Page 928 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

Endocrine system 903

VetBooks.ir T , T is deiodinated to T (or reverse T ) in blood lethargy or work intolerance and alterations in hair
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coat. Horses with experimentally-induced hypothy-
and tissues. T is much more active than T , while
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reverse T has very little activity.
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Alterations in circulating concentrations of thy- roidism have a decreased basal heart rate, cardiac
output, respiratory rate and rectal temperature, and
roid hormones can be mediated centrally by altera- increased serum concentrations of triglycerides, cho-
tions in TRH (tertiary hypothyroidism) or TSH lesterol and very low-density lipoproteins (VLDLs).
(secondary hypothyroidism), by alterations in thy- However, these changes are mild and do not result
roid gland function itself (primary hypothyroid- in resting values clearly outside reference intervals.
ism) or by alterations in peripheral thyroid hormone Mares with experimentally-induced hypothyroidism
metabolism or binding. Primary hypothyroidism have oestrous cycles, can become pregnant and carry
can be caused by interference with iodine uptake by foals to term.
the thyroid gland or by interference with thyroid Hypothyroidism has been suggested as a contrib-
hormone synthesis or release. Iodine deficiency or uting cause to several problems in horses includ-
ingestion of goitrogens (e.g. sulphurated organic ing laminitis, chronic myositis, reduced fertility in
compounds, thiocyanates and isothiocyanates) broodmares and anhidrosis. Documentation for these
causes goitre (defined as an enlarged thyroid gland) claims is currently lacking or incomplete. A recent
in humans and in horses. Selenium deficiency and study of thyroid function in anhidrotic horses dem-
mustard have been suggested as potential dietary onstrated normal serum thyroid hormone concentra-
goitrogens in the horse. Excessive iodine ingestion tions and responses to TRH. However, serum TSH
by the mare (e.g. kelp supplements) can cause goitre response to TRH was mildly enhanced, suggesting
and hypothyroidism in neonatal foals. that there may be some degree of subclinical hypo-
Certain drugs and various physiological or patho- thyroidism, or at least an altered sensitivity of the
logical states can alter thyroid hormone synthesis, thyroid gland to TSH.
metabolism or binding, resulting in altered serum
concentrations of thyroid hormone. In the horse, Differential diagnosis
these include phenylbutazone and other drugs, fast- Differential diagnoses for hypothyroidism with goi-
ing, climate, fitness, iodine intake and strenuous tre in adult horses include a benign thyroid tumour
exercise. Diets high in energy, protein, zinc and cop- or other causes of swelling in the upper neck such as
per have also affected circulating concentrations of an enlarged salivary gland or lymph node.
thyroid hormones. Non-thyroidal illness syndrome
(euthyroid sick syndrome) has been described in the Diagnosis
horse and is similar to the syndrome in other species. Certain drugs and pathophysiological states can
lower serum concentrations of thyroid hormones in
Clinical presentation otherwise euthyroid horses. Resting T and T con-
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Clinical signs of hypothyroidism in adult horses are centrations may not reflect an animal’s actual thy-
not well defined. Overweight horses that are ‘easy roid functional status and therefore it is important
keepers’, with cresty necks, abnormal fat pads and to perform thyroid stimulation tests before a true
a predisposition to recurrent laminitis were once assessment of hypothyroidism can be made. Ideally
described as hypothyroid, probably because clini- any measure of a horse’s thyroid function is made
cians observed clinical improvement when thyrox- at least 4 weeks after thyroid hormone supplementa-
ine was administered. However, thyroid function tion has been terminated.
tests in these horses are invariably normal. It is now Tests that are currently available for the assess-
recognised that horses with the above phenotype ment of thyroid function in the horse include mea-
suffer from insulin dysregulation or pituitary pars surement of total and free fractions of T and T
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intermedia dysfunction (PPID) (equine Cushing’s and response of these hormones to administration
disease) or equine metabolic syndrome. of either TRH or TSH. TRH or TSH stimulation
Clinical signs in horses properly documented to tests are considered essential to document hypothy-
be hypothyroid in published case reports include roidism as baseline thyroid hormone concentrations