Introduction (Causes): Feline hyperthyroidism is a disease of middle and older age cats resulting from excessive amounts of circulating thyroid hormones (T4 and T3). The cause is most often associated with benign but functional adenomatous hyperplasia of one or both lobes of the thyroid gland or of extra-thyroid-gland thyroid tissues. These tissues secrete thyroid hormone autonomously, in the absence of TSH secretion from the pituitary gland (see primer for explanation). Rarely (only 1-2%), hyperthyroidism occurs as the result of a malignant, functional thyroid carcinoma.
Clinical Signs: Excessive thyroid hormone results in the stimulation of metabolic process processes in nearly all tissues of the body. There are many potential clinical signs, (see pictorial montage below too) but the most common are:
- Weight loss, decreased muscle mass, emaciation, weakness
- Increased appetite
- Tachycardia (rapid heart) and hypertension (high blood pressure)
- Heart murmur and/or arrhythmias, secondary hypertrophic cardiomyopathy (heart muscle thickening with compromised cardiac performance, possible congestive heart failure)
- Poor hair coat; self-induced patchy alopecia
- General irritability
Other (less common) clinical signs might include:
- Increased drinking/urinating (thyroid hormones may have some diuretic activity)
- Vomiting, diarrhea
- Poor appetite, depression (about 10% of cats)
- Lethargy, muscle tremors
- Panting
Treatment of Hyperthyroidism:
Treatment options are medical, surgical and radiaton. The appropriate treatment is selected based on concurrent medical "issues", and potential complications of the treatment, which vary animal to animal. For example, treatment of hyperthyroidism sometimes reveals occult kidney disease, which then manifests as overt renal failure. One proposed explanation for this observation is that the hyperthyroidism, by virtue of its effect on blood pressure forces the kidneys to filter more urine than they otherwise would. Correcting the elevated blood pressure (via treating hyperthyroidism) results in poorer perfusion of the kidneys, and less production of urine. All this is theory and it is possible that there are other, more important direct effects of thyroid hormone on renal function. (Otherwise we should be treating all cats with kidney failure with thyroid hormone!!!). Other potential complications are described below.
Of course, in many instances, cost must be considered a mitigating factor as well.
- Medical: In general, medical therapy should be tried before the other (irreversible) options; if overt renal disease is detected after instituting treatment, dosages of medication are adjusted in a manner to achieve control of the thyroid AND to minimize exacerbation and progression of the damage to kidneys. Animals who develop renal disease with medical treatment are NEVER treated surgically or with radiation
- Drugs are used to control the synthesis, release and action of thyroid hormone.
- Methimazole (Tapazole®): this is the drug of choice with fewest side effects., occuring in about 15% of treated cats. Most common of these include lethargy, anorexia, vomiting and/or diarrhea, facial prurits (itching-rubbling at the face).
- Propylthiouracil (PTU): While very effectively blocking thyroid hormone synthesis, the number and severity of potential side effects (the ones described above for methimazole PLUS hemolytic anemia [the immune system actually destroys the circulating red blood cells], hemorrhage due to thrombocytopenia [too few circulating platelets...and thus ineffective blood clotting]) make this an unacceptable choice for therapy.
- Carbimazole (Neo-Mercazole®):This drug is primarily available in Europe, where methimazole is not available. It is metabolized in the cat and converted to methimazole. The bioavailability of converted product is less than that obtained with an equivalent dose of methimazole. There are fewer side effects than with the other previously mentioned products.
- Ipodate (Orografin®): Ipodate is a radiodense organic iodine compound that is usually used for dye-assisted radiographic evaluation of abdominal structures. One unique feature of this material is that presence of the dye prevents the conversion of T4 to T3 (remember, T3 is the biologically active form of thyroid hormone!). If used to treat hyperthyroidism, then one must moniter T3 levels (NOT T4) to assess efficacy.
- Telepaque: At times the availability of Ipodate from suppliers has been a problem and a good alternative is this quite similar substance.
- Surgery:
- The normal position of the thyroid gland lobes is in the neck. Removal of these can be accomplished surgically. Complications of surgery are:
- Inadvertent removal of the tiny parathyroid glands which are in close association with the thyroid. The loss of the parathyroid glands can result in hypocalcemia (low serum calcium levels), resulting in muscular twitching, seizures, and cardiac arrythmias
- HyPOthyroidism. Usually not a significant clinical problem in cats, but if necessary, this is easily treated with thyroid supplementation, as described for dogs (see Hypothyroidism)
- Inadequate removal of thyroid gland tissue. Sometimes thyroid tissue has migrated to areas deep to the neck, and even down in the chest. Surgical removal of glandular tissue in the neck will not effect thyroid tissues that have migrated to other locations. Ideally, a technetium scan to determine the presence of "extrathyroidal" thyroid tissue is recommended prior to surgery.
- Progression of occult renal disease to overt renal failure (see above)
- Radioactive Iodine (I 131): Radioactive iodine is concentrated in the thyroid tissue (wherever that tissue happens to reside!). The radiation irreversibly destroys the active tissue. Usually cats are clincially normal within a few days or a couple of weeks after exposure. Treatment must be performed a a facility licensed to handle and safely dispose of all radioactive materials. There are approximately 25 licensed treatment centers in the United States.
