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Practitioner reality – How I deal with hyperthyroidism (Proceedings)


Hyperthyroidism is caused by excessive production of thyroxine (T4) and triiodothyroxine (T3).

  • Hyperthyroidism (HT4) is caused by excessive production of thyroxine (T4) and triiodothyroxine (T3)

  • >95% of hyperthyroid cats have benign adenomatous growth causing the excessive hormone production

  • Theories for causes for the adenomatous overgrowth include:

o Dietary influences, including soybeans, hydrocarbons

o Herbicides

o Insecticides, including flea medications

o Selenium

o Various other theories


  • Older cats, average age 12-13 years

  • <5% under 8 years old

  • Signs of increased metabolism

o Weight loss

o Polyphagia

o Polyuria/polydipsia

o Increased frequency of vomiting

o Hyperactivity/restlessness

o Diarrhea/increased volume of feces

o Poor hair coat

  • <15% of cats are considered "apathetic hyperthyroid"

o Decreased activity

o Decreased appetite

o Depressed

o Psychological disturbances?

Physical exam findings

  • Palpable thyroid nodule

  • Tachycardia/gallop rhythm/heart murmur

  • Easily stressed

  • Weight loss/cachexia

  • Weakness/ventroflexion

  • Dehydration


  • Elevated T4

  • Elevated FT4

  • Mild increase in PCV

  • Stress leukogram

  • Mild elevation in ALT, AlkPhos

  • +/- hyperglycemia

  • +/- elevated BUN/Creatinine

  • +/- hypertension

  • Rads: +/- cardiomegaly, +/- pleural effusion/pulmonary edema

  • ECG: Tachycardia, increased R wave amplitude

  • Echo: Left ventricular hypertrophy

Differential diagnoses

  • Diabetes mellitus

  • Chronic renal disease

  • Pancreatitis

  • Cholangiohepatits

  • Malabsorption/maldigestion disorders

  • Neoplasia


  • Medication

  • Surgery

  • Radioactive iodine

Practical treatment

  • Start with low doses of methimazole

o 2.5 mg sid for 1-2 weeks, add 2.5 mg/day every other week until T4 is under 2.5 mg/dl and clinical signs have improved

o Recheck T4 and renal values at each recheck until regulated

o Recheck blood pressure as needed

Once regulated, strongly consider permanent treatment. If remaining on methimazole, recheck bloodwork every 4-6 months, or if clinical signs recur. Methimazole dose will need to be increased as thyroid growth continues.

Manage concurrent disease appropriately. Concurrent disease will help determine whether permanent treatment options can be considered. Measure blood pressure as needed if concurrent hypertension. Frequently hypertension will resolve and not need medication if secondary to hyperthyroidism. Caution should be used and close monitoring used when taking a cat off of medication for hypertension. Blood pressure measurements should be taken every few days for 7-10 days to ensure cat does not become hypertensive again.

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