PTH: An Early Predictor of Post-Thyroidectomy Hypocalcaemia

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PTH An Early Predictor of Post-Thyroidectomy
Hypocalcaemia
Lam A, Kerr P The Laryngoscope Dec
20031132196-2200

• Journal Club
• 13th September 2004
• Ben Kwok
• Mentor Janindra Warusavitarne

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Background

• Hypocalcaemia recognised complication after total
  thyroidectomy
• Proposed mechanisms include
• Ischaemia
• Direct trauma
• Haemodilution
• Reversal of negative bone and calcium balance (in
  hyperthyroid patients)

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Predicting Hypocalcaemia

• Slope of Calcium levels post-op can be predictive
  but require repeated blood tests over 24hrs
• Studies looking at PTH levels
• Intra-op (at completion of thyroidectomy or up to
  10 min after thyroidectomy)
• 1 hr post-op

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Objective of study

• Hypothesis that PTH levels as early as one hour
  after total/completion thyroidectomy will
  correlate with development of hypocalcaemia
• If correlation can be shown
• Earlier recognition of potential hypocalcaemia
  and earlier initiation of appropriate calcium
  therapy
• Shorten length of hospital stay

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Methods

• Prospective Series
• Total or completion thyroidectomy from Jan 2001
  to Oct 2002
• Excluding concurrent hyperparathyroidism those
  already taking calcium supplementation
• Clinically significant hypocalcaemia defined as
  ionised calcium (iCa) 0.9 mmol/L
• reference range 1.17-1.33 mmol/L

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Methods

• iCa measured
• pre-op and
• 1, 6, 18, 30, 42 hrs 1 week after surgery
• Intact human PTH measured at 1 and 6 hours after
  surgery
• reference range 7-50 pg/mL
• Analytical sensitivity of 2-3pg/mL
• Univariate relationships between development of
  hypocalcaemia and PTH levels were analyzed using
  t-tests.
• Plt0.05 considered significant

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Results

• 42 patients ? 2 excluded due to underlying
  hyperparathyroidism ? i.e. 40 patients included
• 12/40 (30) patients developed hypocalcaemia
• all described acral paraesthesia
• none had seizures, arrhythmias, carpopedal spasm
  or tetany
• All (12) patients with PTH 8pg/mL 1 hour after
  surgery ? hypocalcaemia
• All (28) patients with PTH 9pg/mL _at_ 1hr
  remained eucalcaemic
• i.e 100 sensitivity and specificity for
  hypocalcaemia at this threshold level of PTH

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Results

• Correlation coefficient of 0.71 b/w 1hr PTH and
  42hr iCa level
• iCa levels appear to stabilize after 6 hrs in
  patients who remain normocalcaemic
• iCa levels in patients who became hypocalcaemic
  continued to decline over the entire 42 hours
• Rate of decline in iCa is greater in
  hypocalcaemic group but not statistically
  significant until 6-18hrs after surgery

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Conclusions

• PTH value at 1hr post-op is a strong early
  predictor of hypocalcaemia after total/completion
  thyroidectomy
• Authors suggest prospective study validating
  predictive level of PTH (8pg/mL) is required
• Authors suggest that available intra-op PTH
  assays not accurate enough to precisely measure
  PTH at these low levels

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Comments

• Positive
• Simple test that could be used in clinical
  setting
• Results encouraging that PTH level _at_1hr could be
  used to identify those at risk of hypocalcaemia
  and those that could be discharged early
• Negative
• Small numbers
• Clinical hypocalcaemia defined numerically as
  0.9 mmol/L
• ?Symptoms correlate with this set level