Thyroid Testing in Dogs & Cats: T4 and TSH Guide¶
Thyroid testing is the most common endocrine workup in small-animal practice — but interpretation differs fundamentally between dogs (hypothyroidism) and cats (hyperthyroidism).
The Two-Species Difference¶
| Species | Primary Disorder | Key Finding |
|---|---|---|
| Canine | Hypothyroidism | Low T4, high TSH |
| Feline | Hyperthyroidism | Elevated T4 |
Test Menu¶
| Test | Use | Species |
|---|---|---|
| T4 (total) | Screening | Canine, Feline |
| TSH | Confirm primary hypothyroidism | Canine |
Interpreting the Dog: T4 + TSH¶
| T4 | TSH | Interpretation |
|---|---|---|
| Low | High | Primary hypothyroidism |
| Low | Normal | Euthyroid sick syndrome OR early hypothyroidism — retest |
| Normal | High | Early/subclinical hypothyroidism |
| Normal | Normal | Euthyroid |
The T4+TSH pair matters: low T4 alone is non-specific (sick dogs suppress T4). A high TSH confirms primary hypothyroidism.
Interpreting the Cat: T4¶
| T4 | Interpretation |
|---|---|
| Elevated | Hyperthyroidism (most cats) |
| High-normal + compatible signs | Early hyperthyroidism — retest or free T4 |
Reference Ranges¶
| Analyte | Species | Reference Range |
|---|---|---|
| T4 (total) | Canine/Feline | 1.0–4.0 µg/dL |
| TSH | Canine | 0–0.6 ng/mL |
FAQ¶
Why is my sick dog's T4 low? Euthyroid sick syndrome — non-thyroidal illness suppresses T4. The normal TSH distinguishes this from true hypothyroidism.
Why do I test T4 and TSH together? T4 alone cannot distinguish primary hypothyroidism from sick-euthyroid; the paired TSH makes the diagnosis.
Case Study: The Lethargic Spayed Female¶
A 6-year-old spayed Labrador presents with lethargy, weight gain, and a thin hair coat. No other systemic illness found.
- T4: 0.8 µg/dL (low; reference 1.0–4.0).
- TSH: 0.9 ng/mL (high; reference 0–0.6).
Interpretation: Low T4 plus high TSH is diagnostic of primary hypothyroidism.
Contrast: A sick dog (e.g., with pneumonia) may show low T4 with normal TSH — euthyroid sick syndrome. The paired TSH is what separates the two.
Lesson: Never diagnose hypothyroidism on T4 alone. The T4+TSH pair is the difference between a correct diagnosis and an unnecessary lifetime of supplementation.
References¶
- Strey S, Mischke R, Rieder J. Hypothyroidism in dogs: an overview. Tierarztl Prax Ausg K Kleintiere Heimtiere. 2021;49(3):195-205. PMID: 34157761
For the review policy, see the Editorial & Review Policy.
Authored by: Migibio Clinical & Scientific Affairs, Guangzhou Magic Biotech Co., Ltd.
Reviewed by: Migibio R&D Quality Committee
Last updated: 2026-08-13
Disclosure: Migibio (Guangzhou Magic Biotech Co., Ltd.) is the manufacturer of the FIA680/FIA880 analyzers and FICT reagents referenced in this content. See our Editorial & Review Policy.