Hyperthyroidism

Hyperthyroidism (Excessive Production of Thyroid Hormone)

OVERVIEW

  • Disease condition caused by high levels of thyroid hormones that increase metabolism in the body
  • The thyroid gland normally produces thyroid hormones in response to stimulation by the pituitary gland, the “master gland” of the body; the thyroid hormones normally increase chemical processes occurring within cells of the body; however, in hyperthyroidism, the excessive hormone levels put the cells and body into “overdrive”
  • Thyroid hormones are known as “triiodothyronine” or “T3” and “tetraiodothyronine” or “T4”

Genetics

  • No known genetic predisposition

Signalment/Description of Pet

  • Cats; most common hormonal (endocrine) disease of cats; one of the most common diseases seen in late middle-aged and old cats

Breed Predilections

  • None

Mean Age and Range

  • Mean age in cats, approximately 13 years; range 4–22 years

Signs/Observed Changes in the Pet

  • Involves many organ systems due to the overall increase in metabolism
  • Weight loss
  • Unkempt appearance
  • Poor body condition
  • Increased appetite (known as “polyphagia”)
  • Vomiting
  • Diarrhea
  • Increased thirst (known as “polydipsia”)
  • Rapid breathing (known as “tachypnea”)
  • Difficulty breathing (known as “dyspnea”)
  • Heart murmur; rapid heart rate (known as “tachycardia”); particular abnormal heart beat, known as a “gallop rhythm”
  • Hyperactivity
  • Aggression
  • Large thyroid gland
  • Thickened nails
  • Less than 10% of pets are referred to as “apathetic”; these pets exhibit atypical signs (such as poor appetite, loss of appetite, depression, and weakness)

Causes

  • Cats—autonomously hyperfunctioning thyroid nodules (where the thyroid nodules produce excess thyroid hormones outside of the control of the pituitary gland, so-called “autonomous” production); rarely, thyroid cancer (known as “thyroid carcinoma”)
  • Dogs—thyroid hormone (T4 or T3) secretion by a thyroid cancer (thyroid carcinoma)

Risk Factors

  • Some reports have linked hyperthyroidism in cats to some canned food diets
  • Advancing age increases risk

Treatment
Health Care

  • Outpatient management usually suffices for cats, if drugs that inhibit the production of thyroid hormones (known as “antithyroid drugs”) are used
  • Treatment using a radioactive form of iodine (known as “radioiodine treatment”) or surgical removal of the thyroid gland (known as “thyroidectomy”) require inpatient treatment and monitoring
  • Rare cases of congestive heart failure require emergency, inpatient intensive care

Activity

  • No alterations recommended

Diet

  • Poor absorption of many nutrients and high metabolism suggest the need for a highly digestible diet, with high availability of protein in untreated hyperthyroidism
  • Resolution of signs resulting from excessive levels of thyroid hormones in the body (condition known as “thyrotoxicosis”) eliminates the need for dietary modifications in many pets
  • Dietary modification may be necessary to treat or control complications (such as kidney damage)

Surgery

  • Surgical removal of the thyroid gland (thyroidectomy) is one recommended treatment for hyperthyroidism in cats
  • Surgical treatment of thyroid carcinoma (dogs and cats) is usually not curative, but can control signs (known as “palliative treatment”)

