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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.

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 daily.

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.

With once-daily 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 therapeutic efficacy.

Initial Response Time To Treatment
Increased activity will be noticed within the first two weeks of treatment in cases of common hypothyroidism.
In animals who are overweight because they are hypothyroid, weight loss usually is seen by eight weeks after normalization of circulating thyroid hormone.
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 possible).

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