The Role of Post -Thyroidectomy PTH in Predicting Hypocalcaemia
Category: Case series
Hypocalcemia is the most common complication after total thyroidectomy. It is also the leading cause for prolonged hospitalization after thyroidectomy. The purpose of this study is to determine the validity of PTH monitoring six and twelve hours after total thyroidectomy in predicting hypocalcaemia. Nineteen patients who underwent total thyroidectomy alone or with neck dissection were included. PTH levels and calcium levels were monitored for all patients at two major points: at 6 and 12 hours post-operatively. In all patients, corrected calcium level was calculated.
At 6 hours post-operatively, 47% of patients had calculated calcium level< 8, with normal PTH level in 5 of them. At 12 hour post-operatively 37% of patients had calculated calcium level< 8, with normal PTH level in 3 of them. The validity of low serum PTH in predicting the post-operative hypocalcaemia and of normal serum PTH in predicting the normocalcemia were calculated.
We conculude that there is a strong correlation of postoperative PTH with post thyroidectomy hypocalcaemia. A single serum PTH level 12 hours post-operatively is an accurate predictor of normocalcaemia. Patients with normal PTH and normal calcium level 12 hours postoperatively can be considered for discharge.
Key Words: Thyroidectomy, hypocalcemia, parathyroid hormone, surgery
Thyroid cancer is the most common endocrine malignancy accounting for approximately 1% of all human cancers and causing approximately 0.5% of all cancer deaths. Total thyroidectomy is the treatment of choice for clinically significant thyroid cancers. Transient hypocalcaemia after total thyroidectomy is reported at 20%-30% in most studies . The search for a blood test that identifies patients who will become hypocalcaemic after total thyroidectomy versus those who will remain normocalcaemic in the early postoperative period has evolved since the late 1980s (1-5) . Transient hypocalcaemia after total thyroidectomy is reported at 20-30% in most studies (2,7). Hypocalcaemia after total thyroidectomy is multi-factorial, but parathyroid dysfunction is the main cause.The incorporation of parathyroid hormone (PTH) levels as a monitor for hypoparathyroidism leading to hypocalcaemia has been the next step in the evolution of this field of research (2). Serum PTH has a half life of 1 to 4 minutes. Any insult to parathyroid glands with impaired PTH secretion leads to an immediate decline in PTH levels.
Serum calcium homeostasis by PTH occurs through a variety of mechanisms:
- It promotes distal tubule calcium re-absorption.
- Bone re-absorption.
- 1,25(OH)2D mediated intestinal calcium absorption.
The nadir of serum calcium post total thyroidectomy is 24-48h postoperatively and it may be delayed as the 4th postoperative day (6). Thus, monitoring calcium levels postoperatively, might lead to patients being hospitalized for longer time than otherwise necessary. The risk of hemorrhage and hematoma is mostly in the early postoperative period, and warrant 24 h in hospital observation.
Inpatient stay beyond 24 h after Total thyroidectomy, in the absence of complications, is not warranted because patients suffer minimal pain (8).
Grodski found that postoperative PTH can be used to stratify the risk of patients developing hypocalcaemia after thyroidectomy (1). In addition, the routine use of oral calcium supplements was shown to lead to decreased incidence and severity of post-thyroidectomy hypocalcaemia (1) .
In this work we aimed to investigate the role and best timing of PTH monitoring post total thyroidectomy in predicting hypocalcaemia.
MATERIALS AND METHODS
In this study we included patients (men and women) that underwent different thyroidectomy procedures: total thyroidectomy or total thyroidectomy with central neck dissection. PTH levels were monitored for all patients at two major points: at 6 and 12 hours postoperatively. Calcium levels were measured twice a day. Corrected calcium level was calculated using the next formula:
Corrected Ca. = serum calcium + 0.8 (4 – serum albumin)
We defined two different points for hypocalcaemia:
While the corrected calcium level was less than 8 mg/dl
While the PTH level was less than 1.6 Pcmoll/l (8).
Nineteen patients were included (13 women and 6 men). Age of patients ranged from 20 to 76, with a mean age of 45 years. Patients underwent total thyroidectomy and 4 patients underwent total thyroidectomy with central neck dissection.
Mean time of hospitalization for these patients was 5.05 days (SD 0.97). The PTH levels, and corrected calcium levels (calculated) at three measurements are shown in Table I.
At 6 hours post-operatively, 47% (9 from 19) of patients had calculated calcium level< 8, with normal PTH level in 5 of them. At 12 hours post-operatively 37% (7 from 19) of patients had calculated.
calcium level< 8, with normal PTH level in 3 of them. At 48 h post-operatively, 2 of those 3 patients, had a normal calculated calcium level and were asymptomatic without calcium supplementation.
Seven of our patients (36.8%) received intravenous calcium treatment after the operation. Six of them received also the calcium supplementation. Five from these patients were symptomatic. Furthermore, one patient who underwent thyroidectomy with central neck dissection required 5 month calcium supplementation.
We calculated the validity of low serum PTH in predicting the post-operative hypocalcaemia and of normal serum PTH in predicting the normocalcaemia. For low serum PTH, the sensitivity of this test was 55% at 6 hours after the operation, and 71% at 12 hours after the operation. The specificity of this test was 90% at 6 hours after the operation, and 100% at 12 hours after the operation.
The sensitivity of normal serum PTH level as a predictor of normocalcaemia was 91% at 6 hours after the operation and 100% at 12 hours after the operation. The specificity of this test was 44% at 6 hours after the operation and 75 % at 12 hours after the operation.
Hypocalcemia is the most frequent complication of total thyroidectomy. The reported incidence in the literature varies from 1-50% (1). The risk of hypoparathyroidism increases in reoperative thyroid surgery and when central neck is associated (11). Patients symptoms and serum calcium levels dictate the necessity for calcium and vitamin D supplementation. The need of monitoring hypocalcemia is the leading reason for patients’ hospitalizaion beyond 23 hours (7) . Within the last decade, numerous studies have been conducted to predict the factors involved in the early prediction of hypocalcemia following thyroidectomy. Parathyroid hormone assay was one of the most reliable tool to predict postoperative hypocalcemia within hours of total thyroidectomy or completion thyroidectomy ( 11-13).
Several published reports have demonstrated the marked utility of the PTH assay in predicting patients who will develop hypocalcemia after thyroidectomy. However, there is a substantial
variability among these reports about the sensitivity and specificity of PTH in accurately predicting hypocalcemia. Lam and ker found that all patients with PTH values less than 8pg/mL measured one hour after surgery became hypocalcemic, and all patients with PTH level greater than 9pg/mole did not (14) . Higgins and his colleagues demonstrated that 64% of those patients who subsequently required calcium supplementation had more than 75% decrease in PTH levels from baseline 20minutes post surgery. Another finding was that most of patients who did not need calcium supplementation (74%) had a decrease of less than 75% from baseline (15). For Lombardi and colleague. PTH less than 10 pg/mL measured four or six hours after surgery predicted hypocalcemia with an overall accuracy of 98% (3). Pattou and colleagues reported that a postoperative PTH level of 12 pg/mL or less was a good predictor of hypocalcemia, but did not state how long after surgery PTH values were obtained (6). Payne and his colleagues found that 6 hours post-operation PTH level is the most accurate predictor for hypocalcaemia and accurately predicts also the normocalcaemia (7).
As we saw earlier, most of the studies are discussing the correct timing of taking the measurements of PTH levels post-operatively. In our study we performed the measurements at 6 and 12 hours after the operation. We saw that the sensitivity of low PTH as a predictor of hypocalcaemia was higher at the 12 hours measurement. Despite this no absolute PTH levels or percentage decline has 100% sensitivity or specificity. Sensitivity of normal PTH levels predicting normocalcaemia was higher than sensitivity of low PTH levels predicting hypocalcaemia.
There are a few limitations to the study. First, it is a retrospective non-random review, which may result in selection bias. Secondly, a small number of participants. Further randomised controlled are required.
In our study we saw that a single serum PTH level 12 hours postoperatively is an accurate predictor of normocalcaemia. Thus, patients with normal PTH and normal calcium level 12 hours post-operatively can be considered for discharge reducing unnecessary hospitalization and patients’ discomfort.
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- Pattou, F., Combemale, F., Fabre. et al. Hypocalcemia following Thyroid Surgery: Incidence and Prediction of Outcome. World Journal of Surgery:1998: 22, 718–724.
- Payne, R. J., Hier, M. P., Tamilia, M. et al. Same-Day Discharge After Total Thyroidectomy: The Value of 6-hour Serum Parathyroid Hormone and Calcium Levels. Head and Neck:2004; 27: 1 – 7.
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Table I. PTH and calcium level measurements at two different post-operation time points
|6 hours after the operation||12 hours after the operation|
|3.36 (2.58)||3.27 (2.47)|
|8.04 (0.42)||8.04 (0.55)|