Thyroid Panel
A blood test measuring thyroid hormone levels (T4, free T4, TSH) used to diagnose hypothyroidism and hyperthyroidism. Thyroid dysfunction is one of the most common endocrine disorders in dogs, affecting metabolism, energy, and longevity.
A thyroid panel is a blood test that measures the concentration of thyroid hormones and related markers to evaluate thyroid gland function. The thyroid gland, located in the neck, produces hormones that regulate metabolic rate, body temperature, heart rate, and energy utilization in virtually every cell.
Components of a Canine Thyroid Panel
| Test | What It Measures | Normal Range (approximate) |
|---|---|---|
| Total T4 | All circulating thyroxine (bound + free) | 1.0-4.0 mcg/dL |
| Free T4 (by equilibrium dialysis) | Unbound, biologically active thyroxine | 0.6-2.5 ng/dL |
| TSH | Thyroid-stimulating hormone from the pituitary | 0.03-0.5 ng/mL |
| T3 | Triiodothyronine (less commonly measured) | Variable |
| Thyroglobulin autoantibodies | Marker of immune-mediated thyroid destruction | Negative |
Free T4 by equilibrium dialysis is the most accurate single test. Total T4 alone has a high false-positive rate for hypothyroidism because non-thyroidal illness, medications (NSAIDs, corticosteroids, phenobarbital), and normal variation can lower it.
When to Test
Thyroid panels are indicated when dogs present with:
- Unexplained weight gain despite normal food intake
- Lethargy, exercise intolerance, cold intolerance
- Bilateral symmetric hair loss (non-itchy)
- Recurrent skin or ear infections
- Elevated cholesterol on routine chemistry
- Bradycardia (slow heart rate)
Routine thyroid screening is recommended for breeds with known predisposition after age 4-5, and for all senior dogs (7+) as part of comprehensive wellness testing.
Hypothyroidism and Longevity
Hypothyroidism is the most common endocrine disorder in dogs, caused in 95% of cases by immune-mediated thyroiditis (autoimmune destruction of the thyroid gland). Undiagnosed hypothyroidism accelerates aging through:
- Increased body fat and reduced lean muscle mass
- Elevated cholesterol and triglycerides (cardiovascular risk)
- Impaired immune function
- Reduced cognitive function and mental sharpness
With appropriate levothyroxine replacement therapy, hypothyroid dogs can return to normal metabolic function and expected lifespan. Early detection through routine screening prevents years of subclinical metabolic dysfunction.