Hyperthyroidism
The thyroid glands sit on either side of the neck in dogs and cats, and these glands are responsible for maintaining normal metabolic rates. With hyperthyroidism, the thyroid glands produce excessive thyroid hormones which leads to a variety of problems. This is mainly a disease of cats, but dogs with thyroid cancers also can be hyperthyroid. Cats usually have what is called benign hypertrophy of the thyroid gland (meaning that the cause of the overproduction usually is not cancer but just an overgrowth of the gland itself) but they can also have thyroid cancer in rare cases.
Symptoms
The hallmark sign of hyperthyroidism is weight loss despite a ravenous appetite. These cats are usually older (average 12-13 years old) and may also have an increased activity level or a behavior change tending toward irritability. An excessive level of thyroid hormone also affects the circulatory system so any symptoms associated with the heart (weakness, collapse, trouble breathing) or with high blood pressure (weakness, collapse, sudden blindness) can signal a problem with the thyroid gland as well.
Often initial symptoms of restlessness and weight loss go unnoticed, and some cats present to us already in heart failure, which makes treatment difficult. If you notice any early symptoms of hyperthyroidism in your cat, please get him/her evaluated as soon as possible!
Diagnosis
We always feel for enlarged thyroid glands in older normal cats as well as in cats with clinical symptoms that are suggestive of hyperthyroidism. One or both glands are usually enlarged in cats with hyperthyroidism and can easily be felt by a veterinarian. A blood test is run to confirm elevated levels of circulating thyroid hormone. Usually this is all that is needed to diagnose hyperthyroidism. In some borderline cases, repeated testing or more advanced testing may be recommended.
Once bloodwork confirms hyperthyroidism, we will recommend full bloodwork to evaluate kidney and liver function as well as chest x-rays or echocardiogram (heart ultrasound) to check for heart damage. This will help us identify the best method of treatment and let us know what conditions we will need to treat in addition to the hyperthyroidism.
Treatment
There are four main ways to treat hyperthyroidism:
Complications of Surgery and Radiated Iodine Therapy (explanations)
Symptoms
The hallmark sign of hyperthyroidism is weight loss despite a ravenous appetite. These cats are usually older (average 12-13 years old) and may also have an increased activity level or a behavior change tending toward irritability. An excessive level of thyroid hormone also affects the circulatory system so any symptoms associated with the heart (weakness, collapse, trouble breathing) or with high blood pressure (weakness, collapse, sudden blindness) can signal a problem with the thyroid gland as well.
Often initial symptoms of restlessness and weight loss go unnoticed, and some cats present to us already in heart failure, which makes treatment difficult. If you notice any early symptoms of hyperthyroidism in your cat, please get him/her evaluated as soon as possible!
Diagnosis
We always feel for enlarged thyroid glands in older normal cats as well as in cats with clinical symptoms that are suggestive of hyperthyroidism. One or both glands are usually enlarged in cats with hyperthyroidism and can easily be felt by a veterinarian. A blood test is run to confirm elevated levels of circulating thyroid hormone. Usually this is all that is needed to diagnose hyperthyroidism. In some borderline cases, repeated testing or more advanced testing may be recommended.
Once bloodwork confirms hyperthyroidism, we will recommend full bloodwork to evaluate kidney and liver function as well as chest x-rays or echocardiogram (heart ultrasound) to check for heart damage. This will help us identify the best method of treatment and let us know what conditions we will need to treat in addition to the hyperthyroidism.
Treatment
There are four main ways to treat hyperthyroidism:
- Diet - Hills Prescription Diets has come out recently with a brand new diet designed for hyperthyroid cats so that they do not have to take medication or have surgery performed. This diet works because there are virtually no levels of iodine present in the food. The thyroid gland is responsible for the body's uptake of iodine, and with a food that is iodine deficient, the overactive thyroid gland no longer functions abnormally, thus alleviating your cat's symptoms. There are some important points to consider when choosing dietary therapy. First, no other food can be eaten, including treats. Any food given besides the thyroid diet will cause the thyroid gland to become abnormal. Second, the diet will not treat any thyroid cancer present.
