Page 927 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

902 CHAPTER 6

VetBooks.ir be noted. When examining a foal, the musculoskel- insufficiency is suspected. Measurement of urine
concentrations of neurotransmitters or hormones
etal system, particularly the carpus, tarsus and jaw,
may be useful in certain cases (e.g. urinary cat-
should be evaluated carefully.
echolamines in the diagnosis of phaeochromocy-
Haematology toma) but these tests are not commonly performed
A complete blood count should be submitted to rule or readily available.
out infectious diseases and may yield subtle changes
compatible with an endocrine disorder. A serum bio- Hormone analysis
chemistry should also be submitted to rule out an Measurement of serum concentrations of hormones
organ system disease that could be responsible for and response of these hormones to evocative test-
clinical signs and can show specific changes typi- ing plays an important role in the diagnosis of
cal for some endocrine diseases. Although not part endocrine diseases. Values of various hormones in
of many routine large animal biochemistry panels, normal healthy horses have been published, but it is
measurement of triglycerides and cholesterol may important to realise that reference intervals will vary
help support a diagnosis of an endocrine disease. slightly among different laboratories using different
assay techniques in different locations.
Urinalysis Hormones have a circadian or seasonal variation
A standard urinalysis can yield abnormal find- and therefore it is important for clinicians to provide
ings only suggestive of an endocrine disorder (e.g. as much information as possible to the laboratories
hyposthenuria or isosthenuria). Measurement of to which they are sending samples for analysis. In
urinary fractional excretion ratios can also be addition, it is important to make certain that the
of interest in some endocrine disorders, particularly laboratories have validated their assay techniques for
when secondary hyperparathyroidism or adrenal horses and supplied adapted reference intervals.

DISEASES OF THE THYROID AND PARATHYROID GLANDS

THYROID DISEASE IN THE ADULT HORSE gland tumours are benign, non-functional adenomas
although C-cell tumours and carcinomas have also
Definition/overview been reported. Serum thyroid hormone concentra-
The hormones produced by the thyroid glands are tions in most horses with thyroid gland tumours are
essential for proper maintenance of resting meta- within reference intervals.
bolic rate and energy metabolism. Thyroid function The thyroid traps and concentrates iodide,
plays an important role in organ growth and matu- which is oxidised and bound to tyrosine, either as
ration. Both hypothyroidism and hyperthyroidism monoiodotyrosine or as diiodotyrosine. Thyroid
have been described in the horse, but true thyroid hormones, primarily thyroxine (T ), are stored
4
gland dysfunction is extremely rare. Thyroid hor- within the gland bound to thyroglobulin and are
mone administration in adult horses is common. released when stimulated by pituitary thyroid-
However, it is given as a pharmacological agent to stimulating hormone (TSH), which is, in turn,
hasten weight loss and treat the effects of equine released when stimulated by thyrotropin-releasing
metabolic syndrome, not as a replacement medica- hormone (TRH) that originates in the hypothala-
tion because those animals have been diagnosed as mus. Control of hormone secretion occurs at the
hypothyroid. level of the hypothalamus, anterior pituitary gland
and the thyroid itself. In response to TSH, T and tri-
4
Aetiology/pathophysiology iodothyronine (T ) are released into the circulation,
3
True hypothyroidism in adult horses has not been where they bind to plasma-binding proteins. Only
well documented. Hypothyroidism resulting from free (unbound) thyroid hormones are metabolically
thyroid neoplasia has been described. Most thyroid active. While the thyroid gland secretes primarily