Hyperthyroidism

Owners usually notice weight loss despite a good or increased appetite, together with increased thirst and urination. Vomiting, diarrhoea or larger faecal volume, hyperactivity, rapid breathing and an unkempt, matted or greasy coat are common. Examination may reveal a fast heart rate, cardiac changes and high blood pressure; a thyroid nodule may be palpable, but its absence does not exclude the disease. Among hyperthyroid cats with cardiac abnormalities, up to 10-15% develop congestive heart failure. Most affected cats have raised serum T4.

In cats, hyperthyroidism results from excess thyroxine (T4) and triiodothyronine (T3) secreted by benign thyroid adenomatous hyperplasia or adenomas; malignant thyroid carcinoma is a rare cause. Hormone excess raises metabolic rate and tissue demand for blood flow, creating a high-output cardiac state with abnormally low systemic vascular resistance. Upregulated beta-1 adrenoceptors on heart muscle increase heart rate, conduction and contractility, and the left ventricle may thicken while the left atrium dilates; systemic hypertension can develop. Many of these cardiac changes improve or resolve once normal thyroid function is restored. Dietary compounds and chronic exposure to thyroid-disrupting chemicals are suggested contributing factors.

Hyperthyroidism is the most common endocrine disorder of middle-aged to elderly cats, affecting around 10% of cats over 10 years of age in the United States. A 2016 editorial described it as having emerged as a new problem in cats just over 35 years earlier. Because geriatric screening panels now routinely include serum T4, many cats are diagnosed early, often before overt signs. Age-related comorbidities, including heart disease, diabetes mellitus, gastrointestinal dysfunction and chronic kidney disease, commonly coexist.

Echocardiography cannot attribute left ventricular thickening to hyperthyroidism, because hypertrophic cardiomyopathy, hypersomatotropism, systemic hypertension and infiltrative myocardial disease look the same; regression after the cat is made euthyroid points to thyroid-driven change. Coexisting kidney disease, heart disease, diabetes and gastrointestinal disease must be considered when interpreting signs and choosing treatment.

Thyroid glands, heart (left ventricle and atrium), blood vessels and blood pressure, kidneys, gastrointestinal tract, skin and coat, whole-body metabolism

Generally good with treatment. Of 231 cats given radioiodine, 86% had cardiac abnormalities; of 57 re-examined at a median 25 months, 12% still did. Methimazole was linked to shorter median survival than radioiodine or thyroidectomy.