BMD and Parathyroid Hormone Levels in Pseudohypoparathyroidism 1B

Normalizing parathyroid hormone (PTH) levels when treating pseudohypoparathyroidism 1B (PHP1B) may avoid adverse effects on bone, according to a large retrospective, sample-sized study published in the Journal of Bone and Mineral Research.

To evaluate bone responsiveness to serum PTH levels in patients with PHP1B, researchers from Beijing identified 48 patients diagnosed with PHP1B and matched them with 55 nonsurgical patients with hypoparathyroidism (HP). Of the patients with PHP1B, 24 had been treated regularly with calcium (with or without vitamin D) and the others were treatment naive. All of those with HP had persistent hypocalcemia and low serum PTH levels.

Results were expressed as bone mineral density (BMD; g/cm²) and/or Z-score (the standard deviations from the mean for chronological age- and sex-matched controls).

The researchers demonstrated that in patients with PHP1B, the BMD Z-score for the lumbar spine was higher than that for the femoral neck and the total hip. Although untreated patients had lower serum calcium levels, the PTH levels, as well as BMD Z-scores, were comparable between treated and untreated patients at baseline. PTH was found to be a negative predictor for lumbar spine BMD Z-score in patients with PHP1B. Although Z-scores for femoral neck and lumbar spine BMDs increased after treatment, the increase in lumbar spine BMD correlated with a decrease in PTH. For femoral neck, lumbar spine, and total hip, all BMD Z-scores were significantly lower in those with PHP1B than in those with idiopathic HP.

The investigators concluded that, “at least portions of bone tissue in PHP1B patients respond to elevated PTH levels to some extent. As a result, in addition to correction of hypocalcemia, it is reasonable to lower the PTH to appropriate levels when treating PHP1B to avoid adverse effects on bone.”

Further research on skeletal microstructure and fracture risk are needed to provide a more comprehensive evaluation of bone metabolism and optimal serum PTH targets in people with PHP1B.  

Reference

Chu X, Zhu Y, Wang O, et al. Bone mineral density and its serial changes are associated with PTH levels in pseudohypoparathyroidism type 1B patients [published online December 14, 2017]. J Bone Miner Res. Doi: 10.1002/jbmr.3360