There is more to hypothyroidism than alopecia...(Proceedings)
May 01, 2011
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.
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.