Nutritional secondary hyperparathyroidismis a metabolic disorder that predominantly affects growing puppies and kittens which are fed with diets that do not contain enough calcium or that contain excessive amounts of phosphorus together with normal or low levels of calcium. These animals are usually fed with unbalanced homemade diets often composed exclusively of meat and/or animal offal (“all meat syndrome”).
A chronic lack in the supply or reduction of absorption of calcium causes a state of hypocalcaemia and, consequently, an increase in the production of parathyroid hormone whose function is to restore the extracellular concentrations of calcium, promoting the reabsorption of the ion in the kidneys, bowel (increasing the synthesis of calcitriol) and bone and, on the other hand, favouring the renal excretion of phosphorus.
In rapidly growing animals whose nutritional requirements of calcium are particularly high, if the nutritional imbalance is protracted, the induced state of hyperparathyroidism causes progressive demineralisation of bone and the development of clinical signs.
SIGNALMENT
Nutritional secondary hyperparathyroidism most frequently affects dogs, particularly those of large breeds, and cats during the growth phase. It can also occasionally occur in adult animals which are fed for long periods with calcium-poor diets.
CLINICAL SIGNS
The clinical signs in puppies with this disorder are caused by the progressive bone demineralisation and include reluctance to move, bone tenderness, lameness, pathological fractures and, in the most severe cases, paralysis because of compression of the spinal cord due to vertebral body fractures. The abdomen of an affected animal can be disproportionately broad because the bone segments grow less than the soft tissues. Radiological examinations may show thinning of cortical bone, wide bone marrow cavities, greenstick fractures and thin metaphyseal edges in which there may be compression fractures (Fig. 1 [3]). In adult animals the disorder is manifested principally by teeth dropping out because of reabsorption of the bone socket.
DIAGNOSIS
Blood-chemistry examinations are usually not of great diagnostic help in confirming nutritional secondary hyperparathyroidism because in most cases the calcium concentrations remain within the norm. A possible confirmation can be obtained from the finding of increased levels of parathyroid hormone and 1,25-(OH)2D (calcitriol); however, in clinical practice, the history, (focused on the diet) and X-rays are usually sufficient to confirm the diagnosis of nutritional secondary hyperparathyroidism.
In young animals the differential diagnoses to consider are vitamin D deficiency and congenital metabolic diseases, including osteogenesis imperfecta and mucopolysaccharidosis. In adult animals, instead, the secondary form of hyperparathyroidism due to chronic kidney disease and periodontal disorders must be excluded.
TREATMENT AND PROGNOSIS
The treatment of nutritional secondary hyperparathyroidism consists essentially of correction of the diet of the animal by using balanced, commercially available diets for dogs and cats such that between 0.8% and 1.2% of the dry weight is calcium and the calcium-to-phosphorus ratio is between 1.1:1 and 2:1. In the first few weeks this diet can be integrated with a calcium carbonate supplement (50 mg/kg per day, administered in several doses) in order to accelerate bone mineralisation. The improvements are usually already visible within 3 to 4 weeks of changing the diet.
For those animals with bone fractures, surgical correction is not advised, at least initially, and the management is limited to preventing new fractures through attentive care and management of the animal.
The prognosis of animals with nutritional secondary hyperparathyroidism depends mainly on the presence and severity of any pathological fractures; animals with vertebral compression and fractures potentially have a worse prognosis if bone marrow damage has already occurred.
Suggested readings
- Feldman E.C., Nelson R.W. : Canine Hypercalcemia and Primary Hyperparathyroidism. In: Bonagura J.D., Twedt D.C. : Kirk’s Current Veterinary Therapy XIV, Sounders Elsevier, pg. 247-251, 2009.
- Towell T.L.: Nutrition-Related Skeletal Disorders. In: In: Ettinger S.J., Feldman E.C.: Textbook of Veterinary Internal Medicine, VII edizione Elsevier Saunders, pg. 1722-1751, 2010.
- Tryfonidou M.A., Hazewinkel H.A.W. : Calciotropic Hormones. In: Kooistra H. S.: Clinical Endocrinolgy of Dogs and Cats, II edizione Schlutesche, pg. 253-295, 2010.