Sunlight Deficiency: a Reversible Cause of Low Serum Phosphate?

Abstract

Vitamin D deficiency is associated with low circulating levels of calcium and phosphate.

A 56-year-old asymptomatic gentleman was referred with laboratory findings of undetectable serum phosphate (< 0.16 mmol/L) and a history of calcified renal calculi. 25-hydroxyvitamin D (25-OHD) was low at 19.0 nmol/L (concentrations of <30 nmol/L are indicative of vitamin D deficiency). Serum adjusted calcium was normal at 2.38 mmol/L (reference range (RR): 2.0–2.6), as was parathyroid hormone. 24-h urine calcium excretion was elevated at 10.8 mmol per 24 h (RR: 2.5–7.5), with normal 24-h urinary phosphate (RR: 12.9–42.0 mmol/24 h). Renal function was normal.

On questioning, it was revealed that he worked indoors between 06.00 h and 15.00 h for approximately 24 days per month, and on rest days he went out mainly in the evenings.

Achievement of adequate sunlight exposure resulted in normalisation of 25-OHD status and serum phosphate levels. In relation to normalisation of vitamin D status, there was a reduction in urinary calcium excretion, which when elevated may have contributed to the renal calculi.

The authors propose that investigation of low phosphate or calcium levels should always include an assessment of time spent outdoors in the daytime and vitamin D status.

Key words

vitamin D - deficiency - phosphate - sunlight - case

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