Endocrine Abstracts

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 (2.0–2.6) as was parathormone (2.2 pmol/l (1.1–6.9)). Twenty-four hour urine calcium excretion was elevated at 10.8 mmol per 24 h (normal range 2.5–7.5) 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 (0.8–1.4). By mid-summer, there was complete normalisation of phosphate levels (0.83 mm mol/l (0.8–1.4)) 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.