High dose topical calcipotriol consistently reduces serum parathyroid hormone levels

Calcipotriol is a vitamin D analogue which is an effective topical treatment for chronic plaque psoriasis. It has been reported to have no effect on systemic calcium homeostasis provided the manufacturer's guidelines are adhered to (maximum 100 g of calcipotriol ointment (50 micrograms/g) per week). However, there have been reports of hypercalcaemia in patients using topical calcipotriol even at recommended doses. The purpose of this study was to investigate the effects of topical calcipotriol in vivo using the recently developed 'intact' PTH assay as a more sensitive index of systemic calcium homeostasis.

Seven patients with extensive psoriasis were recruited for the study. Each patient was admitted to hospital and applied 200 g of calcipotriol (50 micrograms/g) ointment over the first week followed by 300 g over the second week. In the third week of treatment, patients were treated with 2% crude coal tar which served as a biochemical washout phase.

Serum total adjusted calcium was measured at baseline and three times a week during the study. Twenty-four-hour urinary calcium excretion was measured at baseline and twice a week throughout the study. Peak (0400 h) and trough (0900 h) PTH levels were measured at baseline and at the ends of weeks 2 and 3.

Serum PTH levels were reduced in every patient after 2 weeks' treatment with calcipotriol and rose after withdrawal. Mean 0400 h PTH fell by 2.58 pmol/l (95% confidence interval 1.45-3.70) from 5.11 to 2.53 pmol/l (P < 0.01) and mean 0900 h PTH fell by 2.08 pmol/l (0.84-3.36) from 4.04 to 1.96 pmol/l (P < 0.01). Mean serum and urine calcium rose during treatment with calcipotriol and fell after withdrawal. Mean adjusted total calcium rose by 0.14 mmol/l (95% confidence interval 0.10-0.16) from 2.25 to 2.39 mmol/l (P < 0.01). Mean 24 hour urine calcium excretion rose by 2.09 mmol/24 h (0.51-3.26) from 3.40 to 5.49 mmol/24 h (P < 0.05). No patient developed hypercalcaemia at any stage of the study although hypercalcaemia was recorded transiently in three patients.

Topical calcipotriol is likely to have a dose dependent effect on systemic calcium homeostasis.