Biochemical Testing in Hypoparathyroidism: Perioperative Calcium And PTH

• Biochemical Testing: Perioperative Calcium And PTH
  • The ability to predict the occurrence of transient hypoPT:
    • By subjective surgeon assessment during operation:
      • Is highly variable
  • Objective serial measurement of serum calcium levels:
    • Has traditionally been used to stratify for risk of symptomatic hypocalcemia development during the postoperative period
    • And to determine the need for oral calcium and calcitriol administration
  • After thyroidectomy:
    • The absolute value of serum calcium levels
    • As well as the trend and slope or rate of change of these values:
      • Have also been employed to predict hypoPT:
        • But this approach requires sufficient time to observe such a trend
  • In one study:
    • Serum calcium levels were measured at 6 and 12 hours after total thyroidectomy
    • Patients with a positive serum calcium slope (rise in level):
      • Were deemed safe for hospital discharge with or without calcium supplementation
    • For patients with a non-positive slope but with serum calcium levels equal or greater than 8 mg/dL:
      • Discharge with calcium supplementation was also found to be safe
  • Because trending calcium often requires sampling over a 12- to 24-hour or longer time period, and because postoperative calcium levels may be confounded by prophylactic calcium and calcitriol administration or by low preoperative vitamin D levels:
    • Many groups have examined the utility of measuring intraoperative or postoperative intact PTH levels drawn at various time points in the early post-thyroidectomy period (Table) 

• Intraoperative PTH (IOPTH) refers to:
  • Rapid processing of blood specimens drawn during or shortly after thyroid or central neck surgery to determine PTH levels that will influence surgical or postoperative management
  • At many institutions, the IOPTH assay provides a rapid, actionable result:
    • Whereas the routine intact PTH assay may not have a turnaround time that is fast enough for perioperative decision making
  • It is the normal short half-life of PTH (3–5 minutes):
    • That enables decisions based on early postoperative PTH levels
  • The timing of PTH measurements in published studies has ranged from 10 minutes to 24 hours post thyroidectomy:
    • McLeod et al:
      • Found that a low recovery room drawn postoperative PTH level (< 12 pg/mL) predicted the development of hypocalcemia:
        • With a sensitivity of 100% and a specificity of 92%
    • Sywak et al:
      • Reported that a low four-hour postoperative PTH level (3 to 10 pg/mL) had a sensitivity of 90% and a specificity of 84% for predicting post-operative hypocalcemia 
    • Asari et al:
      • Reported that an intact PTH level of less than 15 pg/mL on postoperative day 1 predicted hypoPT, with a sensitivity of 97.7% and a specificity of 82.6%
  • Thus, the earliest opportunity to predict hypoPT reliably is:
    • Through measurement of serum PTH level either at the conclusion of or immediately following surgery
  • A PTH level < 15 pg/mL:
    • Is usually predictive of impending hypocalcemia
  • IOPTH levels measured alone or combined with serum calcium level measurements:
    • May guide the decision to:
      • Discharge versus monitor patients in the hospital setting
      • To prescribe prophylactic oral calcium
      • To take more aggressive measures to prevent or treat hypocalcemia
  • Based on the available evidence, and acknowledging that reference ranges differ based on assay and institution:
    • It is the opinion of the ATA Surgical Affairs Committee:
      • That a PTH value equal or greater than 15 pg/mL measured in adults at equal or greater than 20 minutes following thyroidectomy:
        • Would obviate the need for intensive serum calcium monitoring and/or calcium supplementation
  • A postoperative PTH value of  less than 15 pg/mL:
    • Would suggest an increased risk for acute hypoPT that might prompt preemptive prescribing of oral calcium and calcitriol and/or serial serum calcium measurements until calcium stability has been confirmed
  • The ability to predict transient as opposed to permanent hypoPT following thyroidectomy is uncertain:
    • At least one study found that acute postoperative hypoPT specifically did not predict permanent hypoPT
    • Several parameters, including postoperative PTH values, decline in PTH values between preoperative and postoperative measurements, degree of decrease in magnesium levels, and whether PA was carried out:
      • Have been associated with the risk of permanent hypoPT:
        • However, the utilization of these parameters is not consistently established and requires further investigation

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