Pattern of bone mineral density in patients with sporadic idiopathic hypoparathyroidism

Abstract

Objective: Measurement of bone mineral density (BMD) in patients with hypoparathyroidism directly addresses the effect of chronic under-exposure of bone to PTH. Because post-thyroidectomy hypoparathyroidism is potentially complicated by the pre-existence of thyrotoxicosis and the need for postoperative thyroxine replacement, we have studied a large group of patients with sporadic hypoparathyroidism who have been followed up in our endocrine clinic. Studies conducted in limited number of patients with sporadic idiopathic hypoparathyroidism (SIH) have suggested an increase in BMD in such patients. In this current study, we have measured BMD in a large cohort of patients with SIH and have assessed the relationship of BMD with duration of disease and with the adequacy of treatment, as indicated by follow-up serum calcium, phosphate and alkaline phosphatase levels. Design: Case control study and intra-group comparison. Subjects: Forty-seven patients (M : F ratio 23 : 24) with SIH who had been reviewed during 2003-2004 in our endocrine clinic were recruited for this study. Their mean age (± SD) was 34.6 ± 13.6 years and the duration from the time of initial diagnosis was 9.6 ± 8.5 years. Forty-eight match healthy volunteers were recruited from hospital staff and from normocalcaemic relatives. Methods: Bone mineral density was measured at total lumbar spine (L₁-L₄), hip and forearm by dual energy X-ray absorptiometry (DXA). The relationship of BMD was analysed with duration of disease symptoms (group I, ≤ 1 year, group II, > 1 and < 5 years and group III, ≥ 5 years) and mean serum total calcium observed during follow-up (group A, calcium ≤ 1.79 mmol/l and group B, ≥ 1.80 mmol/l). Results: Patients with SIH showed significantly higher BMD at total lumbar spine and hip when compared to controls (1.098 ± 0.187 vs. 0.936 ± 0.131 g/cm² and 0.967 ± 0.141 vs. 0.882 ± 0.149 g/cm², P < 0.001 for both). BMD in the forearm was not significantly different in patients and controls. The age- and BMI-adjusted lumbar spine BMD showed correlation with duration of disease (r = 0.348 and P = 0.019). Patients with longer duration of hypoparathyroidism had higher BMD at lumbar spine (group I vs. group III, 0.951 ± 0.132 vs. 1.156 ± 0.180 g/cm², P < 0.05). There was no significant correlation between BMD values in patients with SIH and their mean serum total calcium levels during the period of follow-up (r = 0.192, P = 0.206). Neither was the mean BMD significantly different between group A and B. Serum total alkaline phosphatase showed a significant negative correlation with BMD at lumbar spine (r = -0.445, P = 0.012). Conclusions: Patients with sporadic idiopathic hypoparathyroidism have increased mean BMD in the lumbar spine and hip but not in the forearm, compared to normal matched healthy controls. The increase in BMD is related to the duration of the disease rather than the serum calcium levels.