Summary
Hypothyroidism is a very common endocrine disease in which normal thyroid hormone levels decrease. It happens either because of a progressive deficiency of thyroid hormone (usually due to an autoimmune-related destruction of the thyroid gland known as "lymphocytic thyroiditis") or because of a congenital disease process (one that's present from birth).
Hypothyroidism in its non-congenital (acquired) form is the most commonly reported endocrine system problem in dogs. It happens when the thyroid gland atrophies (shrinks) as a result of a poorly understood auto-immune process, leading to a highly treatable disease state that affects multiple organ systems relatively mildly at first but invariably progresses in insidious ways.
While a genetic predisposition has been proposed (and largely assumed) for the acquired version of hypothyroidism, for the congenital version of the disease a hereditary association is fairly obvious. This form of hypothyroidism, however, is considered very rare.
Acquired hypothyroidism is generally seen in middle-aged, medium to large breed dogs.
Symptoms and Identification
Most affected dogs display very subtle signs. Among these vague signs include lethargy, skin and coat abnormalities and a heightened susceptibility to infection. Over time, hair coat abnormalities tend to become exacerbated (dry, scaly, itchy or greasy skin is typical). Additionally, a slow heart rate (bradycardia), dullness, cold avoidance, infertility, constipation, and (most typically) weight gain can become evident.
During basic screening, standard bloodwork findings include the following:
hypercholesteremia (low serum cholesterol)
mild nonregenerative anemia (low red blood cell count)
elevated serum creatine kinase (CK)
and hypoglycemia (low blood sugar)
Definitive diagnosis is undertaken via additional blood testing. A low thyroid hormone level (T4) is a typical finding. TSH levels are typically monitored as well.
For the rare, congenital version of hypothyroidism, most experts recommend running an entire thyroid panel in keeping with the OFA's (Orthopedic Foundation for Animal's) approach to monitoring this disease in dogs. They keep tabs on these blood values related to thyroid gland function: FT4, cTSH, and TgAA.
Affected Breeds
The following breeds seem to be predisposed:
Treatment
Treatment for hypothyroidism is always medical. Synthetic thyroid hormone supplements are readily available in a variety of forms. Once or twice daily dosing with levothyroxine is the drug treatment of choice for dogs.
Veterinary Cost
The cost of diagnosis for hypothyroidism is relatively slight. Testing typically runs between $50 and $150. Treatment is generally considered manageable as well. Monthly expenses for medication tend to run in the $20 to $50 range. Annually, dogs must be re-tested to ensure adequate dosing. Again, this tends to remain in the typically affordable $50 range.
Prevention
There is no known means of prevention save genetic counseling which would serve to eliminate affected dogs from the breeding pool. This measure, however, is rarely undertaken, possibly due to the widely disseminated impression that hypothyroidism is a benign condition. However, the OFA (Orthopedic Foundation for Animals) maintains a registry for the congenital form of hypothyroidism by way of attempting to reduce the incidence of this disease among dogs.
References
Chastain, C.B., Panciera, D.L. 1995. Hypothyroid diseases. In S.J. Ettinger and E.C. Feldman (eds.) Textbook of veterinary Internal medicine. p. 1487-1501. W.B. Saunders Co., Toronto.
Peterson ME, Melian C, Nichols R. Measurement of serum total thyroxine, triodothyronine, free thyroxine, and thyrotropin concentrations for diagnosis of hypothyroidism in dogs. JAVMA 1997; 211(11):1396-1402. This reference clearly explains the different tests available, and the significance and interpretation of the results.
Orthopedic Foundation for Animals: www.ofa.org