Background: Vitamin D3 is generated in the skin of humans from 7-dehydrocholesterol by ultraviolet light. Vitamin D3 is hydroxylated to its metabolically active form in the liver and kidneys. The hydroxylated form of Vitamin D3, is required for both phosphate and calcium absorption in the gut and reabsorption from the kidneys. Vitamin D deficiency is associated with a low circulating calcium and phosphate.
Case report: A 56-year-old gentleman was referred with laboratory findings of undetectable serum phosphate and a history of calcified renal calculi. 25-OH-Vitamin D3 was low at 7.6 ng/ml (concentrations of <12 ng/ml indicative of vitamin D deficiency). Serum corrected calcium was normal at 2.38 mmol/l (2.02.6) as was parathormone (2.2 pmol/l (1.16.9)). Twenty-four hour urine calcium excretion was elevated at 10.8 mmol per 24 h (normal range 2.57.5) with normal 24 h urinary phosphate. Questioning revealed that he worked between 06.00 and 15.00 on 12 out of 16 days. On rest days he went out mainly in the evenings.
Management: Sunlight exposure during the spring was encouraged. Dietitian review indicated that the low serum phosphate was not due to dietary insufficiency. At the next review in early summer, 25-OH-Vitamin D3 level had improved to 22.6 ng/ml (concentrations >20 ng/ml indicative of vitamin D adequacy). Phosphate was near normal at 0.66 mmol/l (0.81.4). By mid-summer, there was complete normalisation of phosphate levels (0.83 mm mol/l (0.81.4)) with 25-OH-Vitamin D3 remaining normal. Urinary calcium excretion decreased to 8.9 mmol/24 h.
Discussion points: In this case, simply encouraging adequate sunlight exposure resulted in normalisation of vitamin D3 status and serum phosphate.
In relation to normalisation of vitamin D status, there was reduction of urinary calcium excretion, which when elevated may have contributed to the renal calculi.
We propose that investigation of low phosphate/calcium levels should always include an assessment of time spent outdoors and vitamin D status.