Radioiodine Therapy May Extend Parathyroid Recovery After Surgery

Persistent hypoparathyroidism may be related to radioiodine treatment

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by Andrea Lobo |

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Treatment with radioiodine extended the recovery of parathyroid function in patients who underwent surgical thyroid removal, in some cases beyond one year, a study in China has found.

Although some studies suggest that hypoparathyroidism might be permanent if it lasts more than six months, this diagnosis “should be cautiously made at least 12 months after surgery, especially in patients who receive [radioiodine] treatment,” researchers wrote.

The study, “The Effects of Radioiodine Therapy on the Recovery of Parathyroid Function in Patients with Protracted Hypoparathyroidism after Total Thyroidectomy for Papillary Thyroid Carcinoma,” was published in the Journal of Investigative Surgery.

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Most adult patients recover parathyroid function within 6 weeks after surgery

Hypoparathyroidism is characterized by low levels of the parathyroid hormone produced in the parathyroid glands. Low hormone levels result in low calcium and high phosphorus in the blood, which causes symptoms like muscle cramps, pain, tingling, or burning sensation in the limbs.

The condition can have different causes, including damage or accidental removal of the parathyroid glands during total thyroidectomy, or surgical removal of the thyroid gland, called acquired hypoparathyroidism.

Nearly 90% of adult patients will recover parathyroid function within four to six weeks following the surgery, so-called transient hypoparathyroidism. For the remaining patients, hypoparathyroidism might be long-lasting, with reports of parathyroid function recovery after more than 12 months post-surgery.

In people with differentiated thyroid carcinoma (a form of thyroid cancer), the treatment involves thyroidectomy, after which treatment with radioactive iodine-131 (131I) has been deemed effective and safe to clear the remaining thyroid tissue. However, this treatment may also damage the parathyroid glands and contribute to hypoparathyroidism.

In this study, we reported for the first time that persistent hypoPT [hypoparathyroidism] might be related to 131I [radioactive iodine-131] treatment.

To determine the effect of 131I treatment on parathyroid function, a team from a hospital in Sichuan, China, studied 260 patients with papillary thyroid cancer, the most common type of thyroid cancer, who underwent thyroid surgery from January 2013 to June 2018. The study included 166 patients who were treated with 131I and 94 patients who did not receive the treatment (control group). In the study, protracted hypoparathyroidism was characterized by low blood serum PTH levels and the need for continued treatment.

The patients receiving 131I treatment had a significantly higher diagnosis rate of N1 stage, meaning the tumor had spread to lymph nodes (72.89% vs. 25.54% in the control group), and persistent hypoparathyroidism than those in the control group (39.16% vs. 25.53%).

In contrast, controls were diagnosed significantly more with T1 stage cancer than patients receiving iodine treatment (60.64% vs 45.78%). Of note, T1 to T4 refer to the main tumor’s size and/or extent — the higher the number, the larger the tumor or the more it has grown into nearby tissues.

Most of the patients (172) recovered from protracted hypoparathyroidism in a period ranging from two to 39 months, whereas for 88 patients, it became persistent. Participants receiving 131I treatment showed a significantly lower rate of parathyroid function recovery than those in the control group (58.72% vs. 73.86%).

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13% of patients recovered parathyroid function after 1 year

Among the patients who recovered parathyroid function, most (58.14%) did so within six months after surgery, with 35.1 % of patients recovering within 12 months and 13.37% after one year. The participants who recovered within six months were diagnosed significantly more often with cancer at the N0 stage, which refers to no evidence of spreading to regional lymph nodes, whereas recovery after one year was more common in the 131I treatment group.

Also, patients who recovered parathyroid function had significantly higher calcium levels at six months (2.17 vs. 2 mmol/L) and 12 months after the surgery (2.2 vs. 1.99 mmol/L), compared to those with persistent hypoparathyroidism.

A multivariate analysis, which takes into account several different variables, showed that the number of parathyroid glands remaining after surgery and 131I treatment significantly affected parathyroid function recovery.

“In this study, we reported for the first time that persistent hypoPT [hypoparathyroidism] might be related to 131I treatment,” the researchers wrote.

“Recovering from postoperative protracted hypoPT in PTC [papillary thyroid carcinoma] patients might occur even 12 months after the surgery,” they concluded. “Parathyroid function should be monitored during routine yearly surveillance in PTC patients who receive 131I treatment.”