Endocrine Abstracts

Background

Primary hyperparathyroidism (PHPT) is a common endocrine disorder accompanied by low total 25-hydroxyvitamin D [25(OH)D] levels. The 25(OH)D is converted to its biologically active form, 1.25-dihydroxyvitamin D [1.25(OH)₂D] that then circulates as bound or free forms. Free 1.25(OH)₂D concentrations in patients with PHPT have not previously been examined and understanding this is important to better assess vitamin D status.

Objective

To evaluate total and free 1.25(OH)₂D and hormone-to-prohormone [1.25(OH)₂D/25(OH)D] ’activation ratio’ in patients with PHPT and healthy controls.

Methods

Female patients with PHPT and healthy controls (n = 30/group), matched for age and body mass index, were enrolled. Serum levels of calcium, intact parathyroid hormone (iPTH), vitamin D binding protein (DBP), albumin, total 25(OH)D and 1.25(OH)2D levels were measured. The activation ratio of vitamin D was calculated as total 1.25(OH)₂D/25(OH)D. Calculated serum free 25(OH)D and 1.25(OH)2D levels were also reported.

Results

Compared to control subject, patients with PHPT had lower total 25(OH)D and DBP levels (P < 0.001). There were no significant differences in free 25(OH)D or total 1.25(OH)₂D levels between two groups; but free 1.25(OH)₂D levels were about 26% higher in the PHPT patients compared to controls (P < 0.001). The free (but not total) 1.25(OH)₂D level was inversely correlated with DBP (P < 0.01). The activation ratio was significantly higher in patients with PHPT (P < 0.01). The free 1.25(OH)₂D levels and activation ratio were positively correlated with iPTH (r = 0.35, P < 0.01) and calcium levels (r = 0.33, P < 0.01). The activation ratio was highly correlated with levels of total and free vitamin D metabolites (P < 0.001).

Conclusion

Patients with PHPT had significantly higher free 1.25(OH)₂D levels and activation ratio compared to control subjects. We suggest that total vitamin D levels may not be an accurate estimate of true vitamin D nutritional or functional status. The activation ratio may be a good predictor of free vitamin D levels in these patients.