Could temporary hypocalcaemia following thyroidectomy be predicted?
A. H. Sam, W. S. Dhillo, M. Donaldson, A. Moolla, K. Meeran, N. S.
Tolley, F. F. Palazzo
Case 1: A 41-year-old woman underwent total thyroidectomy for Gravesí disease. Her serum calcium and phosphate on the evening of surgery were 2.08 mmol/L and 1.38 mmol/L respectively. Her serum calcium and phosphate on day 1 post-surgery were 1.98 mmol/L and 1.27 mmol/L respectively. She had no clinical features of hypocalcaemia. Her serum calcium was 2.1 mmol/L when recheck on day 2. She was discharged without any treatment.
Case 2: A 64-year-old woman underwent total thyroidectomy for a retrosternal symptomatic goitre. Her serum calcium and phosphate on the evening of surgery were 2.35 mmol/L and 1.65 mmol/L respectively. Her serum calcium and phosphate on day 1 post-surgery were 2.38 mmol/L and 1.86 mmol/L respectively. She was readmitted on day 3 post-surgery with tetany and required calcium and alfacalcidol replacement.
Question: Could we have predicted the development of hypocalcaemia in case 2?
Background: Temporary hypocalcaemia occurs in up to 40% of patients following a total thyroidectomy. Serum calcium and parathyroid hormone (PTH) measurements are currently used to predict post-thyroidectomy hypocalcaemia. However, immediate access to PTH measurement is expensive and not widely available. Serum phosphate responds rapidly to changes in circulating PTH levels and its measurement is readily available in all hospitals. We evaluated the use of serum phosphate to predict temporary hypocalcaemia post-thyroidectomy.
Methods: We retrospectively assessed 111 consecutive patients who had total or completion thyroidectomy. Patients had serum calcium and phosphate measured pre-operatively, on the evening of surgery (day 0), on the morning of day 1 and over the following week as clinically indicated. Vitamin D levels were measured pre-operatively.
Results: Seventy-six patients did not develop treatment-demanding hypocalcaemia. In these patients, the mean serum phosphate concentration was lower on the morning of day 1 compared to that on the evening of surgery. Seventeen patients with a vitamin D > 25 nmol/l developed hypocalcaemia requiring treatment from day 1 onwards. All had an overnight rise in serum phosphate to > 1.44 mmol/l (100% sensitivity and specificity for predicting hypocalcaemia). Twelve patients who had a vitamin D < 25 nmol/l also developed hypocalcaemia but had an attenuated rise in serum phosphate.
Conclusion: Serum phosphate is a reliable biochemical predictor of post-thyroidectomy hypocalcaemia in patients without vitamin D deficiency. Use of serum phosphate may facilitate safe day 1 discharge of patients undergoing thyroidectomy.