Sick euthyroid syndrome in the dog: low T4
8-year-old Labrador hospitalised with aspiration bronchopneumonia. It is lethargic and a thyroid panel is requested to rule out hypothyroidism. No previous skin signs or weight gain.
| Parameter | Result | Reference |
|---|---|---|
| Total T4 | 0.5 µg/dL | 1.0–4.0 |
| Free T4 (equilibrium dialysis) | 10 pmol/L | 8–40 |
| Canine TSH | 0.2 ng/mL | < 0.6 |
What is most appropriate?
chevron_right Show answer and explanation
A. Do not supplement and reassess thyroid function after recovery
Correct. Systemic illness lowers total T4 (reduced protein binding, lower TSH, altered peripheral metabolism) without true hypothyroidism. With normal free T4 and TSH, the right approach is to treat the pneumonia and repeat the panel weeks after recovery.
B. Start levothyroxine to aid recovery
Supplementing a sick euthyroid dog does not improve the prognosis and may be harmful. It would also leave the dog with a lifelong misdiagnosis.
C. Central hypothyroidism due to a pituitary lesion
It is very rare in dogs and there are no neurological signs or other pituitary deficiencies. Non-thyroidal illness easily explains the findings.
D. Primary hypothyroidism with normal TSH, which occurs in a third of cases
TSH can indeed be normal in hypothyroid dogs, but with no previous signs, normal free T4 and a severe ongoing illness, the diagnosis cannot be supported.
lightbulb Key takeaway
Do not test the thyroid in a patient with active systemic illness: total T4 falls in proportion to severity. If testing is essential, free T4 by dialysis and TSH are more reliable, but ideally wait until recovery.
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Updated: October 31, 2026 · Educational content for veterinary professionals. It does not replace current clinical guidelines or the judgement of the attending veterinarian.