title: Prevalence and degree of thyroid pathology in hyperthyroid cats increases with disease duration: a cross-sectional analysis of 2096 cats referred for radioiodine therapy.
abstract: OBJECTIVE
  Hyperthyroidism is common in cats, but there are no reports that evaluate its severity or underlying thyroid tumor disease based on disease duration (ie, time from original diagnosis). The objective of this study was to compare serum thyroxine (T4) concentrations and thyroid scintigraphic characteristics of cats referred for radioiodine treatment based on disease duration.
  
  
  METHODS
  This was a cross-sectional study of 2096 cats with hyperthyroidism. Cats were divided into five groups based on time from diagnosis: ⩽1 year (n = 1773); >1-2 years (n = 169); >2-3 years (n = 88); >3-4 years (n = 35); and >4-6.1 years (n = 31). Methimazole, administered to 996 (47.5%) cats, was stopped at least 1 week prior to examination to allow for serum T4 testing. Each thyroid scintiscan was evaluated for pattern (unilateral, bilateral, multifocal), location (cervical, thoracic inlet, chest) and size (small, medium, large, huge) of the thyroid tumor, as well as features suggesting malignancy.
  
  
  RESULTS
  Median serum T4 concentration increased with increasing disease duration from 100 nmol/l (⩽1 year) to 315 nmol/l (>4-6.1 years) (P <0.001). Prevalence of unilateral thyroid disease decreased, whereas multifocal disease (three or more tumor nodules) increased (P <0.001) with increasing disease duration. Median tumor volume in the five groups increased from 1.6 cm(3) (⩽1 year) to 6.4 cm(3) (>4-6.1 years). Prevalence of large (4-8 cm(3)) and huge (>8 cm(3)) thyroid tumors increased from 5.1% (⩽1 year) to 88.6% (>4-6.1 years), while the prevalence of intrathoracic tumor tissue increased from 3.4% (⩽1 year) to 32.3% (>4-6.1 years). Prevalence of suspected thyroid carcinoma (characterized by severe hyperthyroidism; huge, intrathoracic, multifocal tumors; refractory to methimazole treatment) increased with increasing disease duration from 0.4% (⩽1 year) to 19.3% (>4-6.1 years).
  
  
  CONCLUSIONS
  Our results indicate that the prevalence of severe hyperthyroidism, large thyroid tumors, multifocal disease, intrathoracic thyroid masses and suspected malignant disease all increase with disease duration in cats referred for radioiodine therapy.
authors: Animal Endocrine Clinic, New York, NY, USA drpeterson@animalendocrine.com.
authors: Mark E
authors: ME
authors: Peterson
authors: Advanced Veterinary Medical Imaging, Tustin, CA, USA.
authors: Michael R
authors: MR
authors: Broome
authors: College of Veterinary Medicine, Cornell University, Ithaca, NY, USA Veterinary Information Network, Davis, CA, USA.
authors: Mark
authors: M
authors: Rishniw
mesh_terms: D000818:Animals; D013956:Antithyroid Agents / Q000627:therapeutic use; D002371:Cat Diseases / Q000175:diagnosis* / Q000473:pathology*; D002415:Cats; D003430:Cross-Sectional Studies; D006980:Hyperthyroidism / Q000097:blood / Q000662:veterinary*; D007457:Iodine Radioisotopes / Q000627:therapeutic use; D008713:Methimazole / Q000627:therapeutic use; D015995:Prevalence; D013960:Thyroid Function Tests / Q000662:veterinary*
chemical_list: D013956:Antithyroid Agents; D007457:Iodine Radioisotopes; D008713:Methimazole