Derm's Dirty Dozen: These six diabolical diseases are bound to plague your patients and your patience

Derm's Dirty Dozen: These six diabolical diseases are bound to plague your patients and your patience

Nov 01, 2004

In the first part of this two-part series, I outlined six of the more frustrating dermatology cases I see in my practice. But there are still six to entice you, frustrate you and more importantly, teach you!

Canine hypothyroidism

This condition is the great imitator. It is also a disease that can be very difficult to diagnose, yet it seems to be diagnosed commonly in private practice. I believe the most frustrating aspect of this disease is with all the questions that are raised. What test should I perform to diagnose hypothyroidism? Do I perform the test during a certain time of the day? Do specific medications interfere with the results of diagnostic testing? Could the results of thyroid testing fluctuate normally during a given time period and fall below the normal reference values?

In my practice, we uncommonly diagnose hypothyroidism. Most dogs that have hypothyroidism are generally older than 6 years of age. The common cause is spontaneous lymphocytic thyroiditis, and less commonly, thyroid atrophy. However, some of the larger and giant breeds of dogs can have very early age in onset hypothyroidism, sometimes as early as 1 to 2 years of age. Larger breeds affected at this age include Great Danes, Dobermans, Irish Setters, Newfoundlands and Mastiffs. Other breeds that are predisposed to developing hypothyroidism include Cocker Spaniels, Golden and Labrador Retrievers. Dogs affected with hypothyroidism can present in many different forms, but most commonly a poor hair coat with alopecia can be expected. Some dogs, however, can present with what is called "post-clipping alopecia" (lack of hair regrowth in previously shaven areas). In addition, the constellation of clinical signs and physical examination findings can also include weight gain, seborrhea sicca, recurrent superficial pyoderma, facial myxedema, "rat-tail" appearance (alopecia on distal tail) and lethargy.

Most dogs that I see have been treated with corticosteroids recently (orally, by injection or topically), which interferes with results of thyroid hormone measurements. It is well-known that corticosteroids suppress thyroid hormone values and can significantly impact thyroid hormone measurements, thus falsely providing information to the clinician and leading him or her astray. In addition, it has been well established that other drugs, including the sulfa-type antibiotics, will not only reduce thyroid hormone values, but impact the thyroid gland in such a way to produce clinical, clinicopathologic and histologic (thyroid gland atrophy) evidence of hypothyroidism. Finally, inflammatory conditions and other non-thyroidal illnesses can alter thyroid values causing diminished thyroid values (severe illness, sepsis, Cushing's disease, neoplasia), or in the case of inflammatory bowel disease, elevated thyroid values.

First of all, there is no single test that can diagnose hypothyroidism at any point in time with 100 percent accuracy. What I recommend is performing free thyroid testing at any time during the day. Most investigators believe this thyroid hormone is a true reflection of thyroid status in the dog. It is also less effected by medications and stress. Total thyroid hormone measurement is considered to be a less sensitive test, but it can be of value in private practice because of the reduced cost compared to free thyroid hormone measurement. Borderline values (gray zone) should be rechecked in three to six months. Correlation with the clinical signs and history is also very important when evaluating thyroid values. Measurement of thyroid-stimulating hormone can be valuable as well, and theoretically, it should be elevated in the average case of hypothyroidism. I have found this test to be unreliable and the results very inconsistent. I have also found measurement of thyroid auto-antibodies and other thyroid hormone measurements to be of little value.

After the diagnosis is made, supplementation with thyroid hormone is administered every 12 hours (dosage: 0.1 mg/10 pounds ideal body weight). I also recommend retesting in three to four weeks and measuring trough levels. Most clinicians, however, measure thyroid values four to six hours after the morning dosage (four to six hours post-pill). This value is expected to be in the high—normal, or better yet, in the slightly high reference ranges. Do not lower the dosage if your measurements are in these ranges because as these values are expected at this time during the day after supplementation unless clinical evidence of hyperthyroidism are noted. Trough levels measurement of thyroid hormone immediately prior to a dosage should not fall below reference values and should be low-normal or mid-normal.

The outcome of hypothyroidism treatment results in rapid resolution of lethargy (within one to three weeks), and complete regrowth of hair and resolution of seborrhea within eight to 10 weeks or sooner.