Chronic hypocalcaemia due to selective skeletal resistance to parathyroid hormone.

A 71-year-old man was referred for evaluation of asymptomatic hypocalcaemia dating back at least 20 years. There were no somatic abnormalities and Chvostek and Trousseau signs were negative. Serum total calcium varied from 1.88 to 2.03 mmol/l, albumin 37-44 g/l, phosphate 0.54-1.12 mmol/l and ionized calcium 1-1.13 mmol/l. Serum intact PTH levels were 69 and 55 ng/l (10-65), 25-OHD was 40 nmol/l (2.25-107.5) and 1,25-(OH)2D was 54.6 nmol/l (39-156). Serum and urine magnesium and creatinine clearance were normal. Twenty-four-hour urine calcium was 2.15 mmol and calcium/creatinine ratio 0.07. TM phosphate (maximal rate of tubular reabsorption of phosphate in mmol/l glomerular filtrate (GF)) was 0.84 mmol/l GF (0.80-1.34). Bone formation and resorption markers were normal. Bone mineral densities measured by dual-energy X-ray absorptiometry (DEXA) were within normal limits at the hip, forearm and lumbar spine. Infusion of 200 units of synthetic 1-34 PTH was associated with a rise in urinary cyclic AMP from 43 mmol/l GF to 344 mmol/l GF and TM phosphate fell from 0.93 to 0.76 mmol/l GF; 1-34 PTH infusions of 300 units twice daily for 5 days were associated with an increase in serum 1,25-(OH)2D from 80.6 to 114.4 pmol/l but no increase in serum calcium. This is a most unusual case of chronic hypocalcaemia similar to that reported by Frame et al. resulting from isolated skeletal resistance to PTH that is not related to renal insufficiency, osteomalacia or a magnesium-deficient state. These two cases appear to represent a new variant of pseudohypoparathyroidism ?type III.