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Feline Hyperthyroidism

Monday, October 5, 2026 · Small Animal Medicine, lecture 12

Lecture Summary from a 52-minute lecture recording and the uploaded slides

Key Points

• The most common endocrine disease of older cats; most affected cats are over 10 years old.
• Caused by benign adenomatous hyperplasia or adenoma of one or both thyroid lobes in the large majority of cats. Thyroid carcinoma is rare.
• Concurrent chronic kidney disease and hypertension are common and change how the cat is treated.

Clinical Signs

• Weight loss despite a normal or increased appetite.
• Polyuria and polydipsia, vomiting, diarrhea, hyperactivity, vocalizing at night, unkempt coat.
• Tachycardia, gallop rhythm or a murmur. A thyroid nodule can be palpated in most cats.
• A minority are "apathetic": reduced appetite and lethargy instead of hyperactivity.

Diagnosis

• Total T4 is the first test. It is above the reference interval in most hyperthyroid cats and, with consistent signs, confirms the diagnosis.
• High-normal total T4 with consistent signs: repeat total T4 in 2–4 weeks, measure free T4 by equilibrium dialysis together with total T4, or use scintigraphy.
• Free T4 should not be interpreted on its own; it can be high in sick euthyroid cats.
• At diagnosis, also check blood pressure, renal values and urine specific gravity.

Treatment Options

• Methimazole (oral or transdermal): Reversible and widely available, but given for life. Adverse effects include vomiting and inappetence, facial pruritus with excoriation, hepatotoxicity and blood dyscrasias. Monitor CBC, biochemistry and T4 closely in the first three months.
• Radioiodine (I-131): Curative in most cats with a single treatment; requires a licensed facility and a period of hospitalization.
• Thyroidectomy: Curative; main risk is hypocalcemia from parathyroid damage, especially after bilateral surgery.
• Iodine-restricted diet: Works only if it is the cat's only food; useful when the other options are not possible.

The Kidney Question

• Hyperthyroidism increases glomerular filtration rate and can mask chronic kidney disease.
• Creatinine often rises once the cat is euthyroid.
• For cats with borderline renal values, the lecturer recommends a trial of methimazole before definitive treatment, to see kidney function at a normal thyroid level.
• Iatrogenic hypothyroidism after treatment worsens azotemia, so check T4 and renal values together at every recheck.

Study Questions

1. A 14-year-old cat has weight loss, polyphagia and a total T4 at the top of the reference interval. Give two next steps.
   Answer: Repeat total T4 in 2–4 weeks; free T4 by equilibrium dialysis with total T4; or scintigraphy (any two).
2. Why can creatinine rise after a hyperthyroid cat is treated?
   Answer: Hyperthyroidism raises GFR; once the cat is euthyroid, reduced kidney function that was masked becomes visible.
3. Name three adverse effects of methimazole.
   Answer: Vomiting or inappetence, facial pruritus with excoriation, hepatotoxicity, blood dyscrasias (any three).
4. Which treatment is curative without surgery?
   Answer: Radioiodine (I-131).

For personal study. Check against the course notes and the lecturer's slides.