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ABSTRACT. González A.A.P., França T.N., Ramadinha R.R., Nogueira V.A., Mascarenhas M.B., Peixoto T.C., Costa S.Z.R. & Peixoto P.V. [Clinical pathological study on hypothyroidism diagnosis in dogs, with emphasis on changes in piloerector muscles.] Estudo clínico-patológico no diagnóstico do hipotireoidismo em cães, com ênfase nas alterações dos músculos piloeretores. Revista Brasileira de Medicina Veterinária 38(Supl.1):147-155, 2016. Departamento de Epidemiologia e Saúde Pública, Instituto de Veterinária, Universidade Federal Rural do Rio de Janeiro, Seropédica, RJ 23890-000, Brazil. E-mail: firstname.lastname@example.org Given the high frequency of hypothyroidism in dogs in Brazil and the usual inconclusiveness of laboratory exams, the establishment of the real significance of hypertrophy of piloerector muscles can be important for pathologists. This study aimed to determine if there is a correlation between the hypertrophy of these muscles and low levels of thyroid hormones in dogs, assessing the diagnostic significance in case of a positive correlation and describing the clinical and dermato-histopathological findings in dogs with hypothyroidism in Brazil. Two hundred dogs of both sexes, aged between 6 months and 18 years, with skin disease suspected to be related to hypothyroidism, were evaluated at the Dermatology Section of the Small Animal Veterinary Hospital of the Universidade Federal Rural do Rio de Janeiro between November 2001 and October 2002. Cutaneous biopsies, hormone dosage, skin scrapings, trichograms and cytology exams were performed. Dogs between 2 and 4 years old were the most affected, impacting more females (61%) than males (38.9%) from 32 breeds, especially Poodle, Cocker Spaniel and German Shepherd, with the exception of crossbreeds. Lethargy, obesity and reproductive disorders were observed among the clinical signs. Cutaneous alterations such as hypotrichosis, alopecia, dull and brittle coat, pruritus, seborrhea and hyperpigmentation were frequent. Hypopigmentation, skin thickening and facial myxedema were also observed. Concomitant diseases and/or lesions such as otitis, secondary pyoderma and allergic dermatitis were frequently seen. Histopathology revealed acanthosis, hyperkeratosis, follicular alterations (mainly follicles in the telogen phase) and hypertrophy (70.5%) and vacuolization (cervical - 53.8% and lumbar - 89.4%) of piloerector muscles. Our data indicate that the alterations of the piloerector muscles (hypertrophy and eosinophilic vacuolization) are important for the diagnosis of hypothyroidism. The association of these alterations with other histological findings such as dermal thickness, trichilemmal keratinization, predominance of atrophic follicles and follicles in telogen phase makes the histopathological exam even more helpful for the diagnosis of hypothyoroidism.