Issue Description Hypothyroidism is the natural deficiency of
thyroid hormone. This deficiency is produced by immune-mediated
destruction of the thyroid gland, by natural atrophy of the gland, by
dietary iodine deficiency, or as a congenital problem. In the dog, the
first two causes listed account for almost all cases, though currently
the feeling is that atrophy of the gland actually represents the end
result of earlier immune-mediated destruction of the gland.
Hypothyroidism generally develops in middle aged or elderly dogs.
Breeds with definite predisposition to develop hypothyroidism include:
the Doberman pinscher, the Golden retriever, the Irish Setter, the
Great Dane, the Dachshund, and the Boxer.
Symptoms Although the age of onset is variable,
hypothyroidism most commonly occurs in dogs from 4 to 10 years of age.
It usually affects mid- to large-size breeds and is rare in toy and
miniature breeds. The clinical signs of hypothyroidism may be vague
and insidious in onset, therefore hypothyroidism may be considered in
the differential diagnosis of a wide range of medical problems.
Lethargy, mental dullness, weight gain, unwillingness to exercise, and
cold intolerance are classical signs of hypothyroidism and are the
result of a decreased metabolic rate. Dermatological manifestations
occur in 60% of hypothyroid dogs. These may include a dry hair coat,
seborrhea, alopecia, hyperpigmentation, and pyoderma. While hair loss
occurs in a bilaterally symmetrical pattern, it initially occurs in
areas of friction such as the tail, around the neck, lateral trunk,
and ventral thorax. Accumulation of excessive amounts of
glycosaminoglycans (mostly hyaluronic acid) in the dermis results in
the myxoedematous appearance (tragic facial expression) found in some
dogs. Glycosaminoglycan accumulation may also occur in the
gastrointestinal tract, heart, and skeletal muscles. Neurological,
cardiovascular (bradycardia), and reproductive manifestations have
also been recognized. Myxedema coma, a rare syndrome, is the extreme
expression of severe hypothyroidism.
Ocular changes are not
common in hypothyroidism but the high levels of blood cholesterol and
circulating fat can sometimes lead to eye changes. When these changes
are seen, often thyroid testing is recommended.
Corneal dystrophy, an abnormal change in the clear covering of the
eye, is such an eye sign. This finding is usually represented as a
small white spot (sometimes a white circle) on the eye surface. At
this degree it is only a cosmetic problem and does not interfere with
vision. In more severe forms, painful bubbles can erupt on the corneal
surface leading to ulceration. Obviously, this form would require
Treatment The initial treatment of choice is synthetic
L-thyroxine, because it results in normalization of both T4 and T3
concentrations. Risk of iatrogenic hyperthyroidism is low because
physiologic regulation of conversion of T4 to T3 is preserved.
Bioavailability may vary greatly from one product to another, so it is
advisable to use a brand-name product for initial treatment. It also
is advisable to measure TT4 concentrations 4-8 weeks after changing
the brand of supplement, particularly if a generic product is
substituted for a name-brand product. With few exceptions, replacement
therapy is necessary for the remainder of dog's life.
Optimal dose and frequency of supplementation vary among dogs because
of variability in L-thyroxine absorption and serum half-life.
Treatment should be initiated at a dose of 0.02 mg/kg orally every 12
hours, and then the dose should be adjusted based on results of
therapeutic monitoring. Using twice-daily treatment initially improves
the likelihood of response to treatment in all dogs. After clinical
signs resolve and TT4 concentrations stabilize within the therapeutic
range, the majority of dogs can be maintained on 0.02 mg/kg once
The most important indicator of the success of
therapy is clinical improvement. Clinical resolution of metabolic
signs such as lethargy and mental dullness can be expected within two
weeks of starting therapy, while other abnormalities, including
dermatologic signs, may take up to three months to resolve.
Biochemical therapeutic monitoring is required because of variable
individual response to treatment; dose adjustments are required in
approximately half of all patients.6 Therapeutic monitoring of serum
TT4 and cTSH levels should be started beginning four to eight weeks
after starting supplementation. Serum TT4 concentrations should be
measured at six- to eight-week intervals during the first six to eight
months of treatment, because metabolism of T4 will change when the
metabolic rate normalizes and dosage adjustments may be necessary.
Once adequate serum TT4 concentrations are documented and the dog's
dosage has stabilized, frequency of measurement of serum TT4 may be
decreased to once or twice a year.
administration of T4, the peak serum concentration of T4 generally
should be slightly high to high-normal four to eight hours after
dosing and low-normal to normal 24 hours after dosing. Animals on a
twice-daily administration probably can be checked at any time, but
peak concentrations can be expected at the middle of the dosing
interval (4 to 8 hours) and the nadir just before the next dose. Six
hours after Soloxine administration on a once daily administration
program, a median total T4 value of approximately 55 mmol/L is
associated with good clinical control in most dogs, whereas values of
less than 35 mmol/L usually indicate the need for an increase in
dosage. Maintenance of an elevated circulating cTSH concentration is a
reliable predictor of an increased therapeutic requirement but
suppression of cTSH concentration into the reference interval does not
guarantee the adequacy of therapy. A decrease in circulating
cholesterol and triglyceride concentration and an increase in the RBC
count can be used to indicate an overall effect of thyroid hormone
replacement therapy but is not valuable in reliably confirming
Initial Response Time To Treatment Activity Increased activity will be noticed within the
first two weeks of treatment in cases of common hypothyroidism. Weight In animals who are overweight because they are
hypothyroid, weight loss usually is seen by eight weeks after
normalization of circulating thyroid hormone. Skin Notable improvement in coat and regrowth of
hair in dogs that have experienced hair loss (alopecia) takes months.
In fact...early on, there may be some additional loss of hair due to
telogen defluvion as hair follicle activity recommences on therapy,
older, dormant hairs are lost well (months) before newer hairs are
generated. One other important point about skin is that animals with
poor hair coat due to other causes (i.e. animals who are not
hypothyroid) may show improvement on thyroid supplement. Thyroid
supplementation in this instance is inappropriate, however; the
primary cause of the skin problem should be ascertained and treated
accordingly. Laboratory Abnormalities Are usually significantly improved or normal by
four weeks of appropriate thyroid supplementation.
Is The Diagnosis Correct Because of difficulties in diagnostic testing
methods many dogs have been erroneously diagnosed with hypothyroidism
and have been on medication for years. If there is any question about
a patient and one wishes to re-test, thyroid hormone supplementation
must be discontinued at least 2 months for blood testing to be valid.
If possible, medications known to interfere with testing should be
discontinued for testing (though this is obviously not always