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Q1: What is the differential diagnosis?
Laboratory errors must be considered, especially because clinical and laboratory findings are not obviously compatible. Site localisation of a blood sample may determine the parathyroid hormone concentration in patients who have undergone autotransplantation of parathyroid tissue to the forearm. Otherwise, potential differential diagnosis of the normocalcaemic hyperparathyroidism such as pseudohypoparathyroidism and secondary hyperparathyroidism is unlikely in this patient because of known normal biochemical tests taken previously. Primary hyperparathyroidism of transplanted parathyroid tissue seems to be unlikely because of normocalcaemia and normophosphataemia.
Site localisation of the taken blood sample in patients who have undergone autotransplantation of parathyroid tissue to the forearm, may influence the parathyroid hormone concentration. This can be used to determine the biochemical function of transplanted tissue or to prove additional parathyroid tissue (that is, ectopic parathyroid tissue) postoperatively.
Blood samples from patients who have undergone autotransplantation of parathyroid tissue to their forearm should be routinely taken from their unoperated arm.
Q2: What is your assessment?
The site from which the blood sample was taken should be verified first. The blood sample in the described patient was obtained from the right arm, where parathyroid tissue was implanted two years before. A blood sample obtained from the left arm, revealed the laboratory results shown in table 1 above.
The increase in the parathyroid hormone concentration in the described patient was caused by taking a blood sample from the arm on which autotransplantation of parathyroid tissue was performed two years before. To prevent unnecessary diagnostic procedures, blood sampling of patients who have undergone transplantation of parathyroid tissue to the forearm should be performed as a matter of routine on the unoperated arm.
Otherwise, the function of transplanted parathyroid glands (that is, in transitory hypoparathyroidism directly after transplantation) can be determined, when the blood sample is taken proximal of the transplanted parathyroid tissue. Moreover, through compression of blood flow for a few minutes proximal of transplanted parathyroid gland, parathyroid hormone secreted of transplanted tissue is not measured when the blood sample is taken from the opposite arm (circulating half life of parathyroid hormone: 2–4 minutes). This can be helpful in the evaluation of persistent hyperparathyroidism postoperatively to prove biochemically an additional parathyroid hormone source (that is, an ectopic parathyroid gland).