Two forms of vitamin D are measurable in veterinary serum samples: 25-hydroxyvitamin D (25OHD) and 1,25-dihydroxyvitamin D (calcitriol).

25OHD is produced by the liver and its concentration parallels that of available vitamin D that may be dietary in origin (cholecalciferol (D3), ergocalciferol (D2)) or produced in the skin of certain animals under the influence of UV light. Because the enzymatic hydroxylation of vitamin D in the liver depends almost entirely on the availability of the substrate, this test is an excellent marker of overall Vitamin D status. It can be used to diagnose both conditions of vitamin D deficiency and excess and therefore is valuable in the investigation of both hyper- and hypo-calcaemic disorders. 25OHD is also the analyte of choice in verifying adequacy of UV light exposure in reptilian species. Other situations in which we can expect abnormal 25OHD results would include dietary deficiencies, malabsorption syndromes and calciferol-rodenticide toxicities.

Calcitriol is produced by renal tubular cells as a result of enzymatic action (1α-hydroxylase) on 25OHD substrate. The rate of this process is controlled by PTH concentrations (increasing PTH causes increased 1α-hydroxylase activity). Calcitriol is the most biologically potent form of vitamin D and its principle activities are directed at increasing serum calcium concentrations, including increased intestinal uptake of calcium. The failure of renal tubular cells to generate calcitriol in renal disease is one of the contributing mechanisms to renal secondary hyperparathyroidism (and “rubber jaw”). The measurement of calcitriol may be of some value in understanding, renal tubular function and the effects of PTH on the vitamin D system.

In dogs, cats and most veterinary species, very little passive intestinal absorption of calcium occurs. In these species, intestinal calcium absorption is almost exclusively mediated by vitamin D. The situation is different in horses, rabbits (and the hippopotamus!), in which there is significantly more passive absorption of calcium and calcium status is controlled more by renal losses. In these species, the effects of renal disease on calcium status will be different than in most common mammalian species.

Recent research has suggested that low concentrations of 25OHD may be a risk factor for congestive heart failure (CHF) in dogs. There is extensive evidence that Vitamin D has cardioprotective actions by suppressing the renin-angiotensin-aldosterone system, decreasing myocardial hypertrophy, inhibiting pro-imflammatory cytokines and improving endothelial dysfunction and atherosclerosis. (Kraus, Rassnick, Wakshlag et al, J Vet Intern Med, 2014, 28, 109 – 115)