Low PTH Levels After Surgery May Signal Chronic Hypoparathyroidism
Low levels of the parathyroid hormone (PTH) soon after a thyroidectomy is linked to a greater risk of chronic hypoparathyroidism, or that lasting more than six months, a study reported.
Its findings suggest that blood tests to measure PTH levels may allow doctors to discharge sooner patients who are at low risk of developing long-term hypoparathyroidism, while providing timely treatment to those at high risk.
The study, “Risks and prediction of postoperative hypoparathyroidism due to thyroid surgery,” was published in the journal Scientific Reports by a team of researchers in Turkey.
Thyroidectomy — the surgical removal of all or part of the thyroid gland — may be indicated for patients who have a thyroid tumor or hyperthyroidism, which occurs when the thyroid gland makes too much thyroid hormone.
One of the risks of thyroidectomy, however, is accidental injury to or removal of the parathyroid glands, which can result in hypoparathyroidism, or too little PTH being made.
While some of the risk factors for hypoparathyroidism after thyroidectomy are known, the researchers also wanted to understand what may contribute to a patient developing transient (for less than six months) versus permanent hypoparathyroidism (for six months and longer).
The team looked at the medical files of 352 patients (273 women and 79 men) who underwent complete removal of both lobes of the thyroid gland — either alone or in combination with removal of central lymph nodes or lateral neck tissue — in 2019.
Among other information, they collected data on the levels of PTH measured shortly (four to six hours) after thyroidectomy, with post-operative hypoparathyroidism being defined as PTH levels under 15 picomol/L within that time frame.
“Postoperative PTH measurement can diagnose hypoparathyroidism early and prevent unnecessary treatment and allow early discharge,” the scientists wrote.
Of these 352 people, 58 (16.5%) developed temporary hypoparathyroidism and 14 (4%) developed permanent hypoparathyroidism. The average age was similar between participants with hypoparathyroidism (chronic or transient) and those who did not develop this complication (47.4 vs. 49.1).
Hypoparathyroidism was more common in women than in men, with 63 women (23.1%) and nine men (11.4%) having abnormally low levels of PTH after thyroidectomy. It was also more common in patients who developed an infection at the surgery site.
PTH regulates the levels of calcium in the blood, and low levels of this hormone may cause the levels of calcium to drop, leading to the symptoms of hypoparathyroidism. As such, the researchers also looked at calcium levels after thyroidectomy.
According to the investigators, hypocalcemia, or lower-than-normal levels of calcium, was the cause of longer hospital stays for patients with hypoparathyroidism compared with those without it (median of four versus three days).
“Hypocalcemia is corrected by calcium and active vitamin D supplementation, which increases the length of hospital stay,” the researchers wrote.
The team further found that two (33%) patients with hyperthyroidism caused by Graves’ disease (an autoimmune disorder that makes the thyroid overactive) and five (40%) patients with overactive parathyroid glands prior to surgery had hungry bone syndrome — hypocalcemia associated with low phosphate and magnesium.
Patients who underwent a parathyroid autotransplantation — a procedure in which parathyroid tissue is returned via a transplant, typically in the forearm — were less likely to develop temporary hypoparathyroidism, and no one given this transplant had permanent hypoparathyroidism.
“Parathyroid autotransplantation can prevent the development of PoH,” the team wrote.
In patients with temporary hypoparathyroidism, PTH levels returned to normal in a median of 28 days (about one month), ranging from three days to nearly five months. In those who underwent parathyroid autotransplantation, however, these levels returned to normal in one week (median of seven days), as opposed to the 32-day median in patients who did not undergo this procedure.
Finally, the researchers found that patients with a low blood serum level of PTH (equal to or lower than 5.95 picomole/L) shortly after thyroidectomy were at a 135-times greater risk of developing permanent hypoparathyroidism.
In fact, this value could predict long-term hypoparathyroidism “with a positive predictive value of 100%,” the researchers concluded.