Low PTH Levels May Lead to Greater Risk of Fractures in Hypoparathyroidism, Study Suggests

Lower levels of parathyroid hormone (PTH) in people with hypoparathyroidism may lead to decreased bone material strength and increase the risk of fractures, a new study reports.

The study, “PTH and bone material strength in hypoparathyroidism as measured by impact microindentation,” was published in the journal Osteoporosis International.

Among other functions, PTH helps regulate bone growth and turnover. People with hypoparathyroidism experience increases in bone mass. Although this would be expected to reduce rates of bone fractures, existing reports suggest that people with the condition may actually be more likely to experience such fractures.

In the study, researchers at Columbia University hypothesized that bone fragility in hypoparathyroidism may be due to deficits in bone material properties.

To test this idea, they measured the bone material strength index (BMSi) of 34 participants (mean age was about 45 years), 17 with hypoparathyroidism and 17 controls. Each group had 14 women and three men.

In the assessment, a probe connected to a specialized device was placed through the skin (between the tibia, or “shinbone,” and the inner side of the ankle) to the bone surface. Then, a set amount of force was applied, and the device recorded how deeply the needle penetrated the bone to calculate BMSi.

Results showed that these scores were significantly lower (11%) in the group with hypoparathyroidism than in the controls. Also, BMSi scores were strongly correlated with PTH levels in the overall group and in controls only, which means that those with higher PTH levels also had high BMSi scores.

“We found that bone material strength … was associated with PTH levels in controls and was lower in patients with HypoPT [hypoparathyroidism],” the researchers wrote. “[O]ur finding of reduced BMSi suggests that despite ‘thicker’ bone, bone matrix properties are abnormal in HypoPT.”

Lower BMSi scores in the hypoparathyroidism group was associated with a trend toward greater doses of calcium supplements.

In five participants with the condition, BMSi was re-assessed after a year of treatment with recombinant human parathyroid hormone, or rhPTH(1-84) — Natpara in the U.S. Results after treatment revealed a 23% increase in BMSi.

Together, these data suggest that bone material strength is decreased with low levels of PTH. A possible explanation, the team said, is the lower turnover of bone cells in people with hypoparathyroidism.

“Reduced BMSi in HypoPT might suggest an increased fracture risk with this disease,” the scientists said.