Updated: June 26, 2014
Hypothyroidism is a disorder of the thyroid gland. This gland is responsible for producing and secreting thyroid hormone (thyroxine), which affects nearly all body systems. Thyroxine is the hormone that is primarily responsible for regulating the metabolic rate of many different tissues in the body. In hypothyroidism, not enough thyroxine is produced, which causes the metabolism of these tissues to slow down.
Hypothyroidism is a clinical disease that occurs as a result of decreased blood levels of serum thyroid hormones.
Hypothyroidism is very rare in the cat. It arises most often following treatment of hyperthyroidism in the cat. Hyperthyroidism (overactive thyroid gland) is the most common thyroid disorder of cats.Causes Surgical removal of both thyroid glands results in hypothyroidism. Surgical removal of the thyroid glands is done to treat hyperthyroidism in the cat.
Radioactive iodine therapy for hyperthyroidism may result in under production of thyroxine.
Use of the drug, methimazole, to treat hyperthyroidism may also result in low out put of thyroxine. The effects of this drug are usually transient, and thyroid hormone production usually increases once the drug is stopped or decreased.
Thyroid tumors (neoplasia) are an uncommon cause of hypothyroidism. Unless the tumor affects both lobes of the thyroid and the gland is thoroughly destroyed, hormone output usually remains normal.
Dietary iodine deficiency is a rare cause of hypothyroidism. Most commercial cat foods contain adequate levels of iodine. It is possible for the iodine restricted diet produced by Hills called y/d could cause clinical signs of hypothyroidism in normal cats fed this prescription diet.
Theoretically, other systemic illnesses and medications may adversely affect the function of the thyroid gland, but such conditions are poorly described in the cat.
What To Watch For
A deficiency of thyroid hormone affects the metabolic function of many organ systems. As a result, the clinical signs are usually variable, non-specific, and slow to develop. Although there is no one symptom that is diagnostic, several combined signs may make your veterinarian more suspicious. Symptoms may include:
Lethargy, lack of interest in play, increased amounts of sleeping
Weight gain and obesity
Cold intolerance – seeks out warm places to lie down, low body temperature
Chronic skin disorders, such as dry skin, thinning of the hair coat, excessive hair loss
Hypothyroidism is not always a simple, straightforward disease to diagnose. Various tests are available to diagnose the condition and a combination of tests may be required. Proper diagnosis also includes a thorough history, documentation of clinical signs, a thorough physical examination, and diagnostic tests to assess various organ functions, including thyroid function. A diagnostic work-up may include the following:
Complete blood count (CBC)
Thyroxine (T4) level, tri-iodothyronine (T3) level
Serum Free T4 by Equilibrium Dialysis (FT4ED)
Thyrotropin stimulation test
Thoracic (chest) and abdominal radiographs (X-rays) in certain cases
Other tests to rule out other hormonal disorders, such as hyperadrenocorticism (overproduction of cortisone hormone) and acromegaly (over production of growth hormone)
If the use of methimazole is responsible for the signs of hypothyroidism, the drug is stopped for several days or until the T4 level returns to normal. Methimazole may then be re-instituted at a lower dose.
Thyroid hormone supplementation is indicated for the treatment of other forms of hypothyroidism, and it is administered for the life of the individual.
Synthetic (man-made) levothyroxine (T4) is the drug of choice for treating hypothyroidism. The dosage and frequency of administration are determined by your veterinarian. Levothyroxine is usually given once daily in cats.
There are both brand name and generic levothyroxine products available commercially. Use of a brand name product rather than a generic product is usually preferred. Once a cat is stabilized on a particular thyroid medication, it is also better if the cat remains on that product consistently, rather than bouncing from one product to another.
Optimal treatment requires a combination of home and professional veterinary care. At home it is important to administer all medication exactly as prescribed by your veterinarian. With appropriate therapy, most of the clinical alterations associated with hypothyroidism improve within four to six weeks.
Most cats tolerate thyroid supplementation very well, however, over dosage is associated with return of the signs of hyperthyroidism. Watch the cat closely for signs of hyperactivity, increased vocalization (meowing, howling), restlessness, weight loss and diarrhea, and report these signs to your veterinarian promptly.
It is important to follow-up with regularly scheduled visits to your veterinarian so that both your cat's clinical signs and thyroid concentration in the blood can be monitored. Generally, the first follow-up examination is within four weeks after the start of therapy. T4 levels are often measured six to eight hours after the pill is given. Adjustments in the dosage of medication are then recommended depending upon the results of these tests. Additional recheck visits are then scheduled based upon the test results, changes in clinical signs, and any alterations in the medication schedule.
Careful monitoring during the treatment of hyperthyroidism can often prevent hypothyroidism in the cat. If removal of both thyroid glands can be avoided, then normal output of thyroxine should continue. Periodic measurement of T4 during methimazole therapy helps to prevent over treatment with this drug. Careful dosing of radioactive iodine during radiation therapy of hyperthyroidism is also helpful in avoiding the development of hypothyroidism.