Page 932 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

Endocrine system 907

VetBooks.ir 6.5 6.6

Fig. 6.5 Horse with thyroid adenoma causing an
enlargement of the gland. Note how the profile under
the throat latch is distorted.

Fig. 6.6 Advanced fibrous osteodystrophy (big head)
in a horse with primary hyperparathyroidism due to a
PTH-secreting tumour.

hormone concentrations are also sometimes tempo- and lactating mares are more susceptible. Primary
rarily increased in horses exposed to excess iodine, hyperparathyroidism has also been reported in scat-
such as in a topical blister. Hyperthyroidism may tered case reports of parathyroid gland adenoma
be caused by deliberate or accidental overdose of (Fig. 6.6). Clinical signs and results of diagnostic
oral thyroxine administration. Horses with iatro- tests in cases of primary hyperparathyroidism are
genic hyperthyroidism may appear nervous and have similar to those of secondary hyperparathyroidism.
tachycardia.
Horses with hyperthyroidism secondary to thy- Aetiology/pathophysiology
roid gland neoplasia have been successfully treated As the name implies, nutritional secondary hyper-
with thyroidectomy or hemithyroidectomy, fol- parathyroidism is caused by an improper diet
lowed by replacement hormone therapy (Fig. 6.5). (deficient in calcium or excessive in phosphorous).
Iatrogenic hyperthyroidism can be cured by ceasing Ingestion of oxalate-containing plants that bind
or administering the proper amount of the hormone calcium in the gastrointestinal (GI) tract can also
supplement. cause this disease by decreasing calcium absorption.
Specific diets that can cause nutritional secondary
PARATHYROID DISEASE hyperparathyroidism are listed in Table 6.3.
Low dietary calcium or high phosphorous leads
Definition/overview to increased parathyroid hormone (PTH) release
Both hyper- and hypoparathyroidism have been from the parathyroid gland and increased active
described in horses. Nutritional secondary hyper- vitamin D. The combination of increased parathy-
parathyroidism is characterised by bone demin- roid hormone and vitamin D promotes calcium and
eralisation caused by an imbalance of calcium and phosphorous resorption from the GI tract and bone,
phosphorous in the diet. Young, growing horses keeping serum calcium in the normal range. Serum