Medications

  • Medications presented in this section are intended to provide general information about possible treatment. The treatment for a particular condition may evolve as medical advances are made; therefore, the medications should not be considered as all inclusive.
  • Radioiodine (radioactive form of iodine) is a safe and effective treatment; use of radioactive iodine requires special facilities; availability of veterinary facilities offering this treatment is limited, but increasing
  • Methimazole (Tapazole) inhibits the production of thyroid hormones (antithyroid drug) and is recommended most often
  • Methimazole can be administered through the skin (transdermally); transdermal methimazole must be prepared by a pharmacist; resolution of signs resulting from the excessive levels of thyroid hormones in the body (thyrotoxicosis) takes longer with transdermal methimazole than with methimazole given by mouth
  • β-blockers—sometimes used to treat some of the heart and nervous system effects of excess thyroid hormones; can be used in combination with methimazole; mainly used to prepare the pet for surgical removal of the thyroid gland (thyroidectomy) or radioiodine therapy; atenolol is useful for controlling rapid heart rate (tachycardia), but addition of an ACE inhibitor is necessary to control high blood pressure (known as “hypertension”) in cats with hyperthyroidism
  • Carbimazole—another useful antithyroid drug that inhibits production of thyroid hormones; not available in the United States
  • Propylthiouracil—is an antithryoid drug that inhibits production of thyroid hormones; it can be useful if methimazole is unavailable; side effects may be more common and more severe than with methimazole
  • Ipodate—a radiographic contrast agent; can be used to treat some cases of mild hyperthyroidism, but not effective in most hyperthyroid pets; long-term effectiveness has not been established

Follow-Up Care
Patient Monitoring

  • Methimazole—physical examination, complete blood count with platelet count, serum biochemical analysis, and serum thyroid hormone (T4) determination every 2–3 weeks for the initial 3 months of treatment; the dosage is adjusted to maintain serum thyroid hormone (T4) concentration in the low-normal range
  • Surgical removal of the thyroid gland (thyroidectomy)—watch for development of low blood-calcium levels (known as “hypocalcemia”) and/or paralysis of the voice box (larynx) during the initial postoperative period; measure serum thyroid hormone (T4) concentrations in the first week of surgery and every 3–6 months thereafter, to check for recurrence
  • Radioiodine (radioactive form of iodine)—measure serum thyroid hormone (T4) concentrations 2 weeks after treatment and every 3–6 months subsequently
  • Kidney function—kidney filtration rates decline following treatment in most affected pets; therefore, perform a physical examination, serum biochemistry, and urinalysis 1 month after treatment and then as indicated by clinical history

Possible Complications

  • Untreated disease can lead to congestive heart failure; diarrhea that is difficult to treat; kidney damage; detachment of the retina (a layer in the back of the eye) as a result of high blood pressure (hypertension); and death
  • Complications of surgical treatment include low levels of parathyroid hormone (known as “hypoparathyroidism”; the parathyroid glands are small glands adjacent to the thyroid gland, which may be removed at the time of the surgical removal of the thyroid gland); low levels of thyroid hormone (known as “hypothyroidism”); and paralysis of the voice box (larynx)
  • Low levels of thyroid hormone (hypothyroidism) is rare following radioiodine therapy (radioactive form of iodine)

Expected Course and Prognosis

  • Uncomplicated disease—prognosis is excellent; recurrence is possible and most commonly is associated with poor owner compliance with medical management; regrowth of overactive thyroid tissue is possible, but uncommon after surgical removal of the thyroid gland (thyroidectomy) or radioiodine treatment (radioactive form of iodine)
  • Reported mean survival time for cats treated with radioiodine is 4 years; mean survival time for cats treated with methimazole is 2 years; mean survival time for cats treated with radioiodine and methimazole is 5.3 years
  • Cats with pre-existing kidney disease have a poorer prognosis; kidney failure is the most common cause of death in hyperthyroid cats
  • Dogs or cats with thyroid cancer (thyroid carcinoma)—prognosis is poor; treatment with radioiodine (radioactive form of iodine), surgery, or both usually is followed by recurrence of disease; chemotherapy is of questionable benefit

Key Points

  • Disease condition caused by high levels of thyroid hormones that increase metabolism in the body
  • Most common hormonal (endocrine) disease of cats; one of the most common diseases seen in late middle-aged and old cats
  • Rare in dogs
  • Potential side effects of drugs that inhibit the production of thyroid hormones (antithyroid drugs) and surgical complications
  • Be aware of possible (rare) recurrence after treatment

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