Page 796 - Small Animal Internal Medicine, 6th Edition

768 PART VI Endocrine Disorders

Central nervous system
BOX 48.1
VetBooks.ir + – Potential Causes of Hypothyroidism in Dogs

Hypothalamus – ? Primary Hypothyroidism
Lymphocytic thyroiditis
?
– Idiopathic atrophy
TRH + Neoplastic destruction
Iodine deficiency
Iatrogenic causes
Pituitary thyrotropes Surgical removal
– ? Antithyroid medications
– Radioactive iodine treatment
TSH +
Drugs (e.g., sulfamethoxazole)
– ? Secondary Hypothyroidism
Thyroid follicular cells
Pituitary malformation
Pituitary cyst
Pituitary hypoplasia
Pituitary destruction
Neoplasia
T 4 , rT 3 , T 3 Pituitary thyrotropic cell suppression
Circulation Naturally acquired hyperadrenocorticism
Nonthyroidal illness syndrome (Euthyroid sick
FIG 48.1
The hypothalamic-pituitary-thyroid gland axis. TRH, syndrome)
Thyrotropin-releasing hormone; TSH, thyrotropin; T 4 , Iatrogenic causes
thyroxine; T 3 , 3,5,3′-triiodothyronine; rT 3 , Drug therapy, most notably glucocorticoids
3,3′,5′-triiodothyronine; +, stimulation; −, inhibition. Radiation therapy
Hypophysectomy
Tertiary Hypothyroidism
Congenital hypothalamic malformation (?)
with the hypothalamic-pituitary-thyroid gland axis, TSH Acquired destruction of hypothalamus (?)
receptors, or signal transduction (e.g., as occurs with pitu- Congenital Hypothyroidism
itary TSH deficiency).
Thyroid gland dysgenesis (aplasia, hypoplasia, ectasia)
Clinical Features Dyshormonogenesis: iodine organification defect
Deficient dietary iodine intake
Clinical signs of the more common forms of primary hypo-
thyroidism usually develop during middle age (i.e., 2-6
years). Clinical signs tend to develop at an earlier age in
breeds at increased risk than in other breeds (see Table 48.1).
No sex-related predilection is apparent. DERMATOLOGIC SIGNS
Clinical signs are variable and depend in part on the Alterations in the skin and haircoat are the most common
age of the dog at the time a deficiency in thyroid hormone observable abnormalities in dogs with hypothyroidism.
develops (Box 48.2). Clinical signs may differ between Classic cutaneous signs include bilaterally symmetric, non-
breeds. For example, truncal alopecia may dominate in pruritic truncal alopecia that tends to spare the head and
some breeds, whereas thinning of the haircoat may domi- the extremities (Fig. 48.3). Alopecia may be local or gen-
nate in other breeds. In adult dogs the most consistent eralized and symmetric or asymmetric, it may involve only
clinical signs of hypothyroidism result from decreased cel- the tail (i.e., “rat tail”), and it often starts over sites of wear
lular metabolism and its effects on the dog’s mental status and friction. Although nonpruritic endocrine alopecia is not
and activity. Most dogs with hypothyroidism show some pathognomonic for hypothyroidism (see Chapter 46), hypo-
mental dullness, lethargy, exercise intolerance or unwilling- thyroidism is certainly the most likely diagnosis in an affected
ness to exercise, and a propensity to gain weight without dog with lethargy, weight gain, and no polyuria-polydipsia.
a corresponding increase in appetite or food intake. These Seborrhea and pyoderma are common signs of hypothy-
signs are often gradual in onset, subtle, and not recognized roidism. Depletion of thyroid hormone suppresses humoral
by the client until after thyroid hormone supplementation immune reactions, impairs T-cell function, and reduces the
has been initiated. Additional clinical signs of hypothy- number of circulating lymphocytes—defects that can be
roidism typically involve the skin and, less commonly, the reversed by exogenous thyroid hormone therapy. All forms
neuromuscular system. of seborrhea (i.e., sicca, oleosa, dermatitis) are possible.