Page 103 - Clinical Small Animal Internal Medicine

71

VetBooks.ir

9

Canine Hypothyroidism

David S. Bruyette, DVM, DACVIM (SAIM)
Anivive Lifesciences, Long Beach, CA, USA

Etiology/Pathophysiology History and Clinical Signs
and Epidemiology
As thyroid hormone regulates the metabolic rate and
Canine hypothyroidism, while a common endocrinopa- influences the functions of many organs, clinical signs
thy in the dog, may be overdiagnosed due to confusion/ are often nonspecific and insidious in onset. Many other
inconsistencies in establishing a definitive diagnosis. diseases can have similar clinical signs to hypothyroid-
Hypothyroidism is due to decreased thyroidal produc- ism, which may lead to an incorrect diagnosis. As such,
tion of the thyroid hormones thyroxine (T4) and triio- laboratory testing of thyroid function is often performed
dothyronine (T3). Greater than 90% of cases are primary as part of the diagnostic work‐up in animals with non-
and are due to acquired immune‐mediated destruction thyroidal illness.
of the thyroid gland which is preceded by thyroiditis, Common clinical signs include lethargy, mental dull-
idiopathic atrophy or, less commonly, neoplasia. ness, weight gain, exercise intolerance, alopecia, and
Secondary forms of the disease include thyroid‐stimu- obesity. Many metabolic, infectious, neoplastic, congeni-
lating hormone (TSH) deficiency, pituitary neoplasia, tal, degenerative, and inflammatory diseases can cause
and cystic Rathke’s pouch. Tertiary hypothyroidism with similar clinical signs and biochemical abnormalities seen
thyrotropin‐releasing hormone (TRH) deficiency has with hypothyroidism.
not been documented in dogs. Congenital cases have
been reported in both dogs and cats.
Hypothyroidism most commonly occurs in young to Diagnosis
middle‐aged dogs with an average age of 7 years. Dogs
with autoimmune disease tend to develop hypothyroid- Laboratory Diagnosis
ism at a younger age. While thyroid values decrease
within the reference range in senior dogs, hypothyroid- Thyroxine is the major secretory product of the thyroid
ism is very uncommon and other factors (see later) are while the majority of T3 is derived from extrathyroidal
likely responsible for the observed decreased thyroid sources. Both T4 and T3 are highly protein bound to
concentrations in euthyroid older patients. Spayed serum carrier proteins such as thyroid‐binding globulin,
females and neutered males are at increased risk when transthyretin, and albumin. Only unbound (free) hor-
compared to sexually intact animals. Breed predisposi- mone is able to penetrate cell membranes, bind to recep-
tions have been reported for golden retrievers and tors, and result in biologic activity. Protein‐bound
Doberman pinschers. Thyroiditis is heritable in the bea- hormone acts as a reservoir to maintain steady concen-
gle, borzoi, golden retriever, Great Dane, Irish setter, trations of free hormone in the plasma despite rapid
Doberman pinscher, and old English sheepdogs. No alterations in release and metabolism of T3 and T4 and
known environmental factors have been identified. changes in the plasma protein concentrations.

Clinical Small Animal Internal Medicine Volume I, First Edition. Edited by David S. Bruyette.
© 2020 John Wiley & Sons, Inc. Published 2020 by John Wiley & Sons, Inc.
Companion website: www.wiley.com/go/bruyette/clinical