Hypothyroidism is the most common
endocrine disorder in dogs. Dogs diagnosed with hypothyroidism are
usually between 4 and 10 years old. Since both hypothyroidism and diabetes
is common in older dogs, it is not unusual to
have both a diabetic and hypothyroid dog. Hypothyroidism is rare in cats and
is usually the result of a thyroidectomy performed to treat hyperthyroidism. The
information presented here is for dogs.
Hypothyroidism occurs when the thyroid gland, a small double-lobed gland in the neck,
produces abnormally low levels of thyroid hormones. The thyroid gland produces two
hormones, thyroxine (T4) and triiodithyronine (T3).
Hypothyroidism is most commonly due to destruction of the thyroid gland
either from an immune-mediated process where lymphocytes of the immune
system cause inflammation of the thyroid (lymphocytic thyroiditis), or
Less commonly, hypothyroidism can be due to cancer of the thyroid gland.
It can also be due to dysfunction of the pituitary gland. The
pituitary gland secretes thyroid-stimulating hormone (TSH) which in turn
causes the thyroid to produce thyroid hormone. Other illnesses,
glucocorticoids, and malnutrition can also cause decreased TSH levels,
which would lead to hypothyroidism.
Congenital hypothyroidism causes dwarfism. This is because thyroid hormone is
necessary for normal development of the skeletal and central nervous
Signs develop slowly, effect several body systems, and can vary
depending on the individual.
Changes in skin and
- General changes:
- mental dullness
- exercise intolerance or unwillingness to exercise
- tendency to gain weight without a corresponding increase in food
Changes in the
- symmetrical hair loss beginning with the tail ("rat tail")
and spreading over the rest of the body
- dry, dull coat
- easily epilated hair (hair is easy to pull out)
- slow regrowth of hair
- change in coat color
- oily skin (seborrhea), thick, cool, puffy skin
- poor wound healing and easy bruising
Changes in other
- shortened estrus in females,
- lack of libido, low sperm count, testicular atrophy in males
- slow heart rate, constipation, regurgitation
The physical signs of hypothyroidism can be caused by many diseases, so
a complete history, physical findings, and blood tests are needed.
There are several blood tests available, but some are very expensive,
not readily available, or are inconclusive. Decreased T3 and T4
levels can also be caused by other diseases, so a thorough examination
is important. Typically, serum cholesterol and thyroid hormone
concentrations are measured. Since about 65-75% of hypothyroid
dogs also have high serum cholesterol levels, low serum thyroid hormone
levels in combination with increased blood cholesterol levels and
appropriate clinical signs supports a diagnosis of hypothyroidism.
Hypothyroidism is treated by giving a
pill containing synthetic thyroid hormone - Levothyroxine
Sodium. The drug is available under the trade names Soloxine,
Thyro-Form, and Synthroid.
There are differences in how readily the different brands are absorbed, so
it is recommended to use one product and not change brands
Response to thyroid hormone is gradual, with some signs such as activity and
attitude improving as soon as 7-10 days after treatment begins. Other
signs, such as improvement in skin and coat condition may take 6-8
weeks. Thyroid hormone levels are reevaluated periodically by a blood
test and the dose may need to be adjusted.
Since both hypothyroidism and diabetes are common in dogs it is often necessary to
deal with both conditions.
Diabetics should be started on a lower than
normal dose of thyroid hormone. Thyroid hormone therapy
causes dramatic changes in the metabolic rate, therefore insulin
requirements may change and the diabetes must be monitored closely.
Many owners report having to decrease their dog's insulin dose after
thyroid hormone therapy was started. Dramatic changes in the
metabolic rate and metabolic processes occur with thyroid hormone therapy,
so insulin therapy often needs to be adjusted. Some owners have
noticed that as the hypothyroidism is brought under control, the need for
insulin decreases. One possible explanation for this is that
hypothyroidism in dogs causes insulin resistance. When the synthetic
thyroid hormone is given and the thyroid levels are nearer normal, the insulin
resistance begins to resolve and thus the higher insulin dosages which were needed
before treatment are no longer required.
Signs of thyroid hormone overdose are similar to uncontrolled diabetes and
- weight loss
- weakness and fatigue
- seeking cool areas
- increased heart rate, and
can also occur.
Although each individual is unique, these experiences may help you
understand how others have dealt with diabetic and hypothyroid dogs.
||Udi is both diabetic and hypothyroid, and close monitoring of his BG levels as I increased or
decreased his soloxine dosages showed an increased need for insulin when
his soloxine dosage was lowered, and decreased need for insulin as his
dosage was increased.
is diabetic, hypothyroid, and has Cushing's syndrome. You can read how
Marilyn manages Kiri's three diseases.
- Pocket Companion to the Fourth Edition of Textbook of
Veterinary Internal Medicine Stephen J. Ettinger.
- The U.C. Davis Book of Dogs: A complete
medical reference guide for dogs and puppies. Mordecai Siegal (Editor), et
- The 5 Minute Veterinary Consult: Canine and Feline. Larry
Tilly and Francis W.K. Smith, Jr. 1997.
- Veterinary Drug Handbook. Second Edition. Donald
C. Plumb. 1995.
Updated August 2001
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