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Hyperthyroidism
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) Thyroid: T4 assay and tri-iodothyronine (T3) Thyroid: T3 assay.
- 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.
Presenting signs
- Weight loss with polyphagia
. - Vomiting, diarrhea.
- Hyperactivity (restlessness), increased vocalization.
- Polyuria/polydipsia.
- Complications associated with congestive heart failure Heart: congestive heart failure, eg tachypnea, dyspnea, acute onset hindlimb paralysis (thromboembolism).
- Complications associated with hypertension Hypertension, eg hypertensive retinopathy (detached retina, retina hemorhage, hyphaema), neurological signs.
- Unkempt coat.
- Rare cases show 'apathetic disease' with depression, lethargy, anorexia and weight loss (</=10% cases).
Acute presentation
- Signs associated with hypertrophic cardiomyopathy Heart: hypertrophic cardiomyopathy: cardiac failure Heart: congestive heart failure, thromboembolism Thromboembolism: aorta.
- Signs associated with hypertensive complications - acute onset blindness, seizures Seizures.
- Thyroid storm-episodic dyspnea.
Age predisposition
- Middle-aged to old cats (mean 12-13 years, range 4-22 years).
Breed/Species predisposition
- Siamese Siamese and Himalayans less likely to develop disease.
Cost considerations
- Medical treatment - cost of drugs and routine monitoring for complications of drugs and primary disease, eg hypertension.
- Surgical costs.
- Radio-iodine therapy - cost of evaluation to determine suitability and boarding fees Hyperthyroidism: radio-iodine treatment.
Special risks
- Danger of arrhythmia development during anesthesia if pre-stabilization not achieved. Period of stabilization (minimum of 2 weeks) recommended pre-operatively using anti-thyroid drugs +/- beta-blockers.Warn owner of risk associated with anesthesia.
- Post-operative:
- Risks of iatrogenic hypoparathyroidism Hypoparathyroidism if bilateral thyroidectomy performed.
- Rare risk of laryngeal paralysis Larynx disease.
- Unmasking renal disease Kidney: chronic kidney disease with permanent treatment methods (surgery and radio-iodine) (due to decreased glomerular filtration rate as euthyroidism achieved).
- Severely affected hyperthyroid cats can be very fragile - ensure gentle handling and restraint performed by trained staff to avoid excessive stress upon the patient (collapse, syncope, fatal arrhythmias can occur if severely hyperthyroid and put under undue stress).
- Accelerated insulin catabolism in pre-existing diabetic cats may necessitate increased insulin dose.
Pathogenesis
Etiology
- 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).
Pathophysiology
- 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.
Timecourse
- Months to years.
Diagnosis
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Treatment
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Prevention
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Outcomes
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