YH 454 combined pdf

Your horse’s care
horses insIde out l endocrine system
Thyroid hormones are
involved in many
metabolic and
growth processes in
all types of horses
Parathyroid gland In many species of
animals, the parathyroid glands are tiny
glands located next to the thyroid glands
on each side of the neck — if you can find
the thyroid gland, you can find the
parathyroid. In horses, however, there are
four parathyroid glands — two upper glands
located anywhere around the thyroid glands,
and two lower glands, which may be
anywhere along the jugular groove from the
jaw to the shoulder (and, indeed, even just
inside the chest cavity).
This variability makes it very difficult to
locate the normal parathyroid glands in the
horse. However, there are techniques to
locate the glands when they are abnormal.
The parathyroid produces parathyroid
hormone from chief cells, which regulates
calcium levels by increasing the blood
calcium concentration. If calcium levels drop,
then parathyroid hormone levels increase,
and the body removes calcium from bones,
absorbs more from the intestines and
excretes less in urine, resulting in increased
blood calcium levels. This is partly due to the
parathyroid hormone leading to increased
levels of active vitamin D.
This is a really important function, as
calcium is vitally important for muscle
contraction (with the most important muscle
being the heart). When the calcium levels
rise again, the amount of parathyroid
hormone produced decreases.
Sex organs The main hormone produced
by the male testes is testosterone, which
is responsible for the distinguishing
characteristics of the masculine body.
Many of the results of this will be very
familiar to anyone who has worked with
stallions, but some of the effects are less
obvious. For example, testosterone also
leads to an increase in the number of red
blood cells present in the blood.
The female ovarian hormones are
oestrogen and progesterone and are involved
in the normal reproductive cycle of the mare.
Pancreas The pancreas lies next to the
first part of the small intestine and serves
two major functions. First, to secrete
digestive enzymes into the small intestine,
and second, to produce two important
hormones — insulin and glucagon — which
regulate glucose, lipid and protein
metabolism. Insulin is more commonly
measured and talked about in horses, due
to the fact that high levels of insulin lead
to the development of laminitis.
Obese horses with hyperinsulinaemia and
laminitis have a disease termed equine
metabolic syndrome (EMS). Insulin promotes
Adrenaline is released
as part of the ‘fight
or flight’ response
the uptake of glucose into cells, but in horses
with EMS, the cells stop responding to
insulin (insulin dysregulation), resulting in
increased production of insulin.
“The two hormones
produced by the thyroid gland
increase the metabolic rate”
Adrenal gland The adrenal gland sits
just next to the kidneys and consists of
two distinct parts — the adrenal medulla
and the adrenal cortex. The adrenal
medulla is the central part, which is
involved in the “fight or flight” stress
response by producing adrenaline. Around
this is the adrenal cortex, which produces
various corticosteroids.
Corticosteroids are involved in electrolyte
balance (primarily sodium and potassium)
and metabolism of carbohydrates, proteins
and fats. Small amounts of androgenic
hormones are produced that exhibit the
same effects on the body as the male sex
hormone testosterone.
Thyroid gland The thyroid glands are
located on either side of the windpipe,
just behind the horse’s larynx. They
secrete two hormones, thyroxine and
triiodothyronine (commonly referred to as
T4 and T3 respectively) under the control of
the pituitary gland. These hormones
increase metabolic rate and are therefore
involved in a large number of metabolic and
growth processes. They are especially
important in the growth of foals, with levels
being much higher in foals up to three to
four months of age and declining in adults.
The thyroid gland is also involved in
calcium regulation via the hormone
calcitonin, which decreases calcium levels.
The pancreas is important
for your horse’s metabolism
and the regulation of
insulin levels when eating
Playing a part in laminitis
Our understanding of endocrinopathic
laminitis has grown substantially in
recent years. We know the majority of
laminitis cases are caused by either
Pituitary Pars Intermedia Dysfunction
(PPID) or equine metabolic syndrome
(EMS). PPID occurs due to enlargement
of the pars intermedia portion of the
pituitary gland, and results in increased
levels of the hormone ACTH. Clinical
signs include weight loss, a longer coat,
increased drinking and urination,
sweating, lethargy and laminitis.
Diagnosis
Diagnosis involves blood testing for
ACTH levels, either at baseline, or
following the administration of a
hormone to stimulate the pituitary
gland (TRH stimulation test).
Treatment with pergolide results
in decreased ACTH levels and
improvement, or resolution of clinical
signs. Each horse will require a different
dose, so usual practice is to start
treatment at a low dose and gradually
increase if blood tests show ACTH
levels remain high.
EMS involves the presence of obesity
(or regional adiposity), peripheral insulin
resistance or dysregulation, and an
increased risk (or presence) of laminitis.
Diagnosis is based on identifying these
three factors and so requires blood
testing to demonstrate increased insulin
levels, or increased insulin in response to
feed/sugar challenge.
Treatment
Treatment centres on achieving weight
loss and increasing exercise, as this is the
best way to improve insulin sensitivity.
However, in horses and ponies with
active laminitis, the requirement for rest
rules out exercise. This is when we may
reach for medical therapy to assist us,
until exercise can resume.
Medications used include metformin
(which improves peripheral insulin
sensitivity in humans and decreases
intestinal absorption of glucose in horses),
or levothyrozine (which acts as thyroid
hormones do to increase metabolic rate).
In horses with
active laminitis, the
requirement for rest
rules out exercise
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August 2019 your horse 91