Hypothyroidism

Hypothyroidism may be a common primary cause of otitis externa in breeds that are more prone to this disease.

Hypothyroidism is perhaps the most common endocrine imbalance found in dogs.

Thyroglobulin autoantibody assays may be useful in determining which patients may be at risk for developing hypothyroidism due to lymphocytic thyroiditis. Even before the serum thyroxine levels fall, and prior to the onset of clinical disease, the destruction of the thyroid follicles results in increased levels of serum thyroglobulin autoantibodies.

Discussions concerning thyroid hormone assays and interpretation of thyroid test results can be found in numerous internal medicine and endocrinology references. The current standard of thyroxine measurement in the dog is the free or unbound thyroxine (T4) by equilibrium dialysis (fT4) or total thyroxine (tT4). An increased thyroid-stimulating hormone (TSH) level coupled with a low or borderline T4 level is often used to separate thyroidal from nonthyroidal illness.

Clinically, hypothyroidism is often misdiagnosed based solely on laboratory results because of the influence of nonthyroidal factors on the amount of thyroid hormone available to the test reagents.

Certain breeds seem to be predisposed to hypothyroidism and also seem to pose the greatest challenge to the veterinarian dealing with their otitis externa. The Shar-Pei, Poodle, Cocker Spaniel, Golden Retriever, Chow Chow, and German Shepherd seem to be hypothyroid-prone breeds in which otitis externa is frequently a feature of their hypothyroid condition.

Hypothyroidism results in seborrheic dermatitis as well as seborrheic otitis. In the ear, the lower level of thyroid hormone alters the fatty acid composition of the lipids in the cerumen. When a dog is hypothyroid, the sebaceous glands may become overactivated, resulting in ceruminous otitis. On cytologic examination, there is a predominance of cornified epithelial cells, along with the presence of nonstaining sebaceous cellular debris. Yeasts and cocci bacteria are often found in the ears of hypothyroid dogs.

The bacterial degradation of the increased lipids found in seborrheic plaques results in more free fatty acids on the skin. Malassezia can utilize the free fatty acids as metabolic substrates. Malassezia can induce profound erythema and pruritus through the chemoattractant cytokines produced as well as their metabolic by­products, resulting in localized inflammation. Often the first presenting sign in a hypothyroid dog is the presence of a severely pruritic otitis externa, complicated with Malassezia.

Low circulating thyroid hormone levels are also associated with decreased activity of the B-lymphocytes responsible for humoral immunity. This diminishes the ability

of the skin to respond to cutaneous bacteria.

Proper diagnosis and treatment of hypothyroidism essentially removes this primary cause of otitis externa. Treatment for secondary bacterial and yeast infections reduces the perpetuating inflammatory reaction.

Thyroid testing should be done in dogs with otitis externa that have not responded to treatment for other primary causes of otitis, especially in the breeds prone to hypothyroidism. If this disease is diagnosed, future exacerbations of otitis externa can be prevented.

Without attention to the primary causes of ear disease, the veterinarian treating ear diseases in dogs and cats will find them very frustrating diseases to deal with. Successful management of ear cases depends on finding and correcting the primary causes of ear disease. When a case of otitis is presented for examination, the veteri­narian should, rather than getting the otoscope, step away from the patient and look at the entire skin and take a good dermatological history from the owner before proceeding.

Suggested Readings

Boothe HW: Surgery of the tympanic bulla (otitis media and nasopharyngeal polyps), Probl Vet Med 3:254-269, 1991.

Fingland RB, Gratzek A, Vorhies MW, et al: Nasopharyngeal polyp in a dog, J Am Anim Hosp Assoc 29:311-314, 1993.