Intraoperative Parathyroid Hormone Monitoring

Introduction

Parathyroid hormone (PTH) is secreted from the parathyroid glands and is primarily responsible for calcium homeostasis. The mechanism by which this occurs is through renal tubular reabsorption, bone resorption, and formation of calcitriol from calcidiol in the renal tubular cells. The hormone is synthesized as an inactive polypeptide chain, which is then cleaved at the N-terminal in the parathyroid glands to an active form. The half-life of the hormone ranges from 2 to 4 minutes, making it useful for serologic analysis by rapid immunoassay following excision of an abnormally functioning gland.

Primary hyperparathyroidism is a disorder of the parathyroid glands in which excess PTH is secreted. This most commonly results from a single parathyroid adenoma (80% to 85%) or less frequently from four-gland hyperplasia (10% to 15%) or double adenoma (4% to 6%). Rarely, primary hyperparathyroidism can be caused by parathyroid carcinoma (0.1% to 1%). Historically, primary hyperparathyroidism has been treated with a four-gland exploration with removal of abnormal-appearing parathyroid tissue and biopsy verification of normal parathyroid tissue in the case of adenoma versus three and a half gland excision in cases of hyperplasia. However, bilateral neck exploration has a higher incidence of temporary and permanent hypocalcemia, which may lead to longer hospital stays and increased morbidity from symptomatic hypocalcemia. With the advent of improved imaging techniques and radioguided parathyroidectomy, minimally invasive techniques have decreased the morbidity associated with this condition and led to overall better outcomes.

Preoperative imaging with a sestamibi scan has excellent sensitivity in the case of a single parathyroid adenoma. However, in patients who have underlying four-gland hyperplasia or double adenoma, sensitivity decreases significantly to 30% to 44%. This decreased sensitivity may lead to treatment failure and subject patients to an additional surgery. Intraoperative PTH monitoring is a valuable adjunct in the approach to minimally invasive parathyroidectomy. It allows for the rapid detection of PTH prior to and following excision of abnormal-appearing parathyroid tissue. This decreases the need for bilateral neck exploration as well as the overall risk of postoperative hypocalcemia, hematoma, and recurrent laryngeal nerve injury; it also shortens the operative time. Intraoperative PTH monitoring is a rapid assay performed at three intervals throughout the surgery: before incision and at 10 and 15 minutes after excision. If the PTH level drops at least 50% from the highest preincision or preexcision levels, the procedure can safely be concluded without further neck exploration ( Table 83.1 ).

In addition to providing immediate feedback during parathyroid surgery, intraoperative PTH monitoring has been used to predict the likelihood of hypocalcemia following total thyroidectomy. Hypocalcemia is the most frequent complication following total thyroidectomy, with a reported incidence of 20% to 50%. Debilitating and permanent hypocalcemia, though less common, can still occur in close to 2% of patients. Historically patients are admitted to the hospital following total thyroidectomy for the monitoring of hypocalcemia, since symptoms can occur within 24 to 48 hours and can be life threatening. Research in the use of PTH levels following total thyroidectomy is ongoing and a consensus on the timing of blood draw and appropriate levels for discharge has not been clearly elucidated. In general, PTH assay results drawn within 6 hours after total thyroidectomy have been found to be the most reliable. Cutoff values for PTH that accurately predict hypocalcemia unfortunately do not exist. Studies have shown that patients who went on to develop hypocalcemia had 70% to 80% declines in PTH as opposed to 20% to 30% declines in patients who remained normocalcemic.

Key Operative Learning Points

• 1.

  Intraoperative PTH monitoring is a valuable adjunct in minimally invasive parathyroidectomy, especially in the setting of negative preoperative imaging studies.

• 2.

  A blood sample can be easily and rapidly collected from the internal jugular vein, peripheral vein, or an arterial line.

• 3.

  PTH should be sampled at three separate points: before incision and at 10 and 15 minutes following excision. A greater than 50% decrease in the highest PTH value following excision correlates with complete removal of abnormal parathyroid tissue.

Preoperative Period