Hypothyroidism: Testing and Diagnosing an Epileptic Canine on AEDs
by Daryl Briten

TiaHypothyroidism is a very real and obvious condition that can affect canines regardless of what their other afflictions might be. It is important to accurately diagnose hypothyroidism and to treat it once this diagnosis has been made. If a dog is showing the traditional symptoms of hypothyroidism such as, but not limited to, lethargy, obesity, excessive shedding, skin irritations/lesions, increased appetite, it is a matter of routine to have a thyroid panel done to determine if hypothyroidism is present. Sounds easy BUT once a dog is on phenobarbital therapy it becomes an entirely different story.

1. There is current discussion among owners of epileptic dogs as well as some members of the veterinary community whether or not hypothyroidism can be a cause of seizures. In some circles it is being presented as fact, while in others there is reluctance to accept this as anything but a hypothesis. Whether or not hypothyroidism does cause seizures, in my opinion, is currently a moot point. Until there are controlled studies done, this can be argued back and forth forever. In order to determine if Soloxine, the recommended therapy for hypothyroidism, reduces seizure frequency, the dog's medications, environment, and diet MUST remain constant in order to determine if the sole controlling factor is the Soloxine. To my knowledge, no studies have been done, or are currently being done, using these kinds of controls. At this point in time, the best that an owner of an epileptic dog on Pb therapy can do if hypothyroidism is suspected, is to look at the individual dog and treat accordingly. But how to do this?

2. Testing is important, but according to current experts in the field of veterinary endocrinology there is no way to accurately test a dog for hypothyroidism if they are on phenobarbital therapy. This statement is supported in the following research study:

JAVMA Vol 214, No 12, June 15, 1999, "Serum total thyroxin, total triiodothyronine, free thyroxin and thyrotropin concentrations in epileptic dogs treated with anticonvulsants"

In this study dogs on phenobarbital showed significantly decreased levels of total and free T4 concentrations while serum total T3 and TSH remained unaffected. They state in their study that "Unfortunately, the diagnosis of hypothyroidism is not always straightforward, because nonthyroidal illness and administration of many commonly used drugs such as glucocorticoids and anticonvulsants, may cause test results indicative of hypothyroidism in dogs with normal endogenous thyroid capacity". The researcher's conclusions were: "Results indicate that serum total and free T4 concentrations may be low (i.e. in the range typical for dogs with hypothyroidism) in dogs treated with phenobarbital." "This can create a false impression that the dogs have hypothyroidism." The study showed (in my words) that treatment for hypothyroidism for a dog on phenobarbital should follow a course of action which includes:

1. The dog should be exhibiting clinical symptoms of hypothyroidism.
2. A 6 week trial of Soloxine should be administered.
3. If at the end of the 6 week trial, the clinical symptoms have disappeared, then one can assume that the dog is hypothyroid. If they don't disappear, Soloxine therapy should be discontinued.

I would urge anyone struggling with the thyroid issue to read this Study. It was very informative for me. (As a side note, they also studied the Bromides and they did not affect the T4 levels.)

3. A brief history of my experience: Tia is an epileptic female yellow lab who began having seizures at the age of 6.5 yrs. Phenobarbital therapy was begun which gave us control to one seizure every 4 to 6 weeks. With diet changes and added vitamins and supplements, her seizures are well controlled. (She basically doesn't seize any more unless she is given a drug that interferes with the protein binding for the Pb.) Tia shows none of the clinical symptoms of hypothyroidism listed above. She is trim, eats moderately, and has plenty of energy. However her blood was lipemic, and that can be one of the side effects of hypothyroidism. With all of the discussion of seizures being a side effect I bought into this and decided she had to be hypothyroid!!! So I tested and her T4 levels were in the range of a hypothyroid dog. Four veterinary professionals were involved with the interpretation of her tests over a period of 2 years: Her local vet, her vet neurologist, Michigan State's Lab and Dr. Dodds. Opinions were conflicting. Two saying she did not need supplementation, that she was typical of a dog on Phenobarbital showing low levels of T4, one saying she was hypothyroid, and the other on the fence. What does an owner do when there is contradiction among the professionals? I read and researched and asked so many questions that people got very frustrated with me. I ultimately decided to try it. I put her on a 6 week trial of the Soloxine at .4 mg. BID which is a fairly low dose. She became very finicky about eating and cut her consumption in half. She would pick at her food. She was restless at night and just wasn't "Tia". You know how you know your dog and you know when something just isn't right... I stuck with it and after 6 weeks, checked her blood. Still lipemic. I stopped the Soloxine and within a few days she was eating normally again and back to "herself." So for Tia, it was a failed attempt. She was not hypothyroid, but was a perfect example of the above study.

Each owner has to assess their own dog's situation and I highly recommend that you listen to your vet and discuss all of this with him or her. It's a very confusing issue. If your dog is shedding and lethargic and overweight, it's an easy decision. This is an ongoing science and new things come out every day, but as I do with myself, I do with my dogs. Research, learn, consult with your medical sources, and then make your decision.

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