Canine Hypothyroidism: An Overview
Hypothyroidism (low circulating thyrod level) may be one of the most common hormonal problems encountered in canines. Commonly affected breeds include, but are not limited to, Golden Retrievers, Shetland Sheepdogs, Cocker Spaniels, Labrador Retrievers, Boxers, Dobermans, Akitas, Old English Sheepdogs, Irish Setters, Chows, and of course, our beloved Clumbers. With good diagnostic tests and therapy, hypothyroidism can be well managed in dogs. Additionally, more sophisticated testing methods may help us to identify individuals or bloodlines predisposed to hypothyroidism, thereby helping us to make well-informed breeding decisions.
A few basic definitions are in order to start off. Thyroxin (T4) is produced by the thyroid gland, as is triodothyronine (T3). Free T4 and T3 are the actual, available hormones the body can use. T4 is deiodinated to T3, which is used by the body, in the pituitary gland, liver, and kidneys. This process is important to understand, since a condition called euthyroid sick syndrome, or nonthyroidal illness, may affect the deiodination pathway. This syndrome, in which the production of T3 is faulty for reasons other than true thyroidal illness may lead to a mistaken diagnosis of Hypothyroidism. In these cases, proper diagnosis and treatment of the real underlying disease process can also lead to a resolution of the secondary thyroid problem.
Hypothyroidism affects many systems in the body. Common symptoms noted include lethargy, obesity, heat-seeking (the dog who always wants to lie on the register), reproductive difficulty, and skin and coat problems. Less common symptoms, which may be encountered, include neuromuscular changes, cardiac problems, and behavioral changes.
Hypothyroidism very commonly produces some typical skin and coat changes. One of the most important things to remember is that low thyrois levels do NOT lead to itchy skin. While a secondary bacterial infection may cause some pruritus (itchiness), hypothyroidism itself does NOT cause itchiness. Abnormal shedding (from complete failure to excessive) and alopecia (hair loss) are commonly seen. The hair loss is usually symmetrical, and often most noticeable over the rump to start. Sometimes a wooly coat will be seen (reminiscent of the wooly puppy coat), and often there will be noticeable hair loss on the tail (“rat tail”). The hair may lighten in color. The hair loss is often slow in onset.
The skin itself may darken and thicken. Comedones (blackheads) may be seen on the abdomen. In advanced cases,a “tragic” expression may be seen, due to thickening of the facial skin.
Sometimes we will see skin and ear infections as secondary complications. Staph infections are common. Excessive scaliness (either oily or dry) can be seen. Again, hypothyroid dogs are NOT itchy.
Hypothyroidism may also lead to reproductive difficulty, although it does not always affect reproductive success. Bitches who repeatedly fail to catch should probably be checked, as treatment can lead to resolution of the problem. It is, however, also important to remember that hypothyroidism is considered to be a heritable condition, which should be factored into breeding decisions.
Diagnosis of hypothyroidism is not difficult. Clinical signs and physical exam findings will often suggest thyroid testing. Many different protocols are available. Some of the newer in-house tests are useful, but have limitations. The recent International Seminar on Canine Hypothyroidism recommended the following thyroid panel. Total T4 (tT4), free T4 by equilibrium dialysis (fT4ed: a specific, very accurate testing method), thyroglobulin autoantibodies (TgAA), and canine thyroid stimulating hormone (cTSH). OFA registry also requires specific tests. This panel is easily available. Your vet draws blood and submits serum to any of several labs – we use Michigan State lab – for less than $60.00. The MSU screen also measures free and available T3, which can be useful in certain cases. We feel this panel is preferable to the in-house free T4 test now becoming more available. Testing only fT4 may lead to a false diagnosis of illness which is falsely lowering the T4 levels (remember, T3 is the active metabolite the body uses). Additionally, many dogs produce autoantibodies against their own thyroids. a situation which can actually lead to an increase in T4, as the body tries to produce more and more thyroid hormone, only to have it destroyed. These dogs can be successfully treated with thyroid replacement therapy, but must first be properly diagnosed. If your vet uses a free T4 test ONLY, be aware that a really low T4 may mean that the patient is probably hypothyroid (but may be suffering from some other, nonthyroidal illness); however, a normal T4 as a sole test result may be difficult to interpret. Conditions, which can cause a false decline in T4, include Cushing’s Disease, liver disease, sulfa drugs or steroid therapy.
Following an accurate diagnosis, treatment of canine hypothyroidism is usually straightforward. Levothyroxine replacement therapy is the preferred treatment (Soloxine). Generics and human products may not be as effective. Veterinary products are preferred for this condition in canines. Response may be noted within one week. Weight loss may be seen within two to four weeks. Dermatologic (skin and coat) changes within four to six weeks. Complete resolution may take several months. Thyroid levels should be rechecked in approximately sixty days, to ensure therapeutic levels are being achieved.
There is new evidence that dogs with elevated levels of thyroid autoantibodies (the TgAA on your test results), especially those which continue to increase on successive tests, may be predisposed to true hypothyroidism down the road. The current recommendation for apparently healthy dogs with abnormal test results is to withhold them from breeding, and retest in two to six months. Further information can also be found in an excellent article, which was published in the AKC Gazette, February 1997.
© Copyright 1998 by Roe Froman, DVM