  
recently , other authors reported that the cryopreservation process decreased total cell viability   and that decreased cell viability was associated with increased storage time .
both viability and function were drastically reduced with a storage time of 22 months [ 15 , 17 ] . to counterbalance the effects of cellular necrosis
, some authors routinely perform histologic examination of the cryopreserved tissue and autotransplant parathyroid tissue according to the percentage of necrotic cells .
in addition , cryopreserved parathyroid tissue should be utilized as soon as it is deemed necessary , since a longer storage time limits delayed autotransplant success .
the devastating effect of permanent aparathyroidism has been , for the most part , ameliorated by parathyroid gland cryopreservation and delayed autotransplants .
an immediate autotransplant is preferred during thyroid surgery , yet cryopreservation of a portion of the parathyroid tissue can also greatly help patients at high risk of undergoing further surgery .
cryopreservation allows the clinician to make appropriate decisions regarding the status of the remaining parathyroid glands .
such enhanced decision making is important because most patients undergoing parathyroid and thyroid surgery experience only transient hypocalcemia .
differentiating between transient and permanent hypocalcemia is critical , especially after a parathyroidectomy when hypocalcemia may result from bone hunger , rather than insufficiency of pth .
cryopreservation facilitates appropriate surgical and clinical decisions , prevents unnecessary immediate parathyroid autotransplants , and offers a chance to cure aparathyroidism .
 the risk of permanent hypoparathyroidism following thyroid and parathyroid surgery is around 1% in the hands of experienced endocrine surgeons .   
 although this complication is rare , rendering a patient permanently aparathyroid has significant consequences on the health and quality of life of the patient . 
 immediate autotransplantation of parathyroid glands that are injured or unintentionally removed offers the best possibility of graft viability and functionality . 
 however , since the majority of cases of hypoparathyroidism are transient , immediate autotransplantation can complicate postoperative surveillance in certain patients , especially those with primary hyperparathyroidism .   
 cryopreservation of parathyroid tissue is an alternate technique that was developed to treat patients with permanent hypoparathyroidism . 
 this method allows for parathyroid tissue to be stored and then autotransplanted in a delayed fashion once permanent hypoparathyroidism is confirmed . 
 this article provides a contemporary review on cryopreservation of parathyroid tissue and its current role in thyroid and parathyroid surgery . 
1. Introduction
2. Indications for Cryopreservation
3. Cryopreservation Techniques
4. Autotransplant Techniques
5. Functionality Testing Posttransplant
6. Effects of Cryopreservation
7. Conclusion