Calcium Metabolism Evaluated by 47Ca kinetics: Estimation of Dermal Calcium Loss

Abstract

1. Seventy-seven calcium balance and ⁴⁷Ca turnover studies were performed in normal volunteers (n = 15) and in patients with osteoporosis (n = 12), primary hyperparathyroidism (n = 8), osteogenesis imperfecta (n = 5), medullary carcinoma of the thyroid (n = 4), thyrotoxicosis (n = 2) and intestinal bypass for obesity (n = 11). 2. After intravenous injection of 20 μCi of ⁴⁷Ca two retention curves of ⁴⁷Ca were obtained: R₁(t) directly measured on a whole-body counter and R₂(t) calculated from the cumulated daily excretions of ⁴⁷Ca in urine and faeces. The data were fitted to a modification of the continuously expanding exchangeable calcium pool model. 3. Dermal calcium loss was estimated from the serum ⁴⁷Ca specific radioactivity curve and the constantly increasing difference between the two retention curves. The median dermal calcium loss in 77 studies was 1.50 mmol 24 h⁻¹ 1.73 m⁻² (range 0.13-4.60). 4. The dermal calcium loss might be overestimated by redistribution of tracer or by eventual insufficient collection of urine and faeces. The possible influences of these errors have been evaluated. 5. Patients with primary hyperparathyroidism had a greater (P < 0.02) dermal calcium loss (2.64 mmol; range 0.80–4.50) than a control group (1.38 mmol; range 1.25–2.34).