Hypothyroidism is one of the most common endocrine disorders of adult dogs. The diseases is associated with decreased production and secretion of thyroid hormones (i.e. tetra-iodothyronine also called thyroxine (T4) and tri-iodothyronine (T3)), and in most cases (about 95% of hypothyroid dogs) is a result of autoimmune lymphocytic thyroiditis (or idiopatic follicular atrophy, which is, however, considered as the final form thyroiditis). This form of hypothyroidism is also called primary hypothyroidism. Other forms of hypothyroidism such as central hypothyroidism (secondary hypothyroidism or pituitary hypothyroidism; tertiary hypothyroidism or hypothalamic hypothyroidism) and congenital hypothyroidism occur rarely (Panciera 2013; Rijnberk and Kooistra 2010). Primary hypothyroidism occurs mainly in middle-aged dogs. There is no sex predisposition, however, neutered bitches are at higher risk of the disease. Hypothyroidism may reveal in dogs of all breeds. However, it is observed more frequently in Golden Retrievers, Spaniels, Terriers, Dobermans, Setters and dogs of the large breeds (Dixon 2004; Rijnberk and Kooistra 2010).
Dermatological and metabolic changes are the most common clinical signs of canine hypothyroidism. Symptoms such as lethargy, obesity, alopecia (often with hyperpigmentation), skin thickening, and scaling and scurfing of the skin may be observed by the owner of the animal (Dixon 2004; Panciera 2013). Clinical history may also reveal gradual slowing mental (dullness) and physical (weakness and exercise intolerance) activity, reproductive disorders such as persistent anoestrus and loss of libido, cold intolerance (heat seeking), and decreased appetite (Rijnberk and Kooistra 2010). Clinical examination and additional diagnostic tests may also reveal other clinical signs associated with dermatological, cardiological, neuromuscular, ophthalmological, endocrine, gastrointestinal and hematological disorders (Dixon 2004; Rijnberk and Kooistra 2010).
Hypothyroidism and its differentiation from other diseases with similar clinical signs and decreased serum thyroid hormone concentrations is a diagnostic challenge. Diagnosis is based on the clinical history, physical examination, and results of additional diagnostic tests (including thyroid tests). However, there is no one single test which can confirm the recognition of hypothyroidism. It should be emphasized that some drugs (e.g. glucocorticoids, non-steroidal anti-inflammatory drugs, sulphonamides or barbiturates) and other diseases (also with clinical signs similar to hypothyroidism) may influence the concentration of the thyroid hormone and lead to misdiagnosis with a false positive result. If it is possible, a 6-week withdrawal of thyroid function interfering drugs is recommended. Moreover, in some breeds such as Greyhounds, Wolfhounds, Whippets, Scottish Deerhounds, Basenjis, and Sloughis the T4 concentration is lower than in the dogs of other breeds. Also, the level of T4 is lower in large and medium-sized dogs in comparison with small dogs; and it is lower in elderly dogs in comparison with middle-aged and young dogs (Dixon 2004; Rijnberk and Kooistra 2010). Thus, determination of serum T4 concentration is not sufficient for the diagnosis of hypothyroidism. In routine diagnostics, the concentrations of total T4 (and/or free T4) and thyrotropin (TSH) should be determined (Panciera 2013). However, some drugs also affect the concentration of TSH (e.g. sulphonamides and anticonvulsants may lead to increased TSH concentration) (Dixon 2004). Diagnostic imaging methods such as thyroid scintigraphy and ultrasonography are also useful in the diagnosis of hypothyroidism (Panciera 2013; Reese et al. 2005). Yet, access to scintigraphic thyroid examination is not available in Poland. Additionally, determination of auto-antibodies against thyroglobulin may be supportive for the diagnosis of hypothyroidism, although this test confirms only thyroiditis and not disturbed thyroid function, and both positive and negative test results may be observed in both hypothyroid and euthyroid dogs (Dixon 2004). An appropriate response to the therapy confirms the diagnosis (Panciera 2013).
Therapy is based on the oral supplementation of synthetic thyroxine (levothyroxine). Other drugs consisting of desiccated thyroid gland, liothyronine, T3 or a combination of T3 and T4 are not recommended for hypothyroid dogs (Panciera 2013; Rijnberk and Kooistra 2010). Clinical effects of the treatment should be monitored and based on the clinical and laboratory findings, and also taking into consideration other concurrent diseases (such as diabetes mellitus, Addison’s disease, and renal, hepatic and cardiac diseases), the dose of levothyroxine should be adjusted (Panciera 2013).
Dixon R.M. Canine hypothyroidism. In: Mooney C.T. and Peterson M.E. (eds.) BSAVA Manual of Canine and Feline Endocrinology. 3 rd ed. British Small Animal Veterinary Association, Gloucester, 2004, pp. 76-94.
Panciera D. Hypothyroidism in Dogs. In: Rand J., Behrend E.N., Gunn-Moore D. and Campbell-Ward M.L. (eds.) Clinical Endocrinology of Companion Animals. Wiley-Blackwell, Ames, 2013, pp. 263-272.
Reese S., Breyer U., Deeg C., Kraft W., Kaspers B. Thyroid sonography as an effective tool to discriminate between euthyroid sick and hypothyroid dogs. Journal of Veterinary Internal Medicine, 2005, 19, 491-498.
Rijnberk A., Kooistra H.S. Thyroids. In: Rijnberk A. and Kooistra H.S. (eds.) Clinical Endocrinology of Dogs and Cats. An Illustrated Text. 2 nd ed. Schlütersche Verlagsgesellschaft, Hannover, 2010, pp. 55-91.