Hyperthyroidism

f felis

Contributor(s):

Michael Herrtage

Angie Hibbert

Carmel Mooney

Mark Peterson

Introduction

Most common endocrine disease of cats.
First reported in 1979.
Cause: functional thyroid adenoma, ademomatous hyperplasia or rarely thyroid carcinoma Thyroid gland: neoplasia → excessive concentrations of thyroxine (T4) Thyroxine assay and tri-iodothyronine (T3) Endocrinology: tri-iodothyronine (T3).
Signs: weight loss, polyphagia; lethargy, vomiting, diarrhea, hyperactivity, vocalization, inappetence less commonly.
Diagnosis: laboratory demonstration of elevated T4.
Treatment: carbimazole Carbimazole, methimazole Methimazole, radioactive iodine, surgical removal of tumor.
Prognosis: good with treatment and careful aftercare.
Print off the Owner factsheet on hyperthyroidism Hyperthyroidism - disease and treatment to give to your client.

Functional thyroid adenomas/carcinoma in one or both thyroid glands → elevated [T4] and [T3] → increased metabolic rate and heat production → weight loss and increased appetite.
Elevated [T4] and [T3] → increased sympathetic stimulation → tachycardia and hypertrophic cardiomyopathy Heart: hypertrophic cardiomyopathy and behavioral changes.
Benign adenomatous change in 98% cases - functional malignant carcinoma is rare (<3% cases).

In normal thyroid hormone production is under control of hypothalamic-pituitary-thyroid axis .
Increased [T4] affects many body systems.

Development of disease poorly understood and likely multifactorial. Several theories proposed:
- Some thyroid follicular cells have higher growth potential → focal hyperplasia and adenoma development.
- Circulating factors, eg iodine levels in diet or immunoglobulins stimulate thyroid development.
- Environmental goitrogens → thyroid pathology.
- Oncogene mutations.
Epidemiological risk factors:
- Use of a litter box.
- >50% wet food in the diet.
- Diet including fish, liver or giblets.
- Exposure to canned foods.
- Increasing age.
- Non-pure breed.