Parathyroid surgery: Intraoperative PTH monitoring
What is intraoperative PTH monitoring?
While most patients with primary hyperparathyroidism have a single abnormal gland, up to 20% of patients will have more than one hyperactive gland. Pre-operative localization may not identify those cases where multiple glands are diseased. Therefore, when the surgeon plans a focused parathyroidectomyFocused Parathyroidectomy - An operation where the surgeon examines and removes just the hyperactive parathyroid gland and not all four parathyroids. (i.e. "going after" just one parathyroid gland), it is important to have a way to check that there are no other hyperactive parathyroids. Parathyroid hormone (PTH), which is produced by the parathyroid glands, is cleared from the blood stream very quickly (within a few minutes). Within 10 minutes of removing all hyperactive parathyroid tissue, the PTH levels should fall by more than half. By testing PTH levels before removing the hyperactive parathyroid glands (usually before starting the operation and before tying off the blood supply to the gland) and then again after removal of the abnormal gland the surgeon can determine if there are additional hyperfunctioning glands present. Although there are different criteria for a successful operation, most surgeons look for at least a 50% drop in the PTH levels in order to consider the patient cured. With a PTH level that drops by half or more, there is a 98% chance that the patient is cured. If the PTH levels do not drop by 50% then there is likely additional abnormal tissue and the surgeon will likely proceed to identify and evaluate each of the remaining glands. 68 It can take anywhere between 10-40 minutes to get the results back from the PTH testing. The patient is usually kept in the operating room during this time.
While many surgeons consider PTH testing an essential part of a focused parathyroidectomy, not everyone agrees. PTH testing is not 100% accurate, and in some patients it can mislead the surgeon into thinking that there are additional abnormal glands when there are not (called a false negative), or it could lead them believe that a patient is cured when they are not (called a false positive). Some have argued that in patients with both a positive ultrasound and a positive sestamibi scanSestamibi scan - Nuclear medicine test where a small amount of radioactive material is injected into a vein and an X-ray is taken. the likelihood of having a single abnormal gland is so high (around 96%),41 that the PTH test may be more likely to hurt than help. PTH testing also takes additional time in the operating room. Therefore some surgeons only use the test selectively (i.e. when the localization studies are negative or suggest different locations), or do not use it at all.