J. Catharine R. Scott-Moncrieff
Hypothyroidism is the clinical condition caused by
thyroid hormone deficiency. The thyroid hormones, thyroxine (T4)
and triiodothyronine (T3), are produced by the thyroid glands
located in the neck on either side of the trachea. Thyroid hormones
influence the metabolism of most of the organs in the body. Deficiency
of thyroid hormone results in a decreased metabolic rate, which may
cause a wide variety of clinical signs. "Decrease in metabolic
rate" means that the speed at which cells function or
"work" slows down. For example, the heart rate slows, mental
function slows, and body temperature decreases. Decreased secretion of
thyroid hormones by the thyroid gland may be due to inflammation
(thyroiditis) or progressive failure (atrophy) of the thyroid glands. In
rare cases, thyroid tumors may cause hypothyroidism.
Hypothyroidism is the most common endocrine disease
of the dog. Breeds that are predisposed to developing hypothyroidism
include the golden retriever and the Doberman pinscher, but any breed of
dog may be affected.
The most common clinical signs of hypothyroidism are
weight gain, cold intolerance, lethargy, and a variety of skin problems.
The most common skin abnormalities include hair loss, changes in hair
color and quality, and predisposition to skin infections. Other less
common clinical signs include abnormalities of the reproductive and
nervous systems.
A diagnosis of hypothyroidism is mad by measurement
of thyroid hormone concentrations (T3, T4, and
free T4) in the blood. If thyroid hormone concentrations are
low, other tests may be performed to determine whether the decrease is
due to a thyroid gland problem or to the effects of other diseases or
medications. These additional tests may include measurement of
thyroid-stimulating hormone (TSH) and measurement of a variety of
antithyroid antibodies. In some cases, it is
necessary to use trial therapy with thyroid hormone
supplementation to confirm the diagnosis of hypothyroidism.
Fortunately, hypothyroidism is a disease that is
easily treated. Treatment involves daily or twice-daily oral medication
with synthetic thyroxine. Treatment is usually started with two
treatments per day, one in the morning and one in the evening. When the
clinical signs have resolved, the treatment can be reduced to one dose
per day in many dogs. In most cases, treatment is required for the life
of the dog.
It may take several weeks for the clinical signs of
hypothyroidism to resolve. An increase in activity level is usually
observed after 1 to 2 weeks of treatment. It usually takes longer for
skin problems to resolve, but improvement should be observed within 6 to
8 weeks. In some cases the skin may actually appear worse for the first
1 to 2 weeks of treatment as the old haircoat is shed. If a dog was
experiencing reproductive or neurologic problems, it may take several
months of treatment for complete resolution of the problems.
Clinical signs of excessive supplementation with
thyroid hormone include nervousness, weight loss, and increased drinking
or increased urination. These signs are rare, but if they occur it is
important to call your veterinarian for adjustment of the dose.
In order to establish that the dose of thyroid
hormone supplementation is appropriate, it is recommended that blood
samples be collected for measurement of thyroid hormone concentrations 1
to 2 months after the start of treatment. The results of these tests are
used to adjust the dose of thyroid hormone supplementation. Then it is
usually necessary to measure thyroid hormone concentrations only once a
year or if clinical signs of hypothyroidism recur.
