Hyperthyroidism & Diagnostic Challenges in Senior Cats

The clinical monographs available in Standards of Care offer consolidated and comprehensive guidance on hundreds of topics. Newly updated monographs feature specific assistance on presentation, diagnostic, and treatment scenarios that don’t follow the book. 

The following is a curated excerpt from the Standards of Care monograph on Hyperthyroidism. 

Background

• Feline hyperthyroidism was first recognized in 1979 and has steadily increased in prevalence to become the most common endocrine disorder affecting senior cats. The estimated prevalence is over 10% in cats greater than 10 years of age.^1,2

• Hyperthyroidism in cats almost always results from an autonomous condition of excess thyroid hormone production (ie, adenomatous hyperplasia, benign adenoma).^3–5 Less commonly, hyperthyroidism is caused by a functional thyroid carcinoma.⁴

• Due to widespread effects on basal metabolism, hyperthyroidism can affect the cardiovascular, gastrointestinal, ocular, neurologic, and dermatologic systems. As a result, there is a wide range of clinical signs, from mild to severely debilitating, depending on the disease chronicity and severity.

• Widespread recognition of hyperthyroidism, combined with routine thyroid screening of senior cats, has led to earlier diagnosis and intervention.

• Several treatment modalities exist, ranging from medical management (anti-thyroid drugs, diet) to I-131 radiation therapy and even surgical intervention via thyroidectomy.

• The prognosis is favorable for most cats and is influenced by comorbidities and treatment modalities.

Common Comorbid Conditions 

• Due to the older age predisposition of hyperthyroid cats, concurrent diseases are common and include cardiac, renal, and GI diseases.

• Chronic kidney disease (CKD) may be masked by hyperthyroidism-induced accelerated GFR, reducing plasma concentrations of BUN and creatinine.²⁰

• Alimentary lymphoma and chronic enteropathy were the most common concurrent diseases in one study, which documented that 18% of hyperthyroid cats had a concurrent non-renal disorder.²⁴

• Concurrent cardiac disease may occur as a direct effect of hyperthyroidism or may be a primary cardiomyopathy, valvular disease, or other acquired disorder.

• One study found that the majority of cats presented to a referral facility for radioiodine therapy had a heart murmur, with severity of total T₄ (TT₄) elevation correlating with likelihood of cardiac abnormalities.²⁵

Diagnostic Challenges

The main challenges in definitive diagnosis of hyperthyroidism, complications, and comorbid conditions are fluctuating hormone levels, subclinical or early disease, occult disease, and the presence of concurrent non-thyroidal illness affecting test results.

Palpation of a thyroid nodule/slip in a euthyroid patient

Although the finding of a thyroid nodule on physical examination is highly sensitive for clinical hyperthyroidism, it is not specific, as some cats with thyroid nodules are euthyroid. The size of the palpated nodule appears to be predictive of clinical disease, with larger nodules more consistently found in cats with hyperthyroidism.^39,40,104 

Many cats with thyroid nodules will eventually develop clinical disease as the nodule grows and autonomously secretes thyroid hormones, and therefore close monitoring of clinical signs and hormone levels is advised. One study found that 26% of euthyroid geriatric cats had palpable nodules, and 16% of these patients ultimately became hyperthyroid within the 4.5 years of study follow-up.¹⁰⁵ Benign and malignant thyroid cysts also present as a palpable nodule and may occur in both hyperthyroid and euthyroid patients.³⁴ 

Rarely, non-functional parathyroid adenomas and thyroiditis have also been reported to present as palpable thyroid nodules.¹⁰⁴ 

Although less common, it is important to keep in mind that cervical masses can arise from structures outside of the thyroid gland. If extra-thyroidal disease is suspected, a detailed investigation into the origin of the palpated mass would likely involve CT.