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) 
Studies of Post-Thyroidectomy Intact PTH Threshold Levels That Are Predictive of Normocalcemia or Hypocalcemia
  • 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

#Arrangoiz #ThyroidSurgeon #ParathyroidSurgeon #Hypoparathyroidism #HeadandNeckSurgeon #CancerSurgeon

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