Authors almost always refer to hypothyroidism as a common (if not the most common) endocrine disease in dogs. but there is
little evidence to support this claim. In fact, canine hypothyroidism may be one of the most over-diagnosed diseases in small
animal medicine. Just about any medical disorder can be associated with decreases in circulating concentrations of thyroid
hormone, causing hypothyroidism to be over-diagnosed. In addition, it is common practice for veterinarians to treat hypothyroidism
empirically following a presumptive diagnosis. This practice is not recommended for several reasons. First of all, it is not
scientifically sound medical practice. Secondly, it may allow another disorder to progress undiagnosed. Thirdly, unnecessary
supplementation with thyroid hormone disrupts the normal regulation of TRH, TSH, and thyroid hormone production, the consequences
of which are not understood.
Clinical signs of hypothyroidism result from generalized decreases in metabolic functions supported by thyroid hormone action.
The clinical signs of hypothyroidism are multi-systemic and varied. Their onset is gradual. The classic presentation of a
hypothyroid dog includes obesity, inactivity, mental dullness, decreased appetite, dermatological abnormalities, and intolerance
to exercise and cold. These signs may or may not be present in all dogs with hypothyroidism.
Dermatologic abnormalities are the most common presenting complaints of owners of dogs with hypothyroidism. Approximately
85% of hypothyroid dogs are reported to have skin lesions. Care should be taken in interpreting the reports of the prevalence
of dermatologic signs in canine hypothyroidism because many of the reports have been written by dermatologists. Whatever the
true incidence of these signs, they include alopecia (arrest of hair growth cycle in the telogen phase), dryness, scaling,
dull hair which is easily epilated, seborrhea, slow hair re-growth, various degrees of hyperpigmentation, poor wound healing.
In extreme cases, mucopolysaccharides accumulate in the skin causing it to thicken, especially in the face. This condition
is called myxedema. Dogs with myxedema are classically referred to as having "tragic" facial expressions. The typical skin
problem of canine hypothyroidism is usually not itchy, although secondary bacterial infections are possibly, and could cause
a degree of itchiness. Hypothyroidism is also commonly mentioned as an underlying cause of chronic ear infection. There is
little or no scientific support for this claim.
Fact Or Fiction: Hypothyroidism is a Cause of Otitis
Hypothyroidism is commonly cited as a cause of bacterial pyoderma which can occur locally, manifesting as otitis externa (infection
of the outer ear). In a recent paper the author stated, "Recurrent bacterial and yeast infections of the skin and ears often
occur secondary to hypothyroidism and may be the only presenting signs," although the statement is not supported by any reference
to a published study.4
The paper goes on to explain that hypothyroidism can cause decreased function of lymphoid cells, decreased T cell function,
decreased humoral immunity, and decreased barrier function of the skin, making reference to the leading textbook on veterinary
dermatology. Statements to this effect are made in the 6th edition of the textbook, referencing the 5th edition.
The 5th edition cites the 4rd edition. The 4th edition cites the 3rd edition. In the 3rd edition, the effects of hypothyroidism on the function of the immune system are referenced by the author's unpublished data
from 1982, and a research paper in mice published in 1980. Examination of the reference reveals that it is not a study of
hypothyroidism at all, but rather a study of the effects of thyroid hormone excess on the immunologic response to administration
of sheep red blood cells in mice. The effects were minimal. Athyreotic human patients have been shown to have similar white
blood cell function when compared to normal controls. Still, there is no credible evidence to suggest that otitis externa,
especially as the sole clinical sign, is a feature of hypothyroidism in dogs.