Hyperthyroidism is the most common endocrinopathy of older cats. The disease usually results from adenomatous hyperplasia or adenoma of the thyroid gland; carcinomas are relatively rare.
In 70 percent of cases the thyroid gland changes are bilateral. Many clinical signs can be seen, with weight loss being present in about 90 percent of cases and polyphagia in about 50 percent of cases.1 Hyperactivity, polyuria/polydipsia, tachycardia, arterial hypertension, vomiting, diarrhea, panting, generalized weakness and a variety of other signs can be seen. In rare instances, apathetic hyperthyroidism can occur with weakness, inappetance and marked lethargy.
Diagnosis in most cases is straightforward in that most cats will have an elevated total T4 values. In some instances, especially with other underlying disease or in older cats, total T4 may still be in the normal reference range.
In this situation, additional testing can be useful, especially free T4 with a dialysis procedure. Testing for fT4 is less useful as a screening test since falsely elevated fT4 concentrations can be found in normal cats. In rare instances additional testing such as the T3 suppression test will be needed to confirm a diagnosis.
The goal of treatment for hyperthyroidism is to attain a euthyroid state. This can be accomplished with medications, radioactive iodine therapy, ethanol injections or thyroidectomy. Which treatment is chosen will depend upon various factors including cost, other concurrent diseases, especially renal failure, and the pros and cons of the various therapies.
With medical therapy, adverse side effects can make this therapy impossible in a given patient so that other options need to be pursued. In almost all patients, an attempt at medical therapy should be made before more permanent therapies.
Hyperthyroidism results in an increase in GFR, which can mask the severity of underlying renal issues. Medical management will help to identify whether significant renal issues develop with euthyroidism in a manner that is reversible. If renal function deteriorates significantly, a permanent resolution of hyperthyroidism with surgery or I131 therapy may be contraindicated.
Medical management of hyperthyroidism may include management of concurrent disease, especially of the cardiovascular system. The hyperthyroid state, especially if chronic, can result in a variety of cardiovascular abnormalities including hypertension. With hyperthyroidism, tachycardia is common with or without arrhythmias. Murmurs, gallop rhythms, and occasionally signs of congestive heart failure may be seen.
Hypertension is present in approximately 10 to 20 percent of newly diagnosed hyperthyroid cats (personal data).
Interestingly, some studies suggest that a significant number of cats treated successfully for hyperthyroidism go on to develop hypertension after treatment. It is, of course, always difficult to interpret blood pressure measurements in clinic since these cats are especially stress sensitive.
Figure 1 shows a blood pressure graph of a cat with hyperthyroidism in clinic with values that are consistent with a severe risk of TOD based on the elevated systolic blood pressure (191 mmHg), whereas a measurement at home (Figure 2) by the owner revealed the risk to be only mild (156 mmHg systolic). If marked cardiovascular abnormalities are present, these may need to be addressed with medical therapy.
Methimazole or carbimazole are the drugs of choice for medical management of hyperthyroidism, though carbimazole is not available in the U.S. other than through compounding pharmacies. Carbimazole is metabolized to methimazole. Recently a veterinary labeled methimazole in 2.5 and 5 mg tablets (Felimazole) has become available.
Methimazole inhibits synthesis of thyroid hormones by inhibiting thyroid peroxidase. It does not eliminate already formed thyroid hormones, so it may take several weeks before the complete T4 lowering effect is seen. It has been shown that dividing a given dose into two doses daily is preferable to giving that same amount once a day. A reasonable starting dosage is 2.5 mg BID.
In one study, 87 percent of cats given 2.5 mg twice daily normalized their T4 concentration after being treated two weeks, whereas this only happened in 54 percent of the cats given 5 mg once daily.2 In some instances, of course, pilling is a challenge and giving it once a day may be all that is feasible. Dosages are adjusted based on T4 measurement generally after two or better yet four weeks of therapy.
Methimazole can be administered as a topical gel, pluronic lecithin organogel, which facilitates diffusion of the medication from the skin surface into the bloodstream. Gels are a great option in cats that cannot be pilled.
The antithyroid effect achieved with topical gels is less reliable than oral administration, however, and dose titration may take longer with gels, resulting in the need for more blood draws and T4 determinations.3,4 Starting with 5 mg of methimazole in gel twice a day is a reasonable dosage, though many cats can be controlled with 2.5 mg BID.3,4
Adverse Side Effects
Complications do occur with these drugs, some of which respond to dosage decreases or gradual re-introduction; others necessitate stopping the medications permanently. GI side effects (vomiting, diarrhea, anorexia) are the most common signs seen and occur in 20 to 30 percent of the cats given oral methimazole. This percentage is much lower in cats where the gel is used.
This may be because the pills themselves are irritating or because the gels tend to result in much lower drug concentrations than the pills do. GI signs often resolve with short discontinuation of methimazole and gradual reintroduction. The side effects that are of greatest concern, however, are blood dyscrasias (thrombocytopenia, agranulocytosis, hemolytic anemia), intractable pruritus and hepatopathies.
These side effects do not occur frequently but they all necessitate discontinuing the medication permanently as these side effects are suspected to be dose independent. Blood dyscrasias have been seen in 3 to 9 percent, pruritus in 2 to 3 percent and hepatopathy in 2 percent of cats treated with methimazole.5 A bleeding disorder without thrombocytopenia has also been seen infrequently.
Appropriate therapeutic monitoring is absolutely vital given the chances of complications from these drugs as well as the potential for the development of azotemia as GFR decreases.5 A CBC, chemistry panel and T4 should be evaluated every two weeks for the first four to eight weeks of treatment to monitor for azotemia, neutropenia, anemia and thrombocytopenia.
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. Dosage can be titrated upward to effect. These only work with mild hyperthyroidism and can stop working at a later date. They can generally only be obtained through compounding pharmacies and they aren’t always available. <HOME>
|For more information, read Other Ways to Manage Feline Hyperthyroidism|
Dr. Carr is an associate professor at the University of Saskatchewan’s Western College of Veterinary Medicine.
This article first appeared in the April 2010 issue of Veterinary Practice News