Feline hyperthyroidism: thiamazole before definitive treatment
14-year-old Persian cat with newly diagnosed hyperthyroidism. Creatinine low-normal, urine specific gravity 1.018 and loss of muscle mass. The owners want radioiodine as soon as possible.
What is the most appropriate recommendation?
chevron_right Show answer and explanation
A. Give radioiodine straight away: a normal creatinine rules out kidney disease
Hyperthyroidism increases glomerular filtration and muscle loss lowers creatinine, so kidney disease can remain masked until the cat is euthyroid.
B. Run a thiamazole trial until euthyroid and then assess renal function before definitive treatment
Correct. A reversible treatment for a few weeks reveals true renal function. If significant azotaemia appears, medical treatment can be adjusted before committing to an irreversible option such as iodine-131 or surgery.
C. Do not treat the hyperthyroidism, to avoid worsening the kidneys
Untreated hyperthyroidism causes hypertension, hyperfiltration, proteinuria, heart disease and cachexia, which also damage the kidneys. It is treated, but in a controlled way.
D. Perform bilateral thyroidectomy, which does not affect glomerular filtration
Any treatment that normalises T4 reduces glomerular filtration, surgery included, and it is also irreversible.
lightbulb Key takeaway
Before irreversible treatment of feline hyperthyroidism, stabilise with thiamazole and check renal function when euthyroid. Also avoid iatrogenic hypothyroidism, which worsens renal function.
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Updated: October 31, 2026 · Educational content for veterinary professionals. It does not replace current clinical guidelines or the judgement of the attending veterinarian.