Validity of early parathyroid hormone assay as a diagnostic tool for sub-total thyroidectomy related hypocalcaemia.

OBJECTIVE To determine the validity of early (one hour postoperatively) parathyroid hormone (PTH) assay (² 10 pg/ml), keeping gold standard as the serum ionic calcium level, for predicting sub-total thyroidectomy-related hypocalcaemia and to calculate the sensitivity and specificity of latent signs of tetany. STUDY DESIGN Cross-sectional validation study. PLACE AND DURATION OF STUDY Department of General Surgery, Pakistan Institute of Medical Sciences, Islamabad from August 2008 to August 2010. METHODOLOGY Patients undergoing sub-total thyroidectomy were included by convenience sampling. PTH assay was performed 1 hour post sub-total thyroidectomy. Serum calcium levels were performed at 24 and 48 hours, 5th day and 2 weeks after surgery. Cases that developed hypocalcaemia were followed-up for a period of 6 months with monthly calcium level estimation to identify cases of permanent hypocalcaemia. Symptoms and signs of hypocalcaemia manifesting in our patients were recorded. Data was analyzed through SPSS version 10. 2 x 2 tables were used to calculate sensitivity and specificity of PTH in detecting post-thyroidectomy hypocalcaemia. RESULTS Out of a total of 110 patients included in the study, 16.36% (n=18) developed hypocalcaemia including 1.81% (n=2) cases of permanent hypoparathyroidism. The sensitivity of one hour postoperative PTH assay as a predictive tool for post-thyroidectomy related hypocalcaemia was 94.4% while its specificity was 83.6% with 53% positive predictive value and 98.7% negative predictive value. CONCLUSION One hour post sub-total thyroidectomy PTH assay can be helpful in predicting post sub-total thyroidectomy hypocalcaemia. Moreover, it can be useful in safe discharge of day-care thyroidectomy patients.