Feline Endocrinopathies Danie`lle Gunn-Moore, BVM&S, PhD, ILTM, MACVSc, MRCVS, RCVS Feline Clinic, University of Edinburgh Hospital for Small Animals, Easter Bush Veterinary Clinics, Midlothian, Scotland EH25 9RG Hyperthyroidism Pathogenesis Hyperthyroidism is one of the most common feline endocrinopathies [1–9], affecting approximately 1 in 300 cats [10]. Ninety-nine percent of cases result from benign nodular hyperplasia, adenomatous hyperplasia, or adenoma [4,11,12]. The autonomous secretion of thyroxine (T 4 ) and triiodothyronine (T 3 ) produces a negative feedback effect on the pituitary gland, suppressing the release of thyroid-stimulating hormone (TSH) such that any normal thyroid tissue atrophies. In 70% to 75% of the cats with hyperthyroidism, both thyroid glands are affected [4,11,12]. Only 1% to 3% of cases are caused by mild to moderately malignant thyroid carcinoma [13]. Although the incidence of feline hyperthyroidism has increased steadily since the early 1980s, the cause is still unknown [14]. A number of theories have been proposed, however, involving factors related to diet (possibly including the presence of goitrogens, eating canned cat food, iodine content, or frequent changes) [14–17], environmental causes (possibly associated with cat litter, toxins, pollution, or exposure to allergens) [16,18], a genetic mutation [19], abnormal immune responses [20], or altered hormonal responses [4]. Clinical signs Hyperthyroidism is seen mainly in middle-aged to older cats, with the age at presentation ranging from 4 to 23 years (mean ¼ 13 years) [11,21]. It has occasionally been seen in younger cats (including one 8-month-old kitten), The author received funding support for her lectureship from Nestle´ Purina Petcare. E-mail address: Danielle.Gunn-Moore@ed.ac.uk 0195-5616/05/$ - see front matter Ó 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.cvsm.2004.09.002 vetsmall.theclinics.com Vet Clin Small Anim 35 (2005) 171–210