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Name
Hypothyroidism, Canine
Short Description
Hypothyroidism
Affected Animals: Generally, hypothyroidism affects middle-aged
dogs between the ages of two and six years. Any dog can develop
hypothyroidism, but boxers, cocker spaniels, golden retrievers,
miniature schnauzers, Old English sheepdogs, poodles, Shetland
sheepdogs, and Doberman pinschers have a higher incidence of the
disease. Very rarely, cats may develop the disease as well.
Overview: The thyroid gland maintains the body's
metabolism. When there is a decrease in levels of the thyroid
hormones, the dog's body essentially "slows down" because of a lower
cellular metabolic rate. As a result, the body will begin to lose
its ability to function properly. The signs of slowed function vary
and occur gradually, but tend to include drowsiness, weight gain,
personality changes such as depression, and an inability or
unwillingness to exercise. The animal's skin also may be affected,
and patches of hair loss and darkened pigmentation are not uncommon.
In more severe cases, the heart rate may slow and the dog may have
neuromuscular complications such as seizures.
Generally, dogs show signs of hypothyroidism when they are in middle
age, between the ages of two and six years, although some animals
are affected earlier. Thyroid hormone replacement medications
usually are effective in regulating the dog's metabolism and
improving its quality of life, but when the hypothyroidism is a
result of a damaged pituitary gland or cancer, additional
complications may occur
Clinical Signs: Clinical signs vary and are gradual in onset, but
typically include: lethargy, listlessness, or drowsiness; mental
depression; exercise intolerance; weight gain; personality change;
seeking out warm places; and infertility in some breeding animals.
Additional symptoms include hair loss on the tail and along the
trunk or body, resulting in a "rat tail" appearance; dry or oily
skin associated with hyperpigmentation, or darkened skin; and
thickening of the skin. The cardiovascular system can develop
bradycardia, which is a decreased heart rate. Neuromuscular signs
may include seizures; ataxia or stumbling; circling; knuckling or
dragging the feet that results in the dorsal, or top, part of the
nails being worn down; and forelimb lameness.
Symptoms:
See clinical signs.
Description:
The thyroid gland is responsible for cellular
metabolism in the body. A decrease in the thyroid hormones causes a
lower cellular metabolic rate and this affects the body's ability to
function properly.
There are two types of hypothyroidism that can affect dogs. The
first is called primary or acquired hypothyroidism, which is the
most common form of this disorder. Primary hypothyroidism occurs by
either one of two methods: when the thyroid gland is attacked by the
body's immune system, or when the thyroid gland atrophies, or wastes
away, for unknown reasons. The end result in both cases is the
inability of the thyroid gland to secrete important hormones that
regulate the body's metabolism.
The second type of hypothyroidism is called secondary
hypothyroidism, which involves the destruction or malfunction of the
pituitary gland in the brainstem. A poorly functioning pituitary
gland is unable to produce the TSH hormone that stimulates the
thyroid gland's production of the thyroid hormones. When TSH is not
produced, neither will the thyroid hormones.
Diagnosis: The most common way to diagnose hypothyroidism is
to measure baseline blood levels of the thyroid hormone called T4.
Decreased hormone levels are indicative of hypothyroidism; however,
other non-related diseases and medications will also lower levels of
this hormone. When clinical signs exist together with a low T4
level, a definitive diagnosis of hypothyroidism usually can be made.
Commonly, the veterinarian will order a CBC, or complete blood
count, as well as a serum chemistry panel to look for changes
consistent with hypothyroidism. More specialized tests, such as a
complete thyroid hormone panel, may be needed if a normal T4 is
found and the dog has clinical signs of hypothyroidism.
Prognosis: Dogs with primary hypothyroidism that are being
treated appropriately with thyroid hormone supplementation have a
good prognosis and are expected to have a normal life span. Dogs
with secondary hypothyroidism have a guarded to poor prognosis
because of the possibility of cancer or destruction to the pituitary
gland which is located in the brain stem.
Transmission or Cause: Causes of hypothyroidism include congenital
disorders, cancer, infection, and drugs such as glucocorticoids.
Treatment: Treatment involves synthetic thyroid hormone
supplementation with medications such as sodium levothyroxine. Oral
supplementation generally will be given daily for the remainder of
the dog's life. Frequent re-examinations by the veterinarian will be
needed to evaluate the blood levels of the thyroid hormone to be
sure that the appropriate dose is being given. When too much hormone
supplementation is given, a condition called thyrotoxicosis can
occur. This condition, which is quite uncommon, may result in
clinical signs such as panting, nervousness or anxiety, weight loss,
diarrhea, increased drinking and increased urinating, aggressive
behavior, and increased appetite.
Prevention: Prevention of complications of hypothyroidism
includes lifelong supplementation of synthetic thyroid hormone and
periodic re-examinations by a veterinarian to check the blood levels
of the hormone.
