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Hyperthyroidism (Proceedings)


Hyperthyroidism is a common disease of cats over 6 years of age. Feline hyperthyroidism is caused by adenomatous hyperplasia or adenoma of the thyroid gland; carcinomas are rare.


Hyperthyroidism is a common disease of cats over 6 years of age. Feline hyperthyroidism is caused by adenomatous hyperplasia or adenoma of the thyroid gland; carcinomas are rare. The disease is bilateral in 70% of cases. The cause remains unknown. Clinical signs can vary depending upon which stage the disease is identified. Weight loss is present in about 90% of cases with polyphagia in about 50% of cases. Cats can become hyperactivity, anxious, and are prone to this especially when being examined by a veterinarian. Most cases have a thyroid slip.

Other signs:

     • Polyuria and polydipsia

     • Heart murmur or gallop rhythm

     • Tachycardia

     • Systemic arterial hypertension is common.

     • Vomiting

     • Diarrhea or increased fecal volume

     • Panting or dyspnea

     • Generalized weakness can occur because of impaired muscle function.

     • Cervical ventroflexion is occasionally present and may be due to myopathy, hypokalemia, or thiamine deficiency.

In rare cases apathetic hyperthyroidism can occur with decreased appetite, weight loss and severe lethargy.

Routine laboratory tests

     • Elevated liver enzyme (ALT, AST, alkaline phosphatase) activity is commonly present.

     • BUN and creatinine are elevated in some cats, this is generally related to concurrent renal disease and not the hyperthyroid condition. The presence of this increases risk of treatment for the hyperthyroid state as more severe renal dysfunction may be unmasked.

Diagnostic imaging

Thoracic radiographs:

     • Indicated when dyspnea, tachypnea, or muffled heart and lung sounds are present.

Ancillary testing

     • Electrocardiography is indicated when an arrhythmia is suspected.

     • Arterial blood pressure should be measured in all cases.

Specific tests for diagnosis

Hyperthyroidism is usually readily diagnosed by documenting elevated serum total T4 concentration. Normal serum T4 can be seen in hyperthyroid cats with mild hyperthyroidism, hyperthyroidism with a concurrent nonthyroidal illness, or a disease other than hyperthyroidism. If the total T4 is normal and hyperthyroidism is still suspected, serum T4 should be measured again in 1-4 weeks. Alternatively free T4 with a dialysis procedure can be run. This is a more sensitive test (98% vs. 90% with just T4). Falsely elevated fT4 can be seen in cats with nonthyroidal illness.

      Thyrotropin releasing hormone (TRH) stimulation test

      As effective as the T3 suppression test in diagnosing hyperthyroidism in difficult cases.


      Obtain blood sample for serum T4 concentration before and 4 hours after IV administration of 0.1 µg/kg TRH.


      Normal response is an increase in T4 on the 4 hour sample > 60% of the basal concentration.

      Hyperthyroidism is diagnosed when the serum T4 concentration is < 50% of baseline.

      T3 suppression test


      Obtain blood sample prior to initiating test for measurement of serum T4.

      Administer T3 at 25 µg PO q 8 hours for 7 doses.

      Obtain blood sample 2-4 hours after the final dose of T3 for measurement of serum T4 and T3 concentrations.


      Normal response is a decrease in T4 concentration to < 20 nmol/L.

      Hyperthyroidism is diagnosed when the serum T4 is > 20 nmol/L post-T3 administration.

      Serum T3 should be elevated on the post-treatment sample unless the T3 was not administered properly.

Treatment recommendations

Antithyroid drug treatment

Methimazole or carbimazole are the drugs of choice for medical management of hyperthyroidism. Methimazole inhibits synthesis of thyroid hormones. Methimazole should always be used prior to a more permanent treatment in order to assess the effects that resolution of hyperthyroidism has on renal function. Methimazole should be administered initially at 5 mg/day in a single dose or divided twice per day.

Complications do occur with these drugs, some of which respond to dosage decreases or gradual reintroduction, others necessitate stopping the medications permanently. GI side effects (vomiting, diarrhea, anorexia) are the most common signs seen. Blood dyscrasias (thrombocytopenia, agranulocytosis, hemolytic anemia) occur rarely but are life-threatening. Other severe side effects include pruritus and hepatopathies. All the severe side effects necessitate stopping the medication permanently.

Monitoring treatment is vital given the chances of complications from these drugs. A CBC should be evaluated every 2 weeks for the first 3 months of treatment to monitor for neutropenia, anemia, and thrombocytopenia. Renal function should also checked every 2 weeks until serum T4 concentration has decreased into the normal range for at least 2 weeks.

When methimazole is poorly tolerated, the most effective alternative with the fewest side effects is ipodate or ipanoic acid. These oral contrast agents reduce T3 without changing T4. Administer 50 mg twice per day. These only work with mild hyperthyroidism and can stop working at a later date.

Radioactive iodine:

Treatment of choice if available and affordable. Ideally only abnormal tissue will be destroyed, normal tissue should be spared. Equally effective if abnormal tissue is not located in the thyroid gland. Can also be used with adenocarcinomas (higher dose required). Cats do have to be relatively stable clinically to be treated and need to eat in clinic if treatment is to occur. Most cats are euthyroid within 1-2 months of treatment.

Percutaneous ethanol injection:

Has a learning curve, so needs experienced operator. Only one thyroid gland should be treated at a given time even if both thyroid glands are enlarged; the contralateral gland should be treated at a later date. Results with treatment of cats with unilateral involvement has been good, while hyperthyroidism has uniformly reoccurred in those with bilateral disease. Laryngeal paralysis is a common complication, which may be permanent or transient; bilateral laryngeal paralysis may be fatal. Horner's can also occur.

Surgical thyroidectomy:

An effective, permanent treatment of hyperthyroidism with bilateral thyroidectomy being recommended given that 70% of cases have bilateral disease. Surgical removal can be staged if needed. Hypoparathyroidism can occur. Because of this calcium concentration should be monitored daily for 3 days after the procedure. Levothyroxine supplementation (0.1 mg QD) is recommended for 2 months following bilateral thyroidectomy. Laryngeal paralysis and Horner's syndrome occur rarely due to intraoperative trauma.

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