Thyroid Disease in Dogs and Cats

Thyroid disease is a common endocrine (i.e., hormonal) abnormality that affects dogs and cats, although they suffer from different thyroid conditions. Dogs typically are affected by hypothyroidism (i.e., underactive thyroid), whereas cats usually suffer from hyperthyroidism (i.e., overactive thyroid). Our team at Southern Arizona Veterinary Specialty and Emergency Center wants to provide information about these conditions, so you can recognize the signs if your pet is affected.

Thyroid disease in dogs

Skip to hyperthyroidism in cats

What causes thyroid disease in dogs?
Dogs experiencing thyroid issues are typically affected by hypothyroidism, which occurs when the thyroid gland, located in the neck, fails to produce adequate amounts of the hormones thyroxine (T4) and triiodothyronine (T3). This is most commonly caused by a low functioning thyroid or an inflamed thyroid resulting from another disease (e.g., idiopathic thyroid gland atrophy or immune-mediated lymphocytic thyroiditis). T3 and T4 are involved in many metabolic processes, and low thyroid hormone levels can cause a wide array of clinical signs and laboratory abnormalities.

What are hypothyroidism signs in dogs?
Thyroid hormones play a role in many metabolic processes, and decreased levels can result in numerous symptoms.

  • Metabolic signs — Affected dogs frequently experience unexplained weight gain, and tend to be lethargic and cold intolerant.
  • Hair coat signs — Dogs may exhibit hair loss over their trunk, their remaining hair coat may be dull and lusterless, and they may not regrow hair after being clipped.
  • Skin signs — Dogs may have skin changes, including scaling, seborrhea (i.e., dandruff), hyperpigmentation, and recurrent infections.
  • Neuromuscular signs — Facial nerve paralysis, laryngeal paralysis (i.e., difficulty vocalizing), and vestibular signs (i.e., poor balance) may occur in some cases.
  • Gastrointestinal signs — Affected dogs may experience constipation or diarrhea.

How is hypothyroidism diagnosed in dogs?
A dog’s history and physical examination can raise suspicion about hypothyroidism. A complete blood count (CBC) and biochemistry profile can help rule out other potential issues, but abnormal results on these tests are not hypothyroidism-specific. If the condition is suspected, further blood tests are needed.

  • Total T4 concentration — This is a useful screening test for hypothyroidism. If the total T4 concentration is in normal limits, the dog is likely not hypothyroid, but if the value is below the reference range, further testing is required.
  • Free T4 and thyroid stimulating hormone (TSH) — If the total T4 is below the reference range, free T4 and TSH are tested. If the free T4 is low and the TSH is high, the dog is hypothyroid.

How is hypothyroidism treated in dogs?
Affected dogs require lifelong supplementation with a thyroid hormone replacement. A starting dose is recommended based on the dog’s weight, and the levels are re-evaluated in about eight weeks to determine if the dose is appropriate. Signs can take two to three months to resolve, and the dog’s dose may need adjusting over time.

Thyroid disease in cats

What causes thyroid disease in cats?
Hyperthyroidism, which is the most common hormonal imbalance in cats, is caused by a benign growth on the thyroid gland that overproduces T4. Why these tumors develop is unclear, but chemicals in the cat’s diet or environment may play a role. Thyroid tumors are rarely malignant (i.e., not likely to spread); malignancy is found in less than 3% of hyperthyroid cats.

What are hyperthyroidism signs in cats?
Multiple systems are affected when T4 levels are too high. Signs include:

  • Metabolic signs — Many cats experience weight loss, despite a ravenous appetite, and hyperactivity, aggression, and restlessness are common.
  • Gastrointestinal signs — Affected cats can exhibit vomiting and diarrhea.
  • Urinary signs — About 50% of cats have increased thirst and urination.
  • Cardiac signs — Hyperthyroid cats typically have an elevated heart rate, which can lead to increased pumping pressure. This may result in high blood pressure, or a condition called thyrotoxic cardiomyopathy in which the heart enlarges.

How is hyperthyroidism diagnosed in cats?
Total T4 levels are typically extremely high in hyperthyroid cats, making diagnoses straightforward when these levels are tested. However, in some cases, further testing involving free T4 or a T3 suppression test may be required. Additional diagnostics also may be needed before treatment is started.

  • Kidney enzymes — Your cat’s kidney values will be assessed to ensure they are healthy enough to undergo specific treatment protocols.
  • Blood pressure — Your cat’s blood pressure will be measured to ensure the hyperthyroidism has not resulted in them being hypertensive.
  • Chest X-rays — Chest X-rays may be recommended to ensure the cat is not affected by thyrotoxic cardiomyopathy.
  • Thyroid scintigraphy — Further thyroid gland imaging may be necessary, to determine the tumor size before treatment.

How is hyperthyroidism treated in cats?
Treatment in cats can be addressed in several ways.

  • Oral medication — Drug therapy can be used to suppress T4 production, and the dosage may need adjusting over time. Some cats experience kidney issues while on the medication, and are not candidates for this treatment protocol.
  • Radioactive iodine — Radioactive iodine can be injected to destroy the abnormal tissue and provide a permanent cure. This treatment requires hospitalization for one to two weeks, since cats will be radioactive after the injection.
  • Surgery — The abnormal tissue can be surgically removed to provide a permanent cure.
  • Prescription diet — Feeding an iodine-limited diet can also resolve clinical signs.

Thyroid disease can be problematic for dogs and cats, but the condition can be treated if diagnosed properly. If your pet is experiencing thyroid condition complications, contact our Southern Arizona Veterinary Specialty and Emergency Center central location or east side location, so we can provide the care they need.

Endocrine — Referring to the system that produces and controls hormone levels throughout the body

Hyperthyroidism — Overproduction of thyroid hormones

Hypothyroidism — Underproduction of thyroid hormones

Thyroxine (T4) — Hormone produced by the thyroid gland

Triiodothyronine (T3) — Hormone produced by the thyroid gland