Lab Tests Can Pinpoint Risk of Hypoparathyroidism After Surgery

Miriam E. Tucker

May 22, 2018

BOSTON — Patients with low calcium and parathyroid hormone (PTH) levels immediately following total thyroidectomy, and those with significant drops in those values from baseline, are at highest risk for developing permanent hypoparathyroidism, new research suggests.

The findings from a single-center retrospective review of post-thyroidectomy patients were presented May 17 here at the American Association of Clinical Endocrinologists (AACE) 2018 Annual Scientific & Clinical Congress by Steven L. Brown, DO, an endocrine fellow at the University of Arizona College of Medicine, Phoenix.

According to the literature, transient hypoparathyroidism occurs in about 20% to 30% of patients following total thyroidectomy, and the condition becomes permanent in about 3% of individuals.  

"Predicting at-risk patients is key to post-operative management of total thyroidectomy patients. Pre- and post-operative serum calcium and PTH are beneficial in predicting permanent hypoparathyroidism," Brown said during his presentation.

"Calculating change of calcium and PTH can assist in predicting [the need for] supplementation in patients undergoing total thyroidectomy," he stressed. 

Should Routine Testing Be Performed to Predict a Rare Occurrence?

Currently, such measurements are not routine in patients undergoing thyroidectomy. In fact, in their review of 250 patients at the University of Arizona, Brown and colleagues had to exclude 72 patients because calcium and/or PTH values weren't available.

Now the investigators are advocating for such testing, Brown told Medscape Medical News.

"We want to establish protocols to have laboratory data available in order to predict those who may be at risk," he said.

Asked to comment, session moderator Matthew J. Levine, MD, Scripps Clinic, La Jolla, California, said the findings raise an interesting question, as the authors are advocating routine measurements to prepare for a very rare occurrence.

"We wouldn't routinely check calcium or PTH in a patient who didn't have a pre-existing calcium-involving endocrinopathy, but the question is valid."

Levine suggested that perhaps those measurements should be recommended routinely in patients who are going to have lymph node dissection performed as part of the thyroidectomy because the hypoparathyroidism risk increases with the amount of tissue removed. But on the other hand, it's not always known in advance that the surgeon will perform lymph node dissection.

"So, knowing someone is going to have a total thyroidectomy, it might be worth thinking about measuring calcium and PTH at baseline...One could make the argument that while permanent hypoparathyroidism in the hands of a high-volume thyroid surgeon is very uncommon, it still exists frequently enough that we need to be prepared for it."

Four-Fold Risk Difference Found

The current study involved a retrospective single-center chart review of 250 patients who had undergone total thyroidectomy for papillary thyroid cancer between 1999 and 2013. Laboratory measurements were reviewed at baseline, and 1 month, 6 months, and 12 months postoperatively. 

Permanent hypoparathyroidism was defined as PTH < 12 pg/mL, serum calcium < 8.0 mg/dL, and/or requiring calcitriol to maintain normal calcium levels for more than 6 months following total thyroidectomy.   

After excluding the 72 patients who had missing data and two patients who had other conditions (one each with primary hyperparathyroidism and hypervitaminosis D), the study population totaled 176 patients.

Brown and colleagues divided patients into four groups based on immediate post-operative values: (1) 66 with low PTH and low calcium; (2) 30 with low PTH and normal calcium; (3) 31 with normal PTH and low calcium; and (4) 49 with normal PTH and normal calcium.

The overall incidence of permanent hypoparathyroidism was 17%, higher than previously reported in the literature. Those with permanent hypoparathyroidism didn't differ from those without by age, sex, or ethnicity, nor by preoperative vitamin D, serum calcium, or serum PTH.

However, immediate postoperative serum calcium and serum PTH did significantly differ between those who later developed permanent hypoparathyroidism (7.58 vs 8.08 mg/dL; P < .05) and those who didn't (8.0 vs 30.8 pg/mL; P < .05).

In addition, those who developed permanent hypoparathyroidism had a greater percentage drop from baseline in both serum calcium (17.8% vs 14.3%; P = .042) and PTH (69.7% vs 29.7%; P = .016).

The proportions of patients who developed permanent hypoparathyroidism were 30.3% in Group 1 (low PTH, low calcium) compared with just 19% in Group 2, 10% in Group 3, and 2% in Group 4. The odds ratio for Group 1 versus the other three was 4.3 (95% CI, 1.9 - 9.9).   

Taking the Findings Forward

Levine pointed out that the study findings still leave open the question of timing: "When should we be checking in the postoperative period, given what was presented regarding percentage drop in PTH?"

The answer could come soon from a prospective study Brown and colleagues are planning at three clinical sites. As part of it, he explained, "We're going to have to have a close relationship with our surgeons."

Brown has reported no relevant financial relationships. Levine is a speaker for and/or has received honoraria from Novo Nordisk, Janssen, Merck, Boehringer-Ingelheim, and Amgen.

American Association of Clinical Endocrinologists (AACE) 2018 Annual Scientific & Clinical Congress. May 18, 2018; Boston, Massachusetts. Abstract 720.

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