Total T4 (Serum Thyroxine Test)
A quantitative serum immunoassay measuring the total concentration of circulating thyroxine (protein-bound and free T4).
💬 In Plain English
Total T4 is the standard baseline blood test used to check thyroid function in pets. In cats, high Total T4 levels diagnose hyperthyroidism (an overactive thyroid gland that causes older cats to lose weight, eat ravenously, and develop high blood pressure). In dogs, low Total T4 levels raise suspicion for hypothyroidism (an underactive thyroid causing weight gain, sluggishness, and hair loss). However, because many common illnesses (like infections or arthritis) can falsely suppress T4 in dogs—a phenomenon known as "Euthyroid Sick Syndrome"—a low T4 in dogs must often be confirmed with a Free T4 by equilibrium dialysis and a TSH test.
🔬 Clinical & Technical Depth
For Vets & TechniciansFormal Clinical Definition
Quantitative immunoassay determining the concentration of circulating serum thyroxine (L-3,5,3',5'-tetraiodothyronine), evaluating endocrine secretory capacity of the thyroid follicular epithelium.
Why Veterinarians Use It
Veterinarians use Total T4 as the initial screening test for feline hyperthyroidism, canine hypothyroidism, and for monitoring thyroid drug therapies.
Clinical Context Encountered
Included on senior blood panels, weight-loss evaluations in cats, and sluggishness workups in dogs.
Clinical Reference Guide (Diagnostic Test)
Normal vs. Abnormal Results
Canine normal: 1.0 to 4.0 ug/dL; Feline normal: 0.8 to 4.0 ug/dL. High (> 4.0 ug/dL) indicates feline hyperthyroidism. Low (< 1.0 ug/dL) indicates canine hypothyroidism or euthyroid sick state.
Next Steps If Abnormal
High in cats: initiate methimazole or radioiodine (I-131). Low in dogs: run full thyroid panel (Free T4 by equilibrium dialysis and cTSH) before prescribing levothyroxine.
- •A low Total T4 in a sick dog does NOT automatically prove hypothyroidism: non-thyroidal systemic illness routinely pushes T4 low; always test Free T4 and TSH before starting lifelong pills.
❓ Questions to Ask Your Veterinarian
Take these actionable questions with you to your next clinic visit or appointment:
- 1In my cat, is the elevated T4 consistent with hyperthyroidism (requiring methimazole, diet, or I-131)?
- 2In my dog, is the low T4 caused by true hypothyroidism or Euthyroid Sick Syndrome from another illness?
- 3If monitoring thyroid medication, was this blood drawn at the ideal peak time (4–6 hours post-pill)?
Academic & Clinical References
- AAHA Guidelines for the Diagnosis of Canine Hypothyroidism and Feline HyperthyroidismOrganization: American Animal Hospital Association (AAHA) • Accessed: 2026-01-10https://www.aaha.org