- Medical therapy alone - We use a drug called Methimazole to suppress the release of thyroid hormone from the thyroid gland. This will allow the thyroid hormone to return to normal circulating levels and symptoms should resolve. This medication can be applied in a topical ear gel which most cats tolerate well. The cats will need to be on medication for the rest of their lives, and side effects can be common during the first three months. Monitoring will need to be performed several times a year to make sure the medication is being well tolerated. This includes bloodwork, blood pressure monitoring and monitoring blood thyroid levels.
- Surgical removal of the thyroid gland - This involves removing the affected thyroid gland. We will place these cats on Methimazole for a minimum of 15-30 days prior to surgery to get their thyroid levels under control before anesthesia (this helps reduce the risk of complications during surgery). Risks of surgery include anesthetic complications (which increase if heart disease is present - this is why we recommend evaluating the heart prior to any surgery), laryngeal nerve damage, Horner's syndrome, worsening kidney disease, hypocalcemia, and hypothyroidism (see below for explanations of each of these complications). It takes about 1-2 months for thyroid hormone levels to stabilize after surgery. In most cases, surgery is curative and does not require any medication long-term except for underlying diseases (heart and kidney disease). In some cases, the thyroid gland that does not visibly appear to be affected can in fact be in the early stages of hypertrophy. If possible, getting a nuclear scan done prior to surgery will help us determine if one or both thyroid glands need to be removed (see explanation below in radiated iodine therapy). This scan, however, is sometimes cost-prohibitive - in these cases we simply remove the gland that is obviously affected and warn the owners that symptoms may return in the future if the other gland is affected as well. We always submit the thyroid gland for histopathology if a nuclear scan has not been performed to check for malignant cancer. If the thyroid gland comes back cancerous, we recommend the cat be evaluated by a facility that can perform a nuclear scan. Even if the other thyroid gland is not overactive at the time of surgery, there is always the possibility that it may become hyperactive in the future and medical treatment may be necessary.
- Radiated iodine therapy - This is generally considered to be the safest and most effective method of treating hyperthyroidism; however it is also expensive. To confirm hyperthyroidism and to evaluate for malignancy, the cat is given a compound that will be taken up by the overactive thyroid tissue and show up on the nuclear scan. All thyroid tissue that is abnormal will show up on the scan, so we can tell if one or both glands is affected and, in the case of malignant cancers, if the cancer has spread to other places in the body. The cat is then injected with a radioactive iodine compound, which is also taken up into the thyroid glands. Being radioactive, this compound destroys surrounding thyroid tissue, thus eliminating the disease condition. This compound is highly specific for only thyroid tissue so the rest of the body is spared. Because the cat will then be radioactive, hospitalization is required for 4-5 days after the treatment is given. This treatment is only needed once and no ongoing therapy is required. It is also safer for cats with heart disease because no anesthesia is needed, but this may not be a good option for cats with kidney disease.
Complications of Surgery and Radiated Iodine Therapy (explanations)
- Laryngeal paralysis - the laryngeal nerve sits next to the trachea near the thyroid gland and if damaged can cause a temporary or permanent voice change, or have an increased risk for aspiration pneumonia or coughing.
- Horner's syndrome - a bundle of sympathetic nerves also sits near the thyroid gland and if damaged can cause the eye to become squinty, have the third eyelid protruded, and cause the pupil to become constricted.
- Hypothyroidism - if too much thyroid tissue is removed (as sometimes is necessary) then the cat may need to be supplemented with thyroid hormone for the rest of its life. Hypothyroidism, however, has less severe side effects than hyperthyroidism.
- Hypocalcemia - the parathyroid glands sit touching the thyroid glands. We take care not to remove these glands with the thyroid gland, but in some instances these parathyroid glands can be damaged. These glands are responsible for regulating blood calcium levels, and if removed or damaged the cat can develop low blood calcium. When removing one thyroid gland, there is usually not a risk of developing hypocalcemia post-operatively, but when removing both glands this becomes a possible issue following surgery.
- Worsening kidney disease - in some hyperthyroid cats, the overproduction of thyroid hormone can actually be beneficial for kidney disease and mask its signs. When we correct the thyroid disease, these cats often experience a worsening of their kidney disease. This is why we recommend treating with medication for 30 days prior to doing surgery or iodine radiation. If kidney disease worsens on therapy, then we like to keep these cats on medication only or not medicate them at all depending on how severe their signs are.