الفهرس 
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Abstract
Hypoparathyroidism is a common and sometimes a dangerous complication of total thyroidectomy, hence early identification and management of patients at risk of this complication is essential (McLeod et al., 2006).
Recently, measurement of postoperative serum parathyroid hormone level has been used for this sake (Del Río et al., 2011), and this study is a trial to identify the time point for measuring postoperative PTH level that best predicts parathyroid function prognosis.
The basic developmental embryology and surgical anatomy of the thyroid and parathyroid glands are intimately linked, and knowledge of both is essential for successful surgery.
The thyroid gland develops from the floor of the primitive pharynx, between the 1st and 2nd pharyngeal pouches, together with neural crest, and migrates caudally to reach its final position (Larsen, 2001).
The inferior parathyroid glands develop from the 3rd pharyngeal pouch and migrate caudally with the thymus giving a high variation in anatomical location, while the superior parathyroid glands develop from the 4th pharyngeal pouch and are relatively constant in position (Henry, 2003).
The normal thyroid gland weighs 20-25g., and it consists of two lateral lobes connected by the isthmus at the midline (Fancy et al., 2010; Cummings, 1998; Williams et al., 1995).
The intimate relationship between the thyroid arteries and laryngeal nerves is of supreme important for the operating surgeon (Skandalakis, 2004; Cernea et al., 1992).
There are four parathyroid glands that are typically described as golden yellow in colour and weigh approximately 30mg. The blood supply arises from the inferior thyroid artery.
Despite the variability of anatomical position, in most of cases they are located around the junction of the inferior thyroid artery and the recurrent laryngeal nerve (Castleman, 1987).
Calcium homeostasis is a complex process, and parathyroid hormone plays a central role in the regulation of serum calcium levels together with biologically active vitamin D and, to a lesser extent, calcitonin (Mundy, 1990).
The target organs for parathyroid hormone action are bone, kidneys and intestine, and the overall effect is to increase serum calcium level (Juppner et al., 1991).
Complications of total thyroidectomy include: injury of recurrent laryngeal nerve, injury to external branch of superior laryngeal nerve, neck hematoma, infection, thyrotoxic storm and hypoparathyroidism leading to hypocalcaemia (Isozaki et al., 2016).
This is a prospective cohort study done at Endocrine surgery department, Ain Shams University hospitals to identify the best time PTH levels should be checked to predict transient and permanent hypoparathyroidism, and determine the postoperative serum PTH level that most safely predicts the development of permanent hypoparathyroidism after total thyroidectomy.
The study had continued for six months from August 2018 to February 2019.
Thirty patients who underwent total thyroidectomy, done by the same surgical team, were enrolled in the study.
All patients underwent preoperative evaluation (careful history taking, thorough clinical examination and preoperative investigations) and postoperative follow up (early at hospital and then at the outpatient clinic), and we had found the following results:
22 female patients and 8 male patients were enrolled in the study, their age ranged from 34 – 59 years; all had normal preoperative serum PTH and serum calcium levels.
The preoperative provisional diagnoses were toxic goiter in 16 patients and simple nodular goiter in 14 patients.
The operative time for total thyroidectomies in our study ranged from 90 – 130 minutes, and the blood loss ranged from 150 – 250 mL.
As regard postoperative complications, none of the thirty patients had postoperative bleeding, however three patients were complicated by recurrent laryngeal nerve injury manifested as hoarseness of voice; however, their symptoms resolved one to two months after surgery.
Out of the thirty patients, there were thirteen patients (13/30= 43.33%) who had transient postoperative hypoparathyroidism (as indicated by low serum PTH level two hours postoperatively only or one day postoperatively only or both).
Three patients (3/30=10%) had low serum PTH levels two hours postoperatively only, seven patients (7/30=23.33%) had low serum PTH levels on the first postoperative day, three patients (3/30=10%) had low serum PTH level both two hours and one day.
Out of the thirty patients in our study, there were six patients (6/30=20%) who had transient postoperative hypocalcaemia (biochemical transient hypocalcaemia), out of these six patients, three patients (3/10=10%) had symptoms of hypocalcaemia manifested as tingling of the digits and were prescribed oral calcium replacement (symptomatic transient hypocalcaemia).
Two patients (2/30=6.67%) had permanent hypoparathyroidism as defined as persistently low serum PTH level six months after total thyroidectomy and were prescribed oral calcium and vitamin D supplements, the same two (2/30=6.67%) with permanent hypoparathyroidism had persistently low serum calcium six months after total thyroidectomy and were considered permanently hypocalcaemic.
The current study results showed that postoperative day one PTH level was a better predictor of transient hypoparathyroidism (P-value 0.007, highly significant) than two hours postoperative PTH level which was non-significant (P-value 0-075).
The present study also showed that postoperative day one PTH levels predicted safely the development of permanent hypoparathyroidism (P-value 0-001, highly significant), while two hours postoperative PTH level had no significance for the same purpose (P-value 0.13).
Using an ROC curve analysis, we found that the cut-off value of the best time of PTH measurement that safely predicts permanent hypoparathyroidism (postoperative day one) was less than or equal to 3.2ng/mL.