What is Thyroid Testing?

Thyroid testing measures the levels of thyroid hormones in your pet's blood to detect dysfunction of the thyroid gland. The thyroid gland, located in the neck, produces hormones (primarily T4 and T3) that regulate metabolism, affecting virtually every organ system in the body. When thyroid hormone production is too low or too high, it can cause significant health problems.

Thyroid disease is one of the most common endocrine (hormonal) disorders in both cats and dogs, but it manifests very differently between the two species.

Hypothyroidism in Dogs

Hypothyroidism (underactive thyroid) is the most common hormonal disorder in dogs. The thyroid gland produces insufficient hormones, slowing metabolism throughout the body.

Causes

  • Lymphocytic thyroiditis: Autoimmune destruction of the thyroid gland (most common)
  • Idiopathic thyroid atrophy: The gland wastes away for unknown reasons
  • Rarely: thyroid tumors or congenital defects

Symptoms

  • Weight gain without increased appetite
  • Lethargy and mental dullness
  • Cold intolerance (seeking warm places)
  • Dry, dull coat with excessive shedding
  • Skin thickening, especially on the face ("tragic expression")
  • Recurrent skin and ear infections
  • Hair loss (bilaterally symmetric, often on the trunk and tail)
  • Slow heart rate
  • Reproductive issues (infertility, irregular cycles)

Commonly Affected Breeds

  • Golden Retriever
  • Doberman Pinscher
  • Irish Setter
  • Dachshund
  • Cocker Spaniel
  • Boxers
  • Usually diagnosed at 4–10 years of age

Hyperthyroidism in Cats

Hyperthyroidism (overactive thyroid) is the most common endocrine disorder in cats, affecting approximately 10% of cats over 10 years old. The thyroid gland produces excess hormones, accelerating metabolism.

Causes

  • Benign thyroid adenoma: A non-cancerous growth on the thyroid gland (97% of cases)
  • Thyroid carcinoma: Malignant thyroid tumor (rare, approximately 3%)

Symptoms

  • Weight loss despite increased appetite (the hallmark sign)
  • Increased thirst and urination
  • Hyperactivity and restlessness
  • Vomiting and diarrhea
  • Rapid heart rate and sometimes heart murmur
  • Unkempt coat
  • Behavioral changes (irritability, vocalization)
  • Palpable thyroid nodule in the neck
  • In advanced cases: heart disease (thyrotoxic cardiomyopathy)

Diagnostic Tests

Total T4 (Thyroxine)

  • The primary screening test for both hypothyroidism and hyperthyroidism
  • Dogs: Low T4 suggests hypothyroidism
  • Cats: Elevated T4 confirms hyperthyroidism
  • Simple, affordable, and widely available

Free T4 (by Equilibrium Dialysis)

  • Measures the unbound, biologically active form of T4
  • More accurate than total T4 in some situations
  • Useful when total T4 is borderline or when concurrent illness may be affecting results

TSH (Thyroid Stimulating Hormone)

  • Measured in dogs alongside T4
  • Elevated TSH with low T4 strongly supports hypothyroidism
  • Not as useful in cats

T3 (Triiodothyronine)

  • Sometimes measured alongside T4
  • Less commonly used as a standalone test

Thyroid Scintigraphy (Nuclear Scan)

  • Used primarily in cats to identify the location and extent of overactive thyroid tissue
  • Helps plan surgical or radioactive iodine treatment

When to Test

Dogs

  • When symptoms of hypothyroidism are present
  • Breeds predisposed to hypothyroidism: consider baseline testing at 4–6 years
  • During investigation of unexplained weight gain, lethargy, or skin problems
  • Before breeding in predisposed breeds

Cats

  • All cats over 7–8 years: Annual thyroid screening is recommended
  • When weight loss, increased appetite, or hyperactivity develops
  • During routine senior wellness blood panels

Treatment

Hypothyroidism (Dogs)

  • Levothyroxine: Oral synthetic thyroid hormone, given daily for life
  • Inexpensive and highly effective
  • Symptoms typically improve within weeks
  • Regular monitoring (T4 levels every 6 months) to adjust dosing

Hyperthyroidism (Cats)

Several treatment options:

  • Methimazole (Tapazole): Daily medication that blocks thyroid hormone production; requires lifelong administration and regular monitoring
  • Radioactive iodine (I-131): The gold standard; a single injection destroys overactive thyroid tissue with a 95%+ cure rate; requires specialized facilities
  • Thyroidectomy: Surgical removal of the affected thyroid gland
  • Prescription diet (Hill's y/d): Iodine-restricted food that limits hormone production; must be the exclusive diet

Prognosis

With appropriate treatment:

  • Hypothyroid dogs return to normal health and have a normal lifespan
  • Hyperthyroid cats treated with radioactive iodine are typically cured
  • Cats on methimazole can be well-managed for years
  • Untreated thyroid disease in both species can lead to serious complications
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