| HYPOTHYROIDISM IN THE DOG
Canine hypothyroidism seems to be a topic of conversation within all
kinds of dog owning groups. It is no more prevalent in show dogs,
agility dogs, hunting dogs or house pets than any other demographic.
Situated on both sides of the first five to eight tracheal rings, the
thyroid gland secretes into the blood stream two major types of
thyroid hormones. These active hormones are called T3 and T4. Think of
the thyroid gland as the body's thermostat . . . if it is set too high,
too much T3 and T4 are secreted and the dog's metabolism burns too
hotly. The dog may become hyperexciteable and restless, pant and lose
weight. Hyperthyroidism in dogs is rare; in older cats it is
If the thermostat is set too low, subnormal amounts of T3 and T4 are
secreted into the blood stream and the dog's metabolic fire is too
cool. You can see that if a dog is hypothyroid the signs will be
lethargy, weight gain, poor skin and coat condition, shivering and poor
stamina. Many hypothyroid dogs become "watchers" rather than
"doers"... they have little interest in engaging in common daily
Not every dog
will show ALL the signs, of course. Some will display only one or two
of these signs and even then, in borderline cases, some dogs show only
very subtle changes from normal. Hypothyroidism is very rare
in cats but can occur if treatment for hyperthyroidism to reduce
thyroid function isn't monitored closely.
Control of Thyroid Function - The
thyroid gland's activity is directed by outside factors such as the
pituitary gland located at the base of the brain and the hypothalamus,
another area deep in the brain. As the dog's metabolism changes
due to changes in activity, nutrition, environmental temperature,
stress, etc., the need for more or less T3 and T4 is detected by the
hypothalamus and pituitary gland - they then send signals to the
thyroid to "hurry up" or "slow down". The brain signals to
the thyroid gland via production of TSH (Thyroid Stimulating Hormone)
in the pituitary gland at the base of the brain. The thyroid
secretes more or less hormones depending on the signals from TSH.
Now, for the abnormal. Only ten percent of hypothyroidism in dogs is
due to any interruption of this supply and demand relationship between
the brain-pituitary gland and the thyroid gland. For example, if
the pituitary gland fails due to a tumor or other disease of if the
hypothalamus is diseased, the thyroid won't "get the message" to "hurry
up". So the thyroid gland, even though it can function normally,
doesn't receive the order to do so. T3 and T4 production from the
thyroid gland slows down and so does the hypothyroid dog!
Ninety percent of canine hypothyroid cases are due to what is called primary
hypothyroidism. The thyroid gland itself is not functioning up to
par even when told to do so by the pituitary gland and hypothalamus.
The thyroid gland is being told to "hurry up" but simply can't respond.
1. Lymphocytic thyroiditis- Abnormal infiltration of the thyroid gland
with white blood cells. This is likely an autoimmune type of disorder.
2. Idiopathic atrophy - This just means that for unknown reasons the
gland degenerates and shrinks in size or becomes infiltrated with fat
Cancer - Occasionally tumors will occur in
and damage the thyroid gland.
Breeds Most Likely to Develop Hypothryoidism
|| Miniature Schnauzer
| View Michigan State University's
veterinary diagnostic lab ranking of 140 breeds with autoimmune
is disease that does not seem to be due to environmental factors,
vaccinations, nutrition (unless a diet deficient in iodine is fed) or
other management practices. It is seen equally in males and
| Crusty, circular patches that spread
are due to a bacterial infection.
Blood tests are essential
for making an accurate assessment of the patient.
of a dog before and after treatment for hypothyroidism
|An overweight and nearly
due to hypothyroidism
|Two months after starting thyroid hormone replacement therapy
veterinarian may do skin scrapings to be certain the hair loss isn't
due to skin mites such as these demodex mites seen under the microscope.
| Hypothyroidism has inhibited new hair
growth after the dog shed some hair. These areas of scant hair
regrowth have the same pattern on the left and right side of the dog
and is called bilateral, symmetric alopecia.
| Follicular dermatitis with hair loss is
more common in dogs with low levels of thyroid hormone.
|| Most common signs:
With varying degrees of intensity the most
common signs seen are a thinning of the
skin, hair loss over the back and sides without new hair production,
and weight gain in the presence of normal caloric intake.
Proof of hypothyroidism is gained via blood
analysis for the hormones called T3 and T4 and several other forms of
thyroid hormones, if needed. (See this site for full information). Anemia and high
blood cholesterol levels are common. The veterinarian may make a
diagnosis of hypothyroidism based upon the patient's history, physical
exam, and laboratory data.
Correction of hypothyroidism is aimed at increasing the blood levels of
T3 (triiodothyronine) and T4 (tetraiodothyronine). An animal origin
thyroid product ("natural thyroid") has no advantage over synthetic
medications and, in fact, can be unreliable in some cases.
Synthetic T4, called sodium levothyroxine (or just L-thyroxine)
is the best choice for nearly all hypothyroid patients. Giving this
medication in tablet form allows normal levels of both T3 and T4 in
about 90% of cases. The dose is usually given twice a day but the
amount used varies with each patient.
If your dog is hypothyroid and is getting
sodium levothyroxine, be sure you are giving it twice a day at the
proper dose. If the treatment is working, the dog will "perk up" within
a week or two, although skin and coat improvement may take six weeks or
longer. Reproductive status may take six to nine months to
stabilize. Follow up blood tests are a good idea to establish the
correct dose needed.
As with all hormone related disorders, hypothyroidism can have all
sorts of clinical signs, some subtle, some obvious. However, it can be
a rewarding and inexpensive disorder to treat. Set that "thermostat"
right and the patient often returns to an active... and interactive...
happy, healthy dog!
Note: Many dog owners have had a dog "blow their coat". This
excessive shedding over a short period of time is not due to
hypothyroidism; using hormone replacement to prevent this natural
occurrence is unfair to the dog.
Becoming a "watcher" rather than a "doer"
Sparse coat over dorsum on both sides
Dry, scaly skin
Course, dull coat
Bilaterally symmetric alopecia
"Rat-Tail" (sparse, bristly hair on tail)
"Puppy coat" in adult dog
Hyperpigmentation (dark pigment infiltrating the
Seborrhea dermatitis (oily, greasy skin)
Thin skin when pinched
Pyoderma (skin infections
Male lack of libido
Failure to cycle
Prolonged interestrus interval
Weak, dying or stillborn puppies
Silent heat cycles
Prolonged estrual bleeding
Bradycardia (slowing heart rate)
Corneal lipid deposits
Keratoconjunctivitis sicca ("dry eye")
These focal areas of hair loss are not typical of
hypothyroidism and are due to a fungal (dermatophyte) infection.