Maigan Espinili Maruquin
The serum thyrotropin (thyroid stimulating hormone, TSH) concentration is usually measured in human patients as part of the first-line test for the thyroid function and thus considered best for screening overt and subclinical hyperthyroidism (Ross, Ardisson et al. 1989, Bahn Chair, Burch et al. 2011, Peterson, Guterl et al. 2015). With regards to canine and feline, TSH concentration plays an important role as a biomarker to detect failure of thyroid function, as well.
TSH concentration in Canine
The hypothyroidism is known to be a common endocrinopathy in dogs. Although it was believed that some cases of canine hypothyroidism were caused by autoimmune thyroiditis, it may also be a result from an inability of the pituitary gland to synthesize and secrete TSH, which results to secondary thyroid follicular atrophy (Ferguson 2007).
A classic case of adult-onset hypothyroidism present symptoms of lethargy, weakness, dullness of mental attitude and dermatological problems including alopecia or recurrent skin infections however, in some cases, there are only manifestations of general neuropathy or myopathy (Jaggy and Oliver 1994, Jaggy, Oliver et al. 1994, Ferguson 2007). Being considered as a multisystemic disorder, general medical evaluation should be conducted for suspected cases, including clinicopathologic evaluations (Ferguson DC, Hoenig ME, 1991; Ferguson DC, Hoenig M., 2003) (Ferguson 2007).
Fig. 01. A canine manifesting signs of hypothyroidism
Having false-positive test results are common in hypothyroidism in canines and diagnosis can be challenging, thus, diagnostic testing should be considered to animals presenting clinical and clinicopathologic signs that are consistent with hypothyroidism (McCann 2015). The mostly used diagnostic criteria is with low total or free T4 concentration combined with a high TSH concentration in serum (Dixon and Mooney 1999, Shiel, Sist et al. 2010). The elevated concentration of TSH in dogs with primary hypothyroidism is expected in response to negative feedback of thyroid hormones on the pituitary gland. However, TSH as a sole marker of the disease is not effective due to possibilities of normal TSH in hypothyroid dogs and elevated TSH in euthyroid dogs, resulting to poor sensitivity. Thus, it is usually combined with reduced T4 or free T4 for higher specificity in diagnosis (McCann 2015).
TSH concentration in Feline
The hyperthyroidism in cats is a common disease in older cats and like in humans, hyperplastic or adenomatous nodular changes in the thyroid are present and peak at the old age (Wakeling, Elliott et al. 2011). This common feline endocrine disease has multiple factors likely involved, including genetic susceptibility (Taylor 2017) and has been diagnosed in 1.5–11.4% of older cats across the world (Carney, Ward et al. 2016, Taylor 2017).
Felines with this disorder present signs that vary from the described as classic cat with weight loss, polyphagia, polyuria, polydipsia, increased vocalization, agitation, increased activity, tachypnea, tachycardia, vomiting, diarrhea and an unkempt hair coat, lack of appetite, and lethargy to severely hypermetabolic clinical presentation (Carney, Ward et al. 2016, Taylor 2017). Important comorbidities recorded were cardiorespiratory diseases (Fox, Peterson et al. 1999).
A commercial feline-specific thyroid-stimulating hormone (TSH) assay is not yet available in the market, and in some cases, feline samples are analyzed using canine TSH assays, however, delicate differences in TSH at low levels may evade detection. The TSH should always be interpreted with corresponding tT4 or free T4 results (Taylor 2017). An excellent sensitivity is highly suggested if the TSH below the limit of detection is found in a hyperthyroid cat (Peterson ME, 2016)(Taylor 2017).
Ferguson DC, Hoenig ME. Canine hypothyroidism. In: Allen DG, editor. Small animal medicine. Philadelphia: J.B.Lippincott Co; 1991. p. 845–65.
Ferguson DC, Hoenig M. Endocrine system. In: Latimer KS, Mahaffey EA, Prasse KW, editors. Duncan and Prasse’s veterinary laboratory medicine: clinical pathology. 4th edition. Ames (IA): Iowa State Press; 2003. p. 270–303
Peterson ME (2016) Diagnosis and management of iatrogenic hypothyroidism. In: Little SE, ed. August’s Consultations in Feline Medicine Volume 7. Elsevier, St Louis, MO: 260–9
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Carney, H. C., C. R. Ward, S. J. Bailey, D. Bruyette, S. Dennis, D. Ferguson, A. Hinc and A. R. Rucinsky (2016). “2016 AAFP Guidelines for the Management of Feline Hyperthyroidism.” Journal of Feline Medicine and Surgery 18(5): 400-416.
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McCann, T. (2015). “Canine hypothyroidism.” Companion Animal 20(10): 572-578.
Peterson, M. E., J. N. Guterl, R. Nichols and M. Rishniw (2015). “Evaluation of Serum Thyroid-Stimulating Hormone Concentration as a Diagnostic Test for Hyperthyroidism in Cats.” Journal of Veterinary Internal Medicine 29(5): 1327-1334.
Ross, D. S., L. J. Ardisson and M. J. Meskell (1989). “Measurement of thyrotropin in clinical and subclinical hyperthyroidism using a new chemiluminescent assay.” J Clin Endocrinol Metab 69(3): 684-688.
Shiel, R. E., M. Sist, R. F. Nachreiner, C. P. Ehrlich and C. T. Mooney (2010). “Assessment of criteria used by veterinary practitioners to diagnose hypothyroidism in sighthounds and investigation of serum thyroid hormone concentrations in healthy Salukis.” Journal of the American Veterinary Medical Association 236(3): 302-308.
Taylor, S. (2017). “Hyperthyroidism in cats: what’s new?” Companion Animal 22(2): 80-85.
Wakeling, J., J. Elliott and H. Syme (2011). “Evaluation of Predictors for the Diagnosis of Hyperthyroidism in Cats.” Journal of Veterinary Internal Medicine 25(5): 1057-1065.