The hypothyroid patient can present with countless symptoms, all related to the underlying cause. As is the case with allergic
dermatitis, these patients are typically pruritic, however the pruritus in these situations is due to secondary bacterial
and yeast infections, not an allergic disorder. Because thyroid hormone is essential for hair growth, for the process of
keratinization to occur normally and for sebaceous glands to function properly, patients with hypothyroidism frequently present
with secondary infections directly related to these abnormalities of the skin and hair coat. Seborrheic changes occur from
altered keratinization and altered fatty acid concentrations, which can also occur in many patients with allergic dermatitis,
thus one reason for the confusion. Overlap can occur between endocrine disorders and the three main allergic syndromes and
it is not uncommon to come across a patient that suffers from an allergic disorder that has been repeatedly tested for hypothyroidism
or vice versa. Why? Does this make sense? How can we differentiate between those dogs with endocrine disease and those
with allergic disease?
Hypothyroidism in the dog
Multiple etiologies for hypothyroidism in the dog exist, the most common of which is primary hypothyroidism. Primary hypothyroidism
is synonymous with a problem of the gland itself, usually through destruction. Lymphocytic thyroiditis and idiopathic atrophy
both result in progressive damage to the gland and the resulting lack of production of relevant hormones.
**Primary hypothyroidism = ↓T4 and ↑TSH with lack of response to TSH and/or TRH testing, if performed.**
Secondary hypothyroidism is synonymous with a problem of the pituitary gland and impaired secretion of thyroid stimulating
hormone, resulting in lack of secretion of thyroid hormones and thyroid gland atrophy, but without initial damage to the thyroid
gland. The most common cause for secondary hypothyroidism is suppression of pituitary thyrotroph cells via drugs or hormones,
however pituitary neoplasia and pituitary hypoplasia, resulting in disproportionate dwarfism, can also occur.
**Secondary hypothyroidism = ↓T4 and ↓to undetectable TSH with some response to TSH or TRH testing, if performed.**
Tertiary hypothyroidism has not been reported in the dog. Causes for this disorder in humans include congenital defects,
hypothalamic destruction and cellular defects.
Hypothyroidism in the cat
Naturally acquired disease in the cat is quite rare; iatrogenic disease as a consequence of treatment for hyperthyroidism
can certainly occur. Cats that have been reported with naturally acquired hypothyroidism typically suffer from congenital
disease and disproportionate dwarfism.
Disorders recognized as secondary and tertiary hypothyroidism in the cat have not been reported. Most cats with symmetrical
truncal alopecia are in fact pruritic and pulling or barbering their hair, which is most consistent with a parasitic or allergic
condition rather than an endocrine disorder.
Diagnosing hypothyroidism: Who?
First and foremost, before testing suspect hypothyroidism. Not every patient with skin disease needs to be tested, or even
screened. Always treat and resolve ALL secondary infections before continuing with additional testing. If the patient continues
to be pruritic despite complete resolution of all secondary infections, then it is very unlikely the culprit is hypothyroidism.
If the patient is NOT pruritic between episodes of secondary infections, then thyroid testing is warranted as indicated below.
The initial workup
In addition, the presence of a dry, dull hair coat along the dorsum may represent retained telogen hairs that are not being
replaced by growing, anagen hairs. This dull, dry hair coat may progress into multifocal areas of alopecia, especially in
areas of friction, or wear, consistent with a "wear alopecia". Areas over pressure points can develop into focal areas of
deep pyoderma, with or without elbow callus pyoderma. Alopecia along the bridge of the nose can also be an early warning
sign of hypothyroidism along with recurrent unilateral otitis externa.
Other clinical abnormalities include fasting hypercholesterolemia > 400 mg/dl, fasting hypertriglyceridemia (~70% of patients),
normocytic, normochromic, nonregenerative anemia (~30% of patients), elevated liver enzymes as a result of fatty infiltration
of the liver, and neurological signs.