Hyperthyroidism is the most common endocrine disorder of the cat, and increased awareness of the disease has led to earlier diagnosis and treatment. Treatment of feline hyperthyroidism, however, can have complications, some of which are life-threatening and are best averted by careful patient monitoring. The hyperthyroid recheck profile aids veterinarians in monitoring hyperthyroid patients, and includes those values most pertinent to therapy of the uncomplicated patient. This includes: serum T4, BUN, creatinine, SAP, ALT, AST, CBC, and a platelet count. Consider measuring free T4 as well, in cases where concurrent nonthyroidal illness may reduce T4 levels.

Antithyroid drugs are often used for the initial management of hyperthyroid cats. They also may be given to control the condition in preparation for surgical thyroidectomy or radioactive iodine therapy. In cases where additional medical conditions or client preference and cost preclude these latter methods, antithyroid drugs are used on a long-term basis. Treatment complications include side effects of the drugs themselves, or sequelae of the transition to euthyroidism.

The most commonly used antithyroid drug is methimazole (Tapazole®, Jones Med.), a thioureylene which inhibits synthesis of thyroid hormones. Adverse effects of methimazole range from mild to life-threatening. Less serious reactions including anorexia, vomiting, and lethargy may not necessitate cessation of therapy, if transient, but can be persistent enough to preclude continued use of the drug. Methimazole is bitter tasting, and placing it in a gelatin capsule prior to administration may be helpful. Self excoriation, particularly of the head and neck, is another potential side effect, which requires that the drug be discontinued. Some authors recommend use of glucocorticoids in this situation.

More serious adverse methimazole reactions include hematologic and biochemical changes such as: thrombocytopenia, leukopenia, agranulocytosis, positive Coombs' test, antinuclear antibodies, and milder findings like eosinophilia and lymphocytosis. Hepatic toxicity can occur, with elevation of liver enzymes. Potentially life-threatening abnormalities, particularly hepatopathy, thrombocytopenia and significant leukopenia, require withdrawal of therapy. The possibilities of other bleeding complications is currently under investigation.

The deterioration of renal function following initiation of antithyroid therapy has become a rising and significant concern. This may reflect a fall in glomerular filtration rate with return to euthyroidism. Although the relationship is not yet fully understood, monitoring renal function before and following treatment of hyperthyroidism is crucial. Irreversible therapy (e.g., surgical thyroidectomy or radioactive iodine treatment) should be postponed until it has been determined that renal function will be adequate once euthyroidism is achieved. Occasionally, when adverse reaction is severe enough to preclude even short-term administration of antithyroid drugs, owners should be made aware of the risks involved with undertaking other therapies. In some patients, renal failure may prevent use of antithyroid therapy, or necessitate only partial control of hyperthyroidism.

Before initiation of therapy, complete baseline data should be collected, including serum biochemical and thyroid profile, CBC with platelet count, and urinalysis. Blood pressure measurement is ideal if equipment is available, and an echocardiogram may be desired if a heart murmur is present. Hematologic changes associated with hyperthyroidism include erythrocytosis, stress leukogram, hyperphosphatemia, and elevation of liver enzymes (ALT, AST, SAP), and should resolve with therapy, if not due to an unrelated condition. The finding of azotemia necessitates proceeding with extreme caution, with frequent monitoring for deterioration of renal function.

In the absence of complicating abnormalities, treatment with methimazole is begun, and the patient is rechecked in about 2 weeks. Then, a hyperthyroid recheck profile is made, along with monitoring for weight gain and improvement in clinical signs. For the azotemic patient or one where renal dysfunction is suspected, kidney values should be measured earlier (e.g., 1 week or sooner if clinical signs develop). Drug dosage is adjusted, as required, and a recheck profile is collected at 2 week intervals until euthyroidism is achieved, and stable on 2 consecutive measurements. Discovery of serious adverse effects, such as thrombocytopenia or significant leukopenia, requires drug withdrawal. Worsening azotemia may require lowering the drug dose or halting therapy.

When euthyroidism has been satisfactorily achieved, more definitive therapy may be undertaken or medical therapy can be continued. If radioactive iodine therapy is planned, the referral center should be consulted regarding withdrawal of antithyroid drugs before treatment. If long term therapy with antithyroid drugs is elected, the hyperthyroid recheck profile should be evaluated approximately every 3 months. More extensive blood work may be indicated. Higher doses of antithyroid drugs may be needed as the disease progresses. With methimazole, this may be associated with an increased risk of serum antinuclear antibodies. Owner compliance and patient cooperation also may vary as dosage requirements escalate.