Vitamin D

Vitamin D In Dogs & Cats: Uses, Dosage and Side Effects

Overview

• Alongside other hormones like calcitonin and parathormone, vitamin D regulates calcium homeostasis through various intricate mechanisms.
• These mechanisms include the accumulation of calcium in bone reserves and the absorption of calcium from dietary sources.

Uses of Vitamin D

• Chronic treatment of hypocalcemia linked with low parathyroid hormone levels is often seen in conditions like immune-mediated hypoparathyroidism and iatrogenic hyperparathyroidism following thyroid surgery.
• Calcitriol is utilized in managing renal secondary hyperparathyroidism, where it helps decrease serum parathyroid hormone levels.

Dose of Vitamin D in Dogs and Cats

Dogs:

1. Hypocalcaemia/vitamin D deficiency:

• calcitriol: 10–15 ng (nanograms)/kg p.o. q12h for 3–4 days, then decrease to 2.5–7.5 ng/kg p.o. q12h for maintenance; alfacalcidol: 0.01-0.03 μg (micrograms)/kg p.o. q24h and thereafter titrated to effect.

2. Renal secondary hyperparathyroidism:

• calcitriol: 1.5–3.5 ng/kg p.o. q24h; some authors recommend higher doses of up to
  6 ng/kg/day if there is refractory hyperparathyroidism and ionized serum calcium concentrations can be assessed.
• Assess serum calcium and phosphate levels serially and maintain total calcium × phosphate product below 4.2 (calcium and phosphate in mmol/l).
• Do not use it if this is not possible.

Cats:

1. Hypocalcaemia:

• calcitriol: 10–15 ng (nanograms)/kg/day p.o. q12h for 3–4 days, then decrease to 2.5–7.5 ng/kg p.o. q12h for maintenance; alfacalcidol: 0.01-0.03 μg (micrograms)/kg p.o. q24h and thereafter titrated to effect.

2. Renal secondary hyperparathyroidism:

• calcitriol:2.5–3.5 ng/kg/day p.o.

Side Effects of Vitamin D in Dogs and Cats

• Hypercalcaemia, hyperphosphataemia.

Contraindications of Vitamin D in Dogs and Cats

• Do not use in patients with hyperphosphataemia or malabsorption syndromes.
• Do not use it in pregnant animals.

Some Notes:

• Corticosteroids can counteract the effects of vitamin D supplements.
• Sucralfate reduces the absorption of vitamin D.
• Drugs that induce hepatic enzyme systems, such as barbiturates, can increase the metabolism of vitamin D, necessitating higher doses for efficacy.
• Magnesium- or calcium-containing antacids used alongside vitamin D can lead to hypermagnesemia or hypercalcemia.
• Thiazide diuretics may also cause hypercalcemia when used concurrently with vitamin D.
• Hypercalcemia can enhance the toxic effects of medications like verapamil or digoxin, so close monitoring is essential.
• Calcitriol and alfacalcidol (1-alpha-hydroxy cholecalciferol) have a quick onset of action (1–2 days) and a short half-life (<1 day), making them the preferred forms for use.
• Vitamin D2 (ergocalciferol) has a slow onset of action and limited utility in dogs and cats.
• Vitamin D requires two hydroxylations (one in the liver and the other in the kidney) to become active. Therefore, only the active form (calcitriol) should be used in patients with renal failure.
• Vitamin D has a narrow therapeutic range, and toxic doses can be easily reached. Close and frequent monitoring of serum calcium, preferably ionized calcium concentrations, is necessary.
• Avoid using formulations of vitamins A, D3, and E authorized for farm animals, as they are too concentrated for small animal